Amicus Curiae Brief — Bradley Little, Governor of Idaho, et al., Petitioners v. Lindsay Hecox, et al.

No. 24-38

In the Supreme Court of the United States

BRADLEY LITTLE, GOVERNOR OF IDAHO, ET AL.,

Petitioners,

v.

LINDSAY HECOX, ET AL.

On Writ of Certiorari to

the United States Court of Appeals

for the Ninth Circuit

BRIEF OF AMICUS CURIAE

THE AMERICAN COLLEGE

OF PEDIATRICIANS

IN SUPPORT OF PETITIONERS

EDWARD H. TRENT

Counsel of Record

GENE C. SCHAERR

JOHN GREIL

SCHAERR | JAFFE LLP

1717 K Street NW, Suite 900

Washington, DC 20006

Telephone: (202) 787-1060

etrent@schaerr-jaffe.com

Counsel for Amicus Curiae

SEPTEMBER 19, 2025

TABLE OF CONTENTS

TABLE OF AUTHORITIES ....................................... iii

INTRODUCTION AND INTEREST OF

AMICUS CURIAE ................................................... 1

SUMMARY .................................................................. 2

ARGUMENT ................................................................ 4

I.

II.

The Idaho and West Virginia Statutes

Reasonably Classify Sports

Participation on the Basis of Biological

Sex ................................................................... 8

A.

Sex is an Immutable, Objectively

Discernible, Biological

Characteristic. ......................................... 9

B.

Sex is not “assigned” at birth but

rather observed and announced

based on biological reality..................... 11

C.

“Transitioning” to a different sex is

biologically impossible. ......................... 13

The Medical and Physiological Record

Demonstrates Material, Sex-Based

Biological Differences Between Males

and Females .................................................. 14

A.

Significant pre-pubertal differences

between males and females reflect

genetic programming and

structural organization and are not

contingent on pubertal testosterone ..... 15

B.

C.

ii

Testosterone drives lasting male

athletic advantages that begin in

utero and continue through and

beyond puberty. ..................................... 17

Puberty blockers and exogenous

estrogen do not eliminate male

biological advantages. ........................... 20

III. The Record of Sports Performance

Demonstrates Material, Sex-Based

Biological Differences Between Males

and Females .................................................. 26

A.

Males demonstrate higher

performance across diverse sportsrelated metrics. ..................................... 27

B.

Biological males show performance

advantages across a broad range of

sports. .................................................... 29

C.

Testosterone suppression does not

eliminate male performance

advantages............................................. 31

CONCLUSION .......................................................... 33

iii

TABLE OF AUTHORITIES

Cases

Page(s)

B.P.J. v. West Virginia State Bd.

of Educ., 98 F.4th 542

(4th Cir. 2024) ....................... 5, 10, 12-15, 20, 27, 31

Hecox v. Little,

104 F.4th 1061

(9th Cir. 2024) ....................... 5, 10, 12-15, 20, 27, 31

Michael M. v. Superior Ct. of Sonoma Cnty.,

450 U.S. 464 (1981) .................................................. 7

United States v. Skrmetti,

145 S. Ct. 1816 (2025) .............................. 4, 9, 11, 12

Statutes

Idaho Code § 33-6203 .................................................. 8

W. Va. Code § 18-2-25d ......................................... 8, 10

Regulation

34 C.F.R. § 106.41........................................................ 9

Other Authorities

Am. Coll. of Pediatricians,

Gender Dysphoria in Children (Nov. 2018) .......... 13

Am. Coll. of Pediatricians,

Mental Health in Adolescents with

Incongruence of Gender Identity and

Biological Sex (2024) .............................................. 10

Am. Coll. of Pediatricians,

Sex is a Biological Trait of Medical

Significance (Mar. 2021) ............................ 11, 12, 16

iv

Sandro Bartolomei et al.,

A Comparison between Male and Female

Athletes in Relative Strength and Power

Performances, 6 J. Functional Morphology

& Kinesiology, art. 17 (2021) ................................. 28

Gregory A. Brown et al.,

Sex‐based differences in track running

distances of 100, 200, 400, 800, and 1500m

in the 8 and under and 9–10‐year‐old age

groups, 24 European J. Sport Sci. 217 (2024) . 21, 22

Gong Chen et al.,

A Comparative Study on Strength between

American College Male and Female Students

in Caucasian and Asian Populations,

21 Sport Sci. Rev. 153 (2012) ............................. 6, 27

Mandy W. Christensen & Christine M.

Griffiths, Sex Differences in 1600-m

Running Performance and Participation

for Children Aged 6-12 yr,

3 Exercise Sport & Movement e00051 (2025) ....... 22

Comm. on Understanding Biology of Sex

& Gender Differences, Exploring the

Biological Contributions to Human Health.

Does Sex Matter? (Theresa M. Wizemann &

Mary-Lou Pardue eds., 2001) ................................ 15

Lea K. Davis,

Bridging Molecular Genetics and

Epidemiology to Better Understand Sex

Differences in Attention-Deficit/

Hyperactivity Disorder,

83 Biological Psych. e55 (2018) ............................. 16

v

FDA,

Sex as a Biological Variable (Jan. 30, 2025) ......... 12

Genetic All., N.Y.-Mid-Atl. Consortium for

Genetic & Newborn Screening Servs.,

Understanding Genetics: A New York,

Mid-Atlantic Guide for Patients and Health

Professionals (2009) ............................................... 16

Moran Gershoni & Shmuel Pietrokovski,

The landscape of sex-differential

transcriptome and its consequent

selection in human adults,

15 BMC Biology, art. 7 (2017) ............................... 17

Louis J. G. Gooren & Mathijs C. M. Bunck,

Transsexuals and competitive sports,

151 European J. Endocrinology 425 (2004) .......... 33

Lydia C. Hallam & Fabiano T. Amorim,

Expanding the Gap: An Updated Look Into

Sex Differences in Running Performance,

12 Frontiers in Physiology 1 (2022) ...................... 30

David J. Handelsman et al.,

Circulating Testosterone as the Hormonal

Basis of Sex Differences in Athletic

Performance, 39 Endocrine Revs. 803 (2018) ....... 20

Alison K. Heather,

Transwoman Elite Athletes: Their Extra

Percentage Relative to Female Physiology,

26 Int’l J. Envt’l Rsch. & Pub. Health 1

(2022) ........................................................................ 6

vi

Emma N. Hilton & Tommy R. Lundberg,

Transgender Women in the Female

Category of Sport: Perspectives

on Testosterone Suppression and

Performance Advantage,

51 Sports Med. 199 (2021) ... 7, 18, 23, 24, 30, 31, 33

Sandra K. Hunter et al.,

The Biological Basis of Sex

Differences in Athletic Performance:

Consensus Statement for the American

College of Sports Medicine,

55 Med. & Sci. Sports &

Exercise 2328 (2023) .......... 19, 21, 24, 25, 27, 29, 33

Michael V. Lombardo et al.,

Fetal Testosterone Influences Sexually

Dimorphic Gray Matter in the Human

Brain, 32 J. Neurosci. 674 (2012) .................... 18, 19

Carlos C. Marta et al.,

Physical Fitness Differences Between

Prepubescent Boys and Girls,

26 J. Strength & Conditioning Rsch. 1756

(2012) ...................................................................... 22

Kelly L. McNulty et al.,

The Effects of Menstrual Cycle Phase on

Exercise Performance in Eumenorrheic

Women: A Systematic Review and MetaAnalysis, 50 Sports Med. 1813 (2020) ................... 17

vii

Natalie J. Nokoff et al.,

Body Composition and Markers of

Cardiometabolic Health in Transgender

Youth Compared With Cisgender Youth,

105 J. Clinical Endocrinology &

Metabolism e704 (2020) ......................................... 24

Rudolfo Ray & Chrystèle Racine,

Sexual Differentiation, in Endotext

(Kenneth R. Feingold et al. eds., updated

July 24, 2025) ......................................................... 18

Juan G. Ripoll et al.,

Sex differences in pediatric airway anatomy,

105 Experimental Physiology 721 (2020) ............. 26

Timothy A. Roberts et al.,

Effect of gender affirming hormones on

athletic performance in transwomen and

transmen: implications for sporting

organisations and legislators,

55 British J. Sports Med. 577 (2020) .............. 26, 32

Nejc Šarabon et al.,

Establishing Reference Values for Isometric

Knee Extension and Flexion Strength,

12 Frontiers in Physiology 767941 (2021) ........ 6, 28

Miranda Scharff et al.,

Change in grip strength in trans people and

its association with lean body mass and bone

density, 8 Endocrine Connections 1020

(2019) .................................................................. 7, 32

viii

David Schwartz,

Clinical and Ethical Considerations

in the Treatment of Gender Dysphoric

Children and Adolescents: When

Doing Less Is Helping More,

20 J. Infant, Child & Adolescent Psych. 439

(2021) ...................................................................... 10

U.S. Dep’t A.F.,

DAFMAN No. 36-2905, Department of the

Air Force Physical Fitness Program (2022) .......... 32

Ying-Chih Wang et al.,

Hand-Grip Strength: Normative

Reference Values and Equations for

Individuals 18 to 85 Years of Age

Residing in the United States,

48 J. Orthopaedic & Sports Physical

Therapy 685 (2018) ............................................ 6, 28

R. Ann Word et al.,

Testosterone synthesis and adenylate cyclase

activity in the early human fetal testis

appear to be independent of human

chorionic gonadotropin control,

69 J. Clinical Endocrinology &

Metabolism 204 (1989) .......................................... 20

INTRODUCTION AND

INTEREST OF AMICUS CURIAE 1

Millions of American girls and women participate

in competitive sports. From grade school teams to the

Olympic Games, the separation of male and female

divisions has been recognized as essential to fair

competition. This structure reflects both common

sense and biological reality: males, as a class, possess

enduring advantages in speed, strength, endurance,

and power. Until recently, no court had questioned

that basic truth. But the Ninth and Fourth Circuits

have now reached the radical conclusion that

protecting girls’ and women’s sports by limiting them

to females is itself against the law.

This issue is of great concern to the American

College of Pediatricians (the College or ACPeds), one

of the Nation’s leading science-oriented medical

organizations. The College is a national association of

nearly 500 board-certified pediatricians and related

specialists with active practices in 46 states, all

dedicated to the health and well-being of children.

Founded in 2002, the College is a scientific medical

association

committed

to

producing

policy

recommendations grounded in the best available

research. Its mission is to ensure that all children

reach their optimal physical and emotional health and

well-being. Youth sports contribute to that well-being.

1 This brief was not authored in whole or in part by counsel for

any party and no person or entity other than amicus curiae or its

counsel has made a monetary contribution toward the brief’s

preparation or submission.

2

Amicus’s members care for student-athletes every

day and advise families and schools on training, injury

risk, and healthy physical and social development.

They treat all patients with dignity, without

discrimination, and cannot endorse policies that deny

biological facts or expose girls to avoidable harm.

When males compete in female categories, girls

predictably lose roster spots and scholarships, face

unequal competition, and in some sports bear higher

injury risk. Those are real medical and developmental

harms to minors. ACPeds submits this brief to present

the medical record on sex-based differences and to

explain why sex-separated teams are evidence-based

safeguards for fairness and equal opportunity.

SUMMARY

The Court is considering challenges to two state

statutes, from Idaho and West Virginia, that protect

girls’ sports by relying on biological reality. For

decades, it was universally recognized that separating

male and female competition is necessary to ensure

equal athletic opportunity, because males, as a class,

possess enduring physical advantages in speed,

strength, endurance, and power. The Fourth and

Ninth Circuits cast aside that consensus, substituting

ideology for science and treating subjective gender

identity as if it could erase the immutable advantages

of male physiology. Their decisions should be reversed.

I. Idaho and West Virginia reasonably classified

sports participation by biological sex, an immutable

and objectively verifiable trait. That choice reflects

long-standing practice in athletics and common sense:

males and females differ in ways that directly affect

3

performance, and preserving equal opportunity for

girls requires sex-based teams. The courts below erred

by collapsing biological sex into subjective gender

identity, adopting the misleading rhetoric that sex is

“assigned at birth,” and crediting claims that sex can

be “transitioned” in a way that eliminates physical

differences. But sex is not assigned, and it cannot be

changed. It is a stable biological reality, and grounding

athletic classifications in that reality is necessary to

maintain fairness.

II. The medical and physiological record confirms

that males possess enduring advantages in strength,

speed, and endurance that are not erased by hormones

or surgery. These differences are driven by genetic

programming, appear in utero, are amplified during

infant “minipuberty,” magnified by the surge of

testosterone during adolescence, and persist even

after suppression of testosterone or administration of

estrogen therapy. Larger hearts and lungs, more fasttwitch muscle fibers, and denser bones give males

structural and functional advantages that no medical

intervention can undo. By reducing the analysis to

pubertal testosterone, the courts below ignored the

broader body of scientific evidence showing that sexbased differences are permanent, architectural, and

significant in athletic performance.

III. The record of sports performance confirms

what biology predicts: males consistently outperform

females across both discrete metrics and competitive

outcomes. Grip strength, knee extension torque, and

VO₂ max all show substantial male advantages, which

translate directly into faster sprint times, higher lifts,

and superior endurance. Across disciplines, men

4

outperform women by margins ranging from 10% in

swimming and rowing to over 50% in baseball

pitching. And hormone suppression does not close the

gap: even after years of treatment, male athletes

retain strength and endurance levels above those of

female peers.

While

dismissing

the

well-documented

physiological differences between males and females,

the courts below embraced the unproven claim that

identity and hormone therapy can erase sex-based

advantages. But that is false: the evidence shows these

differences are in fact sex-based, not hormone levelbased, and those differences amply justify sex

segregated sports—and associated intimate spaces

such as locker rooms. Their decisions below thus

prioritize gender ideology over biology. It cannot be

true that providing girls with an equal opportunity to

compete violates our guarantees of equality. The

decisions below should be reversed.

ARGUMENT

Equal athletic opportunity for women depends on

recognizing biological reality. Until the last decade, all

have recognized that male and female competitions

should be separated because men, as a class, possess

enduring physical advantages in speed, strength,

endurance, and power. Those advantages begin in

utero, are present before puberty, and persist even

after males are administered puberty blockers and

testosterone suppressants. As this Court recently

emphasized, some medical “treatments and

procedures are uniquely bound up in sex.” United

States v. Skrmetti, 145 S. Ct. 1816, 1820 (2025). The

5

same is true in athletics, where sex-based

physiological differences affect performance and

cannot be erased by identity or intervention.

Recognizing that reality does not violate the

Constitution but upholds it.

Yet the courts below dismissed this reality. The

Ninth Circuit suggested that Idaho’s biologicallybased definition of sex was an “oversimplification”

because it should have included subjective, malleable

“gender identity” in its definition of sex. Hecox v.

Little, 104 F.4th 1061, 1076 (9th Cir. 2024) (internal

citation omitted), cert. granted mem., 145 S. Ct. 2871

(2025). The Fourth Circuit accepted claims that a male

treated with puberty blockers and estrogen

“possess[es]

no

inherent,

biologically-based

competitive advantages over cisgender girls.” B.P.J. v.

West Virginia State Bd. of Educ., 98 F.4th 542, 559

(4th Cir. 2024) (consolidated action), cert. denied sub

nom. West Virginia Secondary Sch. Activities Comm’n

v. B.P.J., 145 S. Ct. 568 (2024) and cert. granted, No.

24-44, 2025 WL 1829164 (July 3, 2025). Those

conclusions rest on ideology, not evidence.

The courts mistakenly reduced the male athletic

advantage to a single variable: pubertal (or

circulating) testosterone, as if pubertal testosterone

determines whether the individual is male or female.

But that view ignores a host of other biological

differences that shape athletic performance and do not

depend on pubertal testosterone. Long before puberty,

male genetics result in sex differences such as bone

6

shape and lung volume. 2 These features translate into

superior

oxygen

transport,

stronger

muscle

contraction, and greater torque in explosive

movements. 3 They are written into the male body at

every stage of development, even before birth, and

cannot be erased by suppressing hormones during

adolescence.

These advantages are evident across core

performance metrics. Grip strength, a simple proxy for

overall strength, sees males aged 18-24 at a 60%

advantage over females. 4 Female triceps and chest

strength is about half of males, 5 while males have a

28% advantage in lower body power as measured by

knee extension torque. 6 In specific events, male

advantages range from 11-13% in rowing, to a 50%

2 Alison

K. Heather, Transwoman Elite Athletes: Their Extra

Percentage Relative to Female Physiology, 26 Int’l J. Envt’l Rsch.

& Pub. Health 1, 4 (2022), 10.3390/ijerph19159103.

3 Id. at 4-5.

4 Ying-Chih

Wang et al., Hand-Grip Strength: Normative

Reference Values and Equations for Individuals 18 to 85 Years of

Age Residing in the United States, 48 J. Orthopaedic & Sports

Physical

Therapy

685,

688

tbl.

1

(2018),

https://tinyurl.com/5chtkr5r.

5 Gong Chen et al., A Comparative Study on Strength between

American College Male and Female Students in Caucasian and

Asian Populations, 21 Sport Sci. Rev. 153, 156 & tbls. 2-3 (2012),

https://tinyurl.com/bdxsffa4.

6 Nejc Šarabon et al., Establishing Reference Values for Isometric

Knee Extension and Flexion Strength, 12 Frontiers in Physiology

767941, at 5 & tbl. 1 (2021), https://doi.org/10.3389/

fphys.2021.767941.

7

advantage in baseball throwing velocity. 7 These are

quantifiable, class-based differences.

Testosterone suppression does not eliminate these

advantages. When males suppressed testosterone for

twelve months and lost 4% of grip strength, their

handgrip still exceeded that of more than 90% of

females. 8

Likewise,

long-term

testosterone

suppression left biological males with higher muscle

mass than women, even though it was lower than that

of other men. 9 This is due to “the large baseline

differences in muscle mass between males and

females” of approximately 40%. 10 Hormone therapy

does not close the gap between male and female

performance or turn a male into a female. And sex

classifications grounded in biological differences do

not violate federal law. Cf. Michael M. v. Superior Ct.

of Sonoma Cnty., 450 U.S. 464, 476 (1981) (statute did

not unlawfully discriminate, because it “reasonably

reflect[ed] the fact that the consequences * * * fall

more heavily on the female than on the male”).

To help the Court evaluate how the States’ laws

are tailored to ensuring equal opportunity for female

7 Emma N. Hilton & Tommy R. Lundberg, Transgender Women

in the Female Category of Sport: Perspectives on Testosterone

Suppression and Performance Advantage, 51 Sports Med. 199,

201-203 & fig. 1 (2021), doi: 10.1007/s40279-020-01389-3;

Erratum, 51 Sports Med. 2235 (2021), doi: 10.1007/s40279-02101480-3.

8 See Miranda Scharff et al., Change in grip strength in trans

people and its association with lean body mass and bone density,

8 Endocrine Connections 1020, 1026 (2019).

9 Hilton & Lundberg, supra note 7, at 207.

10 Id. at 207 & tbl. 1.

8

athletes, ACPeds will focus on (1) the States’

legitimate choice to classify athletic participation by

biological sex, an immutable and objectively

discernible characteristic; (2) the medical and

physiological evidence demonstrating that males

retain enduring athletic advantages that cannot be

erased by puberty blockers or hormone suppression;

and (3) the record of sports performance, which

confirms in practice what science shows in theory: that

males consistently outperform females across events

and metrics, even after hormone treatment.

Idaho and West Virginia have ensured equal

opportunity for girls through a reality-based

categorization. The decisions of the Fourth and Ninth

Circuits should be reversed.

I.

The Idaho and West Virginia Statutes

Reasonably Classify Sports Participation on

the Basis of Biological Sex

Both Idaho and West Virginia took the

straightforward step of classifying sports teams based

on biological sex. West Virginia requires teams to be

designated male, female, or coed, and defines

“[b]iological sex” as “an individual’s physical form as a

male or female based solely on the individual’s

reproductive biology and genetics at birth.” W. Va.

Code § 18-2-25d(b)(1)-(3). Similarly, Idaho’s Fairness

in Women’s Sports Act provides that participation is

determined by a student’s “reproductive anatomy,

genetic makeup, or normal endogenously produced

testosterone levels.” Idaho Code § 33-6203(3).

This is not a novel approach. For decades, states

have sponsored separate teams for each sex in a wide

9

variety of athletic competitions. See 34 C.F.R.

§ 106.41(b) (reiterating that Title IX allows schools to

operate “separate teams for members of each sex”).

The statutes simply codify what has long been

common sense: that equal opportunity in athletics

requires separating competition on the basis of sex,

because male physiology confers enduring advantages

in speed, strength, and endurance.

Just last term, this Court recognized that in the

“medical context * * * some treatments and

procedures are uniquely bound up in sex.” Skrmetti,

145 S. Ct. at 1820. So too in athletics, where the very

purpose of separate male and female teams is to

account for sex-based physiological differences that

directly affect performance.

States that separate physical competitions by sex

do not violate our guarantees of equal opportunity in

sports. They ensure that opportunity is real for sex is

an immutable, objectively discernible biological

characteristic.

A. Sex is an Immutable, Objectively

Discernible, Biological Characteristic.

Biological sex is immutable and identifiable. It is

determined by physical, observable, and measurable

features such as chromosomes and reproductive

anatomy. It is “almost always easily identifiable at

birth (if not before) based upon phenotypic expression

of chromosomal complement” 11—XX for female, and

XY for male.

11 Am.

Coll. of Pediatricians (ACPeds), Mental Health in

Adolescents with Incongruence of Gender Identity and Biological

10

This case also involves a separate issue: gender

identity. Gender identity is a psychological construct

that reflects self-perception and cultural expectations

rather than anatomy or genetics. “Gender does not

exist in the body or in any bodily structure or process.

This is in contrast to sex, which is determined

exclusively

by

bodily

data:

genitals

and

12

chromosomes.”

The decisions below departed from this basic

distinction between immutable biological sex and

psychological gender identity. For instance, the Ninth

Circuit suggested that Idaho’s definition rested on an

“oversimplification of the complicated biological

reality of sex and gender,” and asserted that ‘“a

person’s sex encompasses the sum of several biological

attributes * * * and gender identity.’” Hecox, 104 F.4th

at 1076 (emphasis added) (internal citation omitted).

The Fourth Circuit determined it was discriminatory

to define sex based on ‘“reproductive biology and

genetics at birth’” because it “exclude[s] transgender

girls from the definition of ‘female.’” B.P.J., 98 F.4th

at 555-556 (quoting W. Va. Code § 18-2-25d(b)(1)).

What both courts refused to recognize is that sex is

biological and not influenced by a psychological

concept of “gender identity.” That is why “transgender

Sex 2 (2024) (citing extensive

https://tinyurl.com/u5wrbak8.

12 David

scientific

research),

Schwartz, Clinical and Ethical Considerations in the

Treatment of Gender Dysphoric Children and Adolescents: When

Doing Less Is Helping More, 20 J. Infant, Child & Adolescent

Psych. 439, 439 (2021).

11

girls” (males) are not included in the definition of

“female.”

By conflating immutable sex with variable gender

identity, the Fourth and Ninth Circuits improperly

inserted ideology into a biological and medical

definition. Cf. Skrmetti, 145 S. Ct. at 1851 (Barrett, J.,

concurring) (“[T]ransgender status does not turn on an

immutable characteristic.” (cleaned up)); accord id. at

1861 (Alito, J., concurring in part and concurring in

the judgment) (“Transgender status is not

‘immutable,’ and as a result, persons can and do move

into and out of the class.”). Proper analysis of the

question of sex classifications in sports requires an

accurate approach.

B. Sex is not “assigned” at birth but rather

observed and announced based on

biological reality.

That approach includes the reality that

“[p]ediatricians do not ‘assign’ an infant’s sex; they

announce it based upon the physical reality of the

infant’s body before them.” 13 Sex is dimorphic and

innate, determined at fertilization by chromosomal

complement and revealed in distinctive reproductive

anatomy and unambiguous genitalia. This biological

reality is not altered by cultural perception, identity,

or later psychological discomfort.

Describing sex as “assigned” falsely suggests that

it is arbitrary. It is anything but. Biological sex is “a

stable and universally applicable definition that

13 Am. Coll. of Pediatricians (ACPeds), Sex is a Biological Trait of

Medical Significance 1 (Mar. 2021), https://tinyurl.com/28xjxwrv.

12

allows the consistent differentiation of males from

females.” 14 It is the reason that there are “variations

seen in the safety and efficacy of drugs, biologics, and

medical devices” between men and women. See

Skrmetti, 145 S. Ct. at 1829-1830 (quoting FDA, Sex

as a Biological Variable (Jan. 30, 2025)). And if sex

were assigned arbitrarily at birth, it would not be

marked by the sort of “obvious, immutable or

distinguishing characteristics” that characterize a

suspect class. Cf. id. at 1851 (Barrett, J., concurring)

(cleaned up).

Yet both courts below adopted the misleading

terminology that sex is “assigned at birth,” rather than

rooted in biological reality, observed and announced.

The Ninth Circuit stated that “[a] person’s ‘sex’ is

typically assigned at birth based on an infant’s

external genitalia.” Hecox, 104 F.4th at 1068.

Likewise, the Fourth Circuit described West Virginia’s

statute as creating “a rule that people whose sex was

assigned at birth as female may play on any team but

people whose sex was assigned at birth as male may

only play on male or co-ed teams.” B.P.J., 98 F.4th at

556.

People are not assigned a sex; they have one, and

that biological reality creates real athletic advantages

between the sexes.

14 Id. at 2.

13

C. “Transitioning” to a different sex is

biologically impossible.

Sex, moreover, is not malleable. It is a biological

and immutable characteristic determined at

conception and revealed through chromosomal

complement and corresponding reproductive anatomy.

“From a purely scientific standpoint, human beings

possess a biologically determined sex and innate sex

differences. No sexologist could actually change a

person’s genes through hormones and surgery. Sex

change is objectively impossible.” 15

Describing medical or surgical interventions as

“sex changes” is thus scientifically inaccurate; they

may alter outward appearance but not chromosomal

reality. Proponents of “transitioning,” typically

concede that chromosomes and gonads remain

unchanged. Like the Ninth Circuit below, they may

attempt to redefine “gender” as a subjective identity

distinct from biology, and then smuggle that definition

into discussions of sex. See Hecox, 104 F.4th at 1076

(including gender identity in definition of sex); accord

B.P.J., 98 F.4th at 555-556.

But gender is a socially and politically constructed

concept, not a biological trait. Linguistic moves do not

alter scientific facts that have direct implications for

sports. Athletic performance is tied to skeletal

structure, muscle composition, lung capacity, and

other features determined by sex, not by selfperception. A male athlete may undergo hormone

suppression or surgeries, but his XY chromosomes

15 Am.

Coll. of Pediatricians (ACPeds), Gender Dysphoria in

Children 3 (Nov. 2018), https://tinyurl.com/mu5etsp9.

14

that result in larger bones, greater muscle mass, and

other sex-based traits remain. The suggestion that

medicine can erase or “transition” these biological

realities is not science, but fanciful ideology.

II. The Medical and Physiological Record

Demonstrates

Material,

Sex-Based

Biological Differences Between Males and

Females

Driven as it is by these scientific realities, the

medical evidence makes plain that males retain

significant physiological advantages over females in

strength, speed, and endurance. Yet the decisions

below downplayed this scientific consensus. The Ninth

Circuit dismissed as a “false assumption” the claim

that biological males have “physiological advantages”

over “cisgender women.” Hecox, 104 F.4th at 1082. The

Fourth Circuit credited Respondent’s assertions that

males with the plaintiff’s “background and

characteristics possess no inherent, biologically-based

competitive advantages over cisgender girls when

participating in sports.” B.P.J., 98 F.4th at 559. In

reaching this conclusion, the court relied on the fact

the plaintiff was placed on puberty blockers at the

beginning of Tanner Stage 2 and then on cross-sex

hormones, thus holding that keeping the plaintiff out

of girls’ sports was not appropriate. Id. at 560-561. But

neither hormone treatment makes the male plaintiff a

female.

The reality is that male advantages arise before

puberty, are amplified by puberty, and are not erased

by medical interventions—including puberty blockers

and exogenous estrogen.

15

A. Significant

pre-pubertal

differences

between males and females reflect

genetic programming and structural

organization and are not contingent on

pubertal testosterone

First, both decisions below rested on the mistaken

idea that testosterone is the only factor tied to athletic

advantage. See Hecox, 104 F.4th at 1075-1076

(“circulating testosterone is the one sex-related factor

that a consensus of the medical community appears to

agree actually affects athletic performance” (cleaned

up)); B.P.J., 98 F.4th at 560-561 (plaintiff had no

“benefit from increased strength and speed” because

he “has never felt the effects of increased levels of

circulating testosterone” due to the administration of

puberty blockers).

The medical record shows, however, that sexlinked genetic programming produces significant

differences between boys and girls well before puberty,

independent of testosterone exposure.

Genes found on sex hormones—not testosterone—

primarily drive the differences between males and

females. Many of these differences do not arise from

hormonal exposure at all, but are a “direct result of the

genetic differences between the two sexes.” 16 At least

three genetic mechanisms contribute to these sexbased differences: (1) the effects of sex chromosomes,

16 Comm. on Understanding Biology of Sex & Gender Differences,

Exploring the Biological Contributions to Human Health. Does

Sex Matter?, at 4 (Theresa M. Wizemann & Mary-Lou Pardue

eds., 2001), https://tinyurl.com/2pzfxffp.

16

(2) sex-dependent genetic liability thresholds, and

(3) sex-differential gene expression. 17

Sex chromosomes associated with males and

females contribute to sex-based differences. 18 The

impact of the Y chromosome, for example, is not

limited to the reproductive system. It determines male

sex across multiple organs. Among females, Xchromosome inactivation exerts a genetic influence

that males do not experience. 19 Because one X

chromosome switches off at random in each female

cell, women are less likely to suffer X-linked diseases

like Hemophilia A and Duchenne Muscular

Dystrophy, which mostly strike boys and men. 20

Sex also shapes genetic risk through so-called

“liability thresholds.” This means that males and

females face different odds of developing certain

conditions. 21

Finally,

sex-differential

gene

expression

contributes to many of the physiological differences

between males and females. Researchers have

17 For

an extended discussion, see generally ACPeds, Sex is a

Biological Trait of Medical Significance, supra note 13.

18 See id. at 2.

19 Id. at 3.

20 Genetic All., N.Y.-Mid-Atl. Consortium for Genetic & Newborn

Screening Servs., Understanding Genetics: A New York, MidAtlantic Guide for Patients and Health Professionals 70-71 app.

E (2009), https://tinyurl.com/5n98w4jv.

21 See

Lea K. Davis, Bridging Molecular Genetics and

Epidemiology to Better Understand Sex Differences in AttentionDeficit/Hyperactivity Disorder, 83 Biological Psych. e55, e55

(2018).

17

identified at least 6,500 shared genes that are

expressed differently in males and females. 22 Notably,

the superior development and strength of the male

skeletal muscles stem from this sex-differentiated

gene expression. 23

The upshot is that genetic differences between

males and females are significant and objectively

observable even before puberty. Puberty adds further

changes through rising sex hormones.

B. Testosterone drives lasting male athletic

advantages that begin in utero and

continue through and beyond puberty.

Puberty magnifies the pre-existing biological gap

between males and females. The surge of sex

hormones, especially testosterone, reshapes the body

in ways that drive lasting athletic advantages for

males.

The two major sex-hormones that influence

athletic performance are estrogen and testosterone. A

meta-analysis of 51 studies showed that variations in

estrogen levels likely only have trivial effects on

athletic performance. 24 In contrast, it is well accepted

that testosterone greatly influences athletic

22 Moran Gershoni & Shmuel Pietrokovski, The landscape of sex-

differential transcriptome and its consequent selection in human

adults, 15 BMC Biology, art. 7, at 2-3 (2017),

https://doi.org/10.1186/s12915-017-0352-z.

23 Id. at 3.

24 See

Kelly L. McNulty et al., The Effects of Menstrual Cycle

Phase on Exercise Performance in Eumenorrheic Women: A

Systematic Review and Meta-Analysis, 50 Sports Med. 1813, 1821

(2020).

18

performance. And as males age, sexual differences

brought about by natural increased testosterone result

in greater advantages in athletic performance that do

not dissipate even with testosterone suppression

before, during, or after puberty. 25

Contrary to the idea that sex is “assigned” at birth,

testosterone production begins in the male testes at

nine weeks post conception and begins to decrease

around twenty weeks. 26 This rise in testosterone

contributes to the development of the male

reproductive organs and helps organize the male

brain. The female fetus is exposed to testosterone, but

at much lower levels. 27

Studies show that fetal testosterone leaves lasting

marks on the brain. For instance, “variation in fetal

testosterone (FT) predicts later gray matter volume of

specific brain regions in a direction that is congruent

with sexual dimorphism observed in a large

independent sample of age-matched males and

25 Hilton & Lundberg, supra note 7, at 200-201.

26 See Rudolfo Ray & Chrystèle Racine, Sexual Differentiation, in

Endotext, at tbl. 1 (Kenneth R. Feingold et al. eds., updated July

24, 2025), https://www.ncbi.nlm.nih.gov/books/NBK279001/; see

also ibid. (“In the human fetus, Leydig cells can be identified in

the interstitial tissue by the beginning of the 8th week—after

testicular cords have completely formed—and soon begin to

produce testosterone, which plays an essential role in the

stabilization of Wolffian ducts and the masculinization of

external genitalia.” (citations omitted)).

27 See,

e.g., Michael V. Lombardo et al., Fetal Testosterone

Influences Sexually Dimorphic Gray Matter in the Human Brain,

32 J. Neurosci. 674, 679 (2012) (observing “robust sex differences”

in fetal testosterone measured in amniotic fluid, attributable to

the fetus), https://tinyurl.com/52exvwb7.

19

females.” 28 In other words, testosterone exposure

before birth helps shape male and female brains

differently, leaving structural differences that persist

later in life.

Testosterone continues to drive sex-based

differences after birth. During a developmental stage

known as “minipuberty,” there is an increase in serum

testosterone in infant boys with a peak at one to three

months after birth, followed by a decline to

prepubertal levels by six to nine months. 29 Infant girls

at the same age experience an increase in estrogen. As

a result of “minipuberty,” by mid-childhood, girls have

accumulated more body fat than boys, and this

persists and increases during puberty. 30

By age eighteen, male testosterone levels have

surged, rising more than thirty-fold during puberty. 31

After puberty, male circulating testosterone

concentrations are 15 times greater than those of

females at any age. The result is a clear male

advantage in muscle mass, strength and circulating

28 Id. at 674.

29 See

Sandra K. Hunter et al., The Biological Basis of Sex

Differences in Athletic Performance: Consensus Statement for the

American College of Sports Medicine, 55 Med. & Sci. Sports &

Exercise 2328, 2337 (2023), https://tinyurl.com/yj29967d

[hereinafter “Hunter et al., Biological Basis”].

30 Id. at 2338.

31 Id. at 2338 & fig. 7.

20

hemoglobin levels even after adjusting for sex

differences in height and weight. 32

C. Puberty

blockers

and

exogenous

estrogen do not eliminate male biological

advantages.

Both courts below adopted the mistaken premise

that suppressing testosterone eliminates male athletic

advantages.

The Ninth Circuit credited testimony that a male

who “received hormone therapy to lower [his]

circulating levels of testosterone would likely not have

‘physiological characteristics’ that would lead to

enhanced athletic prowess when compared to a

cisgender woman.” Hecox, 104 F.4th at 1084.

Similarly, the Fourth Circuit emphasized that the

plaintiff had “never experienced elevated levels of

circulating testosterone” because he received puberty

blockers, and concluded that hormone therapy would

cause him to develop “physical changes to [his] bones,

muscles, and fat distribution that are typically

experienced by cisgender girls.” B.P.J., 98 F.4th at

560-561.

But that is not true. Males experience increased

circulating testosterone from their testes beginning at

eight weeks post conception, 33 and go through a surge

32 See David J. Handelsman et al., Circulating Testosterone as the

Hormonal Basis of Sex Differences in Athletic Performance, 39

Endocrine Revs. 803, 805 (2018).

33 R. Ann

Word et al., Testosterone synthesis and adenylate

cyclase activity in the early human fetal testis appear to be

independent of human chorionic gonadotropin control, 69

J. Clinical Endocrinology & Metabolism 204, 204 (1989).

21

of testosterone during “minipuberty” at one to three

months after birth, which differentiates the fat

distribution of boys from girls. 34 The lower courts

offered nothing to suggest why, from a constitutional

perspective, putting a child on puberty blockers and

cross-sex hormones entitles the child to be considered

of the opposite sex other than to suggest a “no harm,

no foul” approach to competition, which is not a

constitutional standard.

Recent research shows there are differences in

athletic abilities between boys and girls even before

puberty. Brown et al. evaluated finalist times from the

USA Track and Field National Youth Outdoor

Championships and the National Junior Olympic

Championships for the years 2016-2023 for various

running distances in the 8 and under age group as well

as the 9-10-year-old age group. 35 In the younger age

group, males were faster than females in all events by

4.0% to 6.7%). 36 Specifically, males were faster in the

100m (4.0%), 200m (4.7%), 400m (5.3%), 800m (6.7%),

and 1500m (6.1%). 37 Similarly, boys were faster than

girls in all events in the 9-10-year-old age group. “In

each distance and age group between 2016 and 2023,

34 Hunter et al., Biological Basis, supra note 29, at 2337-2338.

35 Gregory A. Brown et al., Sex based differences in track running

‐

distances of 100, 200, 400, 800, and 1500m in the 8 and under

and 9–10‐year‐old age groups, 24 European J. Sport Sci. 217,

217 (2024), https://onlinelibrary.wiley.com/doi/epdf/10.1002/ejsc.

12075.

36 Ibid.

37 Ibid.

22

the individual fastest male was faster than the

individual fastest female by 3.7 +/- 2.3%.” 38

Another research project in 2025 evaluated “sexbased differences in aerobic running performance at

1600 m for children aged 6-12 yr” while also studying

whether sex-based differences in participation

affected the results. 39 Utilizing the runnercard.com

website, they evaluated the running velocities and

participation for 3,621 children and found “male

children were faster * * * than female children * * * at

every grade level, with an average difference of

7.7%[.]” 40 Their finding “points to an innate

physiological difference underpinning the sexual

dimorphism of children.” 41

Additionally, a cross-sectional study of 312

prepubescent children’s physical fitness (aerobic

fitness, strength, flexibility, speed, agility, and

balance) revealed that boys had higher scores in all

tests, except balance and flexibility. 42 The greatest sex

differences were found in the explosive strength of

upper and lower limbs. 43

38 Ibid.

39 Mandy

W. Christensen & Christine M. Griffiths, Sex

Differences in 1600-m Running Performance and Participation

for Children Aged 6-12 yr, 3 Exercise Sport & Movement e00051,

at 1 (2025), https://tinyurl.com/3xt75tst.

40 Ibid.

41 Id. at 5.

42 Carlos C. Marta et al., Physical Fitness Differences Between

Prepubescent Boys and Girls, 26 J. Strength & Conditioning Rsch.

1756, 1756 (2012).

43 Id. at 1763.

23

Since there are around 6,500 genes that are

expressed differently between males and females, an

estimated 3,000 of which likely influence the

composition and function of skeletal muscle, the

“minipuberty” of males soon after birth may be

responsible for these prepubertal sex-related

differences. 44 Given these differences in athletic

performance between prepubertal boys and girls,

there are legitimate concerns that biological males

who have their puberty blocked will still have innate

advantages over females in athletic performance.

Further, the plaintiffs’ claims defy what we see

with our eyes, what we know from common sense, and

what the scientific evidence plainly shows.

Suppressing testosterone in adolescence cannot undo

the male body already shaped by genes and early

development.

In short, obvious anatomical differences between

males and females cannot be eliminated by puberty

blockers. These differences impact the body’s response

to acute exercise, training, and athletic performance

beginning in infancy and throughout adulthood.

Testosterone’s impact on the brain, skeletal structure,

44 Hilton & Lundberg, supra note 7, at 200-201; see also id. at

201, citing a study of over 85,000 Australian children between 9

and 17 years demonstrated differences between young boys and

girls, with 9-year-old boys running faster than girls in short

sprints (9.8%), running faster in the longer distance one mile

(16.6%), and jumping farther from a standing start (9.5%). In

addition, the boys could complete 33% more push-ups in 30

seconds and had a more powerful grip by 13.8%, and other

international studies.

24

muscle mass, muscle fiber type, muscle memory, and

cardiorespiratory system is architectural.

In layman’s terms, these changes are permanent

and not modifiable, and therefore they do not change

with sex-hormone supplementation or suppression. In

medical terms, “superior anthropometric, muscle mass

and strength parameters achieved by males at

puberty, and underpinning a considerable portion of

the male performance advantage over females, are not

removed by the current regimen of testosterone

suppression permitting participation of transgender

women in female sports categories.” 45 Males do not

become females, and after about a year of hormone

treatment, transgender individuals develop body

composition (levels of fat and lean muscle) that falls

between that of male and female peers matched for

body size. 46

The permanent structural differences between

males and females are numerous and significant. To

identify a few, males are generally taller than females

and have greater lean body mass, lower percentage of

fat, and longer upper and lower limbs with larger and

denser bones. 47 Even at birth, the average male is

heavier and taller than the average female. And this

advantage, for most athletic endeavors, continues

throughout life.

45 Id. at 209.

46 Natalie

J. Nokoff et al., Body Composition and Markers of

Cardiometabolic Health in Transgender Youth Compared With

Cisgender Youth, 105 J. Clinical Endocrinology & Metabolism

e704, e712-e713 (2020).

47 Hunter et al., Biological Basis, supra note 29, at 2335.

25

Variation in muscle mass between males and

females is also a permanent difference that cannot be

reversed by hormone treatment. “Males have larger,

stronger, faster, and more powerful skeletal muscles

than females[.]” 48 Muscle mass compared to body mass

is generally greater in men than in women. And males

possess a more even distribution of muscle mass

between their upper and lower body. These anatomical

differences provide greater leverage for muscular limb

power exerted in jumping, throwing, and other

explosive power activities. Consequently, “[t]he

muscle mass and limb power of males can be twice that

of females,” 49 an advantage retained even years after

hormone therapy. 50

Additional anatomical differences between males

and females include males having a higher proportion

of fast twitch muscles, males having a narrower pelvis,

allowing for greater generation of force during

extension—improving their ability to squat, kick, or

pedal—and males having a larger lung capacity,

leading to greater cardiovascular ability.

These

permanent

sex-based

differences

demonstrate that the gap between males and females

in athletic ability cannot be erased by sex-hormone

treatment. Consider lung size, which allows males to

take in more oxygen, meaning more fuel for muscles

during exercise. Males’ larger lung and airway size

48 Id. at 2334.

49 Id. at 2335.

50 Id. at 2345.

26

emerges by age 14, resulting in life-long greater

maximum expiratory flows than females. 51

More general metrics, like mean run speed, are

also impacted. In one longitudinal study, males who

receive cross-sex hormones “still had a 9% faster mean

run speed after the 1 year period of testosterone

suppression that is recommended by World Athletics

for inclusion in women’s events.” 52

The Ninth and Fourth Circuits erred in treating

testosterone suppression as a reset button that erases

male advantage and effectively turns a boy into a girl.

The scientific record demonstrates the opposite: males

retain permanent structural and physiological

differences that make them, as a class, stronger,

faster, and more powerful than females.

III. The

Record

of

Sports

Performance

Demonstrates

Material,

Sex-Based

Biological Differences Between Males and

Females

Discrete biological features detailed above,

including muscle mass, lung capacity, skeletal

structure, and hormonal profile create significant

male physical advantages. These advantages manifest

both through discrete measurements such as grip

51 Juan

G. Ripoll et al., Sex differences in pediatric airway

anatomy, 105 Experimental Physiology 721, 722 (2020).

52 Timothy A. Roberts et al., Effect of gender affirming hormones

on athletic performance in transwomen and transmen:

implications for sporting organisations and legislators, 55 British

J. Sports Med. 577, 577 (2020), https://tinyurl.com/yc5afcx3.

27

strength, as well as performance outcomes such as

swim heat times.

Yet the courts below ignored this evidence. The

Ninth Circuit was satisfied that not “all transgender

women, including those like [the plaintiff] who receive

hormone therapy, have a physiological advantage over

cisgender women.” Hecox, 104 F.4th at 1085; accord

B.P.J., 98 F.4th at 559 (Fourth Circuit).

No accurate understanding of fair competition can

rest on this absurd foundation, which contravenes the

scientific evidence.

A. Males demonstrate higher performance

across diverse sports-related metrics.

First, across core measures of strength, power, and

oxygen delivery, males show higher values.

Muscle fibers. “[M]ales persist in possessing a

larger area of type II fibers compared with

females[.]” 53 Type II muscle fibers are the body’s “fasttwitch” fibers that generate quick, powerful bursts of

strength necessary for sprinting, jumping, or

throwing. Because men have more of these fibers, and

the fibers themselves are larger, they can produce

more force at higher speeds. The result is that female

upper body strength measures in the range of 50–60%

of males. 54

53 Hunter et al., Biological Basis, supra note 29, at 2335.

54 See, e.g., Chen et al., Comparative Study, supra note 5, at 156

& tbls. 2-3 (female triceps strength 48-53% of males; female chest

strength 39-41% of males); accord Hunter et al., Biological Basis,

supra note 29, at 2335 (collecting sources).

28

Grip strength. Grip strength is an effective,

simple measurement of overall strength and power. In

a comparison of males and females age 18-24, males

had a median grip strength of 47.8 kg, while women of

the same age had a median grip strength of 28.4 kg. 55

This amounts to an almost 60% difference.

Knee extensions. The quadriceps drive knee

extension, which is critical for sprinting, jumping,

cycling, and nearly every explosive lower-body

movement in sport. Data from a large meta-analysis

show that teenage boys held a roughly 26% advantage

over teenage girls in average produced knee extension

torque. 56

Cross-lift performance. Controlled comparisons

of trained male and female athletes confirm the male

advantage in strength and power even when adjusting

for body size and other variables. In one study,

researchers measured lower-body power through

jumping tests, upper-body power with the bench press,

and overall strength with repetitions in the bench

press, deadlift, and squat. Even after accounting for

body mass and muscle thickness, men outperformed

women on every measure of strength and power, with

differences large enough that the researchers

concluded they were “significantly different”—

meaning could not be explained by chance. 57

55 Wang et al., Hand-Grip Strength, supra note 4, at 688 tbl. 1.

56 Šarabon et al., supra note 6, at 6.

57 Sandro

Bartolomei et al., A Comparison between Male and

Female Athletes in Relative Strength and Power Performances, 6

J. Functional Morphology & Kinesiology, art. 17, at 5-6 & tbl. 2

(2021).

29

Unsurprisingly, these measurable advantages in

muscle power, grip strength, and leg torque translate

into advantages in actual sports performance, from

sprint times to lifting totals.

B. Biological males show performance

advantages across a broad range of

sports.

The measurable male physical advantages

outlined above also manifest in actual sports

performance outcomes across disciplines. Research on

world records, elite competitions, and controlled

comparisons consistently shows males outperforming

females.

In sports that rely on skill, such as archery, the

competitive difference between the sexes is minimal

but that does not mean that sex segregated categories

offends the Constitution or does not further fair

competition. The reality remains that “the top adult

males almost always outperform the top females in

events that rely on muscle power, strength, speed,

and/or endurance.” 58

Competitive running is particularly suited to

objective comparison. In 2019, “[o]ver 10,000 men

(including boys [less than eighteen-years-old]) ran

faster than the three fastest recorded women in that

year” in the 400-m dash. 59

A separate two-decade review of Olympic running

events compared the top twenty male and female

58 Hunter et al., Biological Basis, supra note 29, at 2329.

59 Id. at 2329.

30

finishers in races from 100 meters to 10,000 meters.

The analysis showed consistent male advantages

across sprint, middle-distance, and long-distance

events, each tied to different physiological factors. The

authors identified males’ larger muscle volumes for

sprints, superior anaerobic reserves for middledistance running, and higher VO₂ max for male

marathoners. The authors concluded that, at best,

“[s]ex gaps as low as 10-11% are biologically possible

for the best male and female runners” and that

“females historically and presently are the

disadvantaged sex within sport.” 60

A comprehensive cross-sport review of elite-level

sport also shows that sex-based performance gaps

vary by discipline but are present across the board. 61

The smallest gaps appeared in rowing, swimming, and

running, where men maintained a still significant 1113% advantage over women. The gap widens to 16% in

cycling and 18% in jumping events. 62 The disparity

grows larger in sports that rely heavily on upper-body

strength, because men have greater absolute muscle

mass in the arms and shoulders, longer limbs, and

superior torque production in throwing, punching, and

pushing movements. Not surprisingly, the study

documented male advantage exceeding 20% in tennis

serve speed and more than 50% in the velocity of

60 Lydia C. Hallam & Fabiano T. Amorim, Expanding the Gap:

An Updated Look Into Sex Differences in Running Performance,

12 Frontiers in Physiology 1, 9 & tbl. 1 (2022), https://doi.org/

10.3389/fphys.2021.804149.

61 Hilton & Lundberg, supra note 7, at 209.

62 Id. at 201-203 & fig.1.

31

pitched baseballs. 63 The authors conclude that “there

are few sporting disciplines where males do not

possess performance advantage over females[.]” 64

Across disciplines, the data point to the same

conclusion: when sports rely on strength, speed,

power, or endurance, males as a class are advantaged

over females.

C. Testosterone suppression does not

eliminate male performance advantages.

Both the Ninth and Fourth Circuits accepted the

claim that suppressing testosterone erases male

athletic advantages. See Hecox, 104 F.4th at 1084;

B.P.J., 98 F.4th at 560-561 (claiming that by not going

through male puberty due to puberty blockers and

hormone treatments, the male plaintiff did not obtain

any physiological advantage over the plaintiff’s female

peers). But that reasoning confuses a relative

reduction of competitive advantage among males with

the elimination of the absolute advantage over

females. It is true that males who suppress

testosterone see decreased performance compared to

other males. But the relevant point for this case is that

males that have undergone hormone suppression

remain stronger, faster, and more explosive than

females because of their male genetics, as discussed

above. And, of course, they are still males competing

in girls’ sports.

Remember that grip strength is a simple but

powerful proxy for overall muscular strength. Males

63 Ibid.

64 Id. at 201.

32

who suppressed testosterone for twelve months saw a

4% decrease in grip strength. 65 Yet those individuals

maintained a significant advantage over females: even

after hormone therapy reduced their handgrip

strength to the 25th percentile for males, their

strength remained over the 90th percentile for

females. 66 That means an unexceptional male rises to

the top decile of females even when actively

suppressing his testosterone.

The same is true of heart and lung endurance. The

Air Force measures cardiorespiratory fitness through

a 1.5 mile run in its Physical Fitness Assessment. 67 On

that test, biological men who underwent hormone

therapy “retain[ed] an advantage in endurance * * *

over female controls for over 2 years after starting”

hormones. 68 Receiving estrogen did worsen the

biological males’ run times, yet even then they

“remained faster than [biological women] at all time

points.” 69 In other words, even after hormone therapy,

males still had greater endurance capacity than

women, though less than other males.

Males also retain superior oxygen-carrying

capacity

even

after

hormone

suppression.

Testosterone increases hemoglobin, which boosts the

blood’s ability to deliver oxygen to working muscles

65 See Scharff et al., supra note 8, at 1026.

66 Ibid.

67 See U.S. Dep’t A.F., DAFMAN No. 36-2905, Department of the

Air Force Physical Fitness Program § 3.1.1 (2022) (also allowing

a 20 meter High Aerobic Multi-shuttle Run).

68 Roberts, supra note 52, at 582.

69 Id. at 580.

33

and is central to endurance performance. 70 While

testosterone suppression may be able to eliminate a

male’s hemoglobin advantage over females, 71 even

then, other factors including “total blood volume, heart

size and contractility” result in aerobic capacity

advantages for males who have suppressed

testosterone over biological females. 72 Even if

medicine can narrow one biological factor, the

multitude of other male advantages ensures that

males retain performance advantages over females,

even after those males have undergone testosterone

suppression.

In short, testosterone suppression simply cannot

eliminate the athletic performance advantages that

males carry over females. Nor can it transition a male

into a female.

CONCLUSION

Idaho and West Virginia protect the opportunity

for girls to compete on a fair playing field. The statutes

rest on science and common sense that shows that

males retain enduring athletic advantages that

hormone suppression and surgeries cannot erase. The

contrary rulings below erase the foundation of fair

competition in service of unscientific ideology.

The judgments of the Ninth and Fourth Circuits

should be reversed.

70 Hunter et al., Biological Basis, supra note 29, at 2336.

71 See Louis J. G. Gooren & Mathijs C. M. Bunck, Transsexuals

and competitive sports, 151 European J. Endocrinology 425, 426427 (2004).

72 Hilton & Lundberg, supra note 7, at 208.

34

Respectfully submitted,

EDWARD H. TRENT

Counsel of Record

GENE C. SCHAERR

JOHN GREIL

SCHAERR | JAFFE LLP

1717 K Street NW, Suite 900

Washington, DC 20006

Telephone: (202) 787-1060

etrent@schaerr-jaffe.com

Counsel for Amicus Curiae

SEPTEMBER 19, 2025