In the article targeted by this Commentary, Breedlove (2017) chronicles the progression
of his thinking about hormonal influences on human behavior, including sexual orientation,
across the span of his career. He considers the question in terms of expectations,
based on what has been known at various points in time, versus the accumulation of
data to present day. He broadly frames his argument within the commonly applied architecture
of pre- versus postnatal factors, specifically considering prenatal androgen exposure
and postnatal socialization influences. A critical distinction which quickly becomes
evident is that potential mechanisms underlying the development of human sexual orientation,
regardless of theoretical framework, likely differ for men and women. In this regard,
Breedlove reaches conclusions that differ by sex: fetal androgen exposure contributes
to sexual attractions in females while no such evidence exists for men. In this Commentary,
I reconsider some of the same evidence and present further evidence for a revised
interpretation of the extant literature.
With respect to ultimate conclusions in the target article, Breedlove explores several
lines of evidence relevant to sexual orientation in the context of fetal hormonal
exposure and social learning theories. Among these, he considers: (1) findings from
studies of sexual orientation in individuals exposed to atypical levels of prenatal
androgens, i.e., congenital adrenal hyperplasia (CAH; Pasterski & Hughes, 2016) and
androgen insensitivity syndrome (AIS; Hughes et al., 2012); (2) the reliability and
usefulness of specific physiological traits, i.e., otoacoustic emissions and 2nd to
4th finger–length ratios, as retrospective biomarkers of fetal androgen exposure with
potential for studies of sex development; and (3) findings from studies using these
retrospective biomarkers to assess potential influences of fetal androgen exposure
on human sexual orientation. Evidence for potential mechanisms not discussed in the
target article and that bear additional consideration include findings from longitudinal
studies and investigations looking at the effects of androgens in the early postnatal
period, i.e., mini-puberty. I will consider each of these in turn.
Prenatal Hormones and Postnatal Socialization in CAH
Based on the organizational hypothesis (Phoenix, Goy, Gerall, & Young, 1959), hundreds
of studies have established a role for fetal androgens in sexually dimorphic neurobehavioral
development observed both in humans and in nonhuman vertebrates (Hines, 2010; Morris,
Jordan, & Breedlove, 2004). While the animal literature is replete with experimental
evidence of the potential to directly manipulate sexually dimorphic behavior, in both
males and females, by changing exposure to androgens during critical periods of neural
development, evidence in humans remains quasi-experimental due to ethical constraints.
In humans, potentially confounding factors cannot be controlled. In considering the
bulk of evidence that masculinized patterns of behavior occur in girls exposed prenatally
to excess androgens due to CAH (Hines, 2011; Meyer-Bahlburg, Dolezal, Baker, & New,
2008), Breedlove acknowledges the potentially confounding factor that these girls
are also born physically virilized. Masculinized behavior, including development toward
a homosexual sexual orientation, could be due to socialization by parents affected
by the appearance of the child at birth. Even so, understanding the degree of relative
influences requires a nuanced understanding of such mechanisms. As Breedlove rightly
points out, the majority of women with CAH are not lesbian. He fails, however, to
integrate two important pieces of evidence that inform the interpretation and integration
of the body of findings.
The first comes from a study which directly assessed parental socialization of children
with and without CAH (Pasterski et al., 2005). We found that while parents showed
expected reinforcement of sex-typical toy play, parents of girls with CAH showed increased
reinforcement for play with girls’ toys compared to parents of unaffected girls. Furthermore,
there was a significant inverse relationship between level of sex-typed reinforcement
by parents and the time their daughters with CAH spent playing with girls’ toys. This
suggests that the more these girls played with boys’ toys, or ignored the girls’ toys,
the more the parents encouraged girl-typical play. Alternatively, greater encouragement
by parents was met with greater resistance. Either way, there was no evidence of socialization
toward a masculine presentation. Social desirability was ruled out as a possible explanation
as well, as parents were not explicitly made aware of their own behavior in this study
of childhood sex-typed behavior.
Effects of parental socialization influences notwithstanding, we know from the social
learning literature that peers and other models are also sources of influence in the
development of sex-typed behavior (Pasterski, Golombok, & Hines, 2011b). Further clarification
on the intersection of hormonal influences and social learning comes from a study
assessing imitation of same-sex models and responsiveness to explicit sex-typed labeling
in children with and without CAH (Hines et al., 2016). Children were exposed to modeling
of sex-typed preferences by adults, where males and females demonstrated sex-differentiated
preferences for neutral objects, and to explicit labeling of neutral items as “for
boys” or “for girls.” While unaffected boys and girls showed the expected effects
of imitation and learning from gendered labels, girls with CAH showed reduced imitation
of female models and reduced responsiveness to information that particular objects
were for girls. Implications for the role of increased fetal androgen exposure in
the context of subsequent social learning experiences are important. Even in the face
of explicit socialization influences, girls with CAH did not engage in the self-socialization
typical of unaffected children. Once again, however, we must consider that most women
with CAH are not lesbian, and masculinization of other sex-typed behaviors, such as
those described above, is not “complete.” That is, though the behavior of girls with
CAH is more masculine than that of unaffected girls, they generally are not as masculine
as typical boys (Pasterski et al., 2005, 2007, 2011a). Finally, though sexual function
may be impaired in women with CAH, especially in cases of repeated surgical interventions
to correct genital anomalies (Callens et al., 2012), fundamental physiological arousal
patterns are likely not influenced to the point of reversal to same-sex attractions.
Taken together, these studies suggest that sex-related development is even more nuanced
and complex than previously considered in the framework of prenatal hormones versus
postnatal socialization.
Normative Development: Longitudinal Data, Biomarkers, and Twins
Studies of variability in fetal androgen exposure in normative populations provide
another line of evidence for influences on sex-typed behavior, including sexual orientation.
To that end, investigations are limited to intensive longitudinal studies beginning
with measurement of fetal androgens in pregnancy and following offspring into (early)
adulthood, or on retrospective biomarkers of early androgen exposure. On the former
point, there are reports from the AVON Longitudinal Study of Parents and Children
(ALSPAC; Golding, 2001), which include maternal blood testosterone measurements and
longitudinal follow-up of children who are now in their teens and who have reported
on sexual orientation (Li, Kung, & Hines, 2017). While there are no reports directly
linking fetal testosterone to sexual orientation, an early ALSPAC study (Hines, Golombok,
Rust, Johnston, & Golding, 2002) found that sex hormone binding globulin, a correlate
of fetal testosterone, was related to sex-typed behavior in girls, but not boys, at
age 3.5; and 15 years later, the Li et al. study showed that behavior measured in
a sample of children drawn from the same population at ages 3.5 and 4.7 years was
predictive of sexual orientation at age 15 years in both males and females. These
findings are consistent with Breedlove’s conclusion that fetal testosterone plays
a role in female, but not male, sexual orientation. Nevertheless, direct evidence
confirming this conclusion remains forthcoming.
With respect to the use of biomarkers as proxies for fetal androgen exposure, there
is a wealth of literature linking 2nd to 4th finger–length ratio (2D:4D) to various
behaviors that also show a sex difference, including sexual orientation. Before commenting
on the merits of those studies, or a recent conclusive meta-analysis (Grimbos, Dawood,
Burriss, Zucker, & Puts, 2010), however, the reliability of the biomarker itself as
a proxy for fetal androgen exposure bears scrutiny. Despite thousands of studies assuming
fetal androgen effects based on divergent 2D:4D measurements, there has been no clear
or consistent evidence directly linking sexual dimorphism in 2D:4D to variance in
fetal androgen exposure. Indirect evidence is not sufficient. A fundamental principle
of the scientific process warns against the interpretation of a causal relationship
between variables which have only been shown to covary. Van Hemmen et al. (2017) make
this point in reporting on digit ratio in women with CAIS, where androgen action is
impaired despite a male karyotype. Though Van Hemmen et al. found that proband-control
2D:4D measurements varied in a pattern consistent with a hormonal influences interpretation,
closer inspection of within group variance suggested that fetal androgen alone could
not explain the observations. They explicitly cautioned against the use of digit ratio
as a dependable proxy and suggested that non-androgenic (e.g., sex chromosome) factors
are also needed to establish the male-typical phenotype.
Reliability is the primary problem with evidence directly linking fetal androgen exposure
to variance in 2D:4D. There is the early Manning, Bundred, Newton, and Flanagan (2003)
report which suggested that 2D:4D finger–length ratios covaried meaningfully with
a polymorphic repeat (CAG) sequence in the gene coding for androgen receptors in men;
however, two subsequent and larger studies failed to find the same effect (Hampson
& Sankar, 2012; Hurd, Vaillancourt, & Dinsdale, 2011). Manning et al. have never been
replicated. Then, there are three further studies where a number of different hormones
were measured at various points in pregnancy, including testosterone, estrogen, DHEA,
and insulin-like factor 3 (NSILF3) among others (Lutchmaya, Baron-Cohen, Raggatt,
Knickmeyer, & Manning, 2004; Mitsui et al., 2015, 2016). One of the three studies
found a negative relationship between INSL3 and 2D:4D in a sample of 135 males (Mitsui
et al., 2015), but not in females; another report described a negative relationship
with DHEA, but again in males only (N = 135; Mitsui et al., 2016). Though the third
study found a negative relationship between a ratio of fetal testosterone to fetal
estrogen (FT/FE) and 2D:4D in a combined sample of males and females (N = 33), it
is worth noting that this group of participants did not show the sex difference in
2D:4D which underpins the hypothesis driving such investigations (Lutchmaya et al.,
2004).
Reliability is also a problem in studies of 2D:4D in girls with CAH, who would have
been exposed to high levels of testosterone beginning early in gestation (Brown, Hines,
Fane, & Breedlove, 2002; Buck, Williams, Hughes, & Acerini, 2003; Ökten, Kalyoncu,
& Yariş, 2002; Rivas et al., 2014). Though Brown et al. found that girls with CAH
had lower 2D:4D than unaffected girls, their sample size was small (N = 13 girls with
CAH), making it difficult to generalize to much larger normative populations. Ökten
et al. also found that girls with CAH had lower 2D:4D than control girls; however,
they also had a small sample (9 boys and 17 girls) and included two patients with
non-classical CAH (i.e., onset in later childhood), who would not have been exposed
to high levels of androgen prenatally (note that it was not stated whether these were
male or female participants). Only nine children with CAH (males/females) in their
cohort were diagnosed in the neonatal period, suggesting that the 17 participants
who were diagnosed later may have had a milder form of the condition. Similarly, in
a study of Brazilian children with CAH, Rivas et al. found masculinized 2D:4D in female
patients (N = 31). Once again, however, it was noted that none of these participants
had received hormone replacement therapy prior to the study (M age = 10.7 years),
suggesting that they probably had a milder form of the condition (subtypes of CAH
were not stated). The issue of disease severity in CAH is important in light of evidence
for a dose–response relationship between fetal androgen exposure and masculinized
behavioral traits (Nordenstrom, Servin, Bohlin, Larsson, & Wedell, 2002). Furthermore,
and in contrast to the three studies above, a fourth study included a large sample
of 66 girls with CAH, all of whom were diagnosed with the classical form in the early
neonatal period (Buck et al., 2003). Though the expected sex difference in 2D:4D was
found for controls (N = 69 females, N = 77 males), no significant difference was found
between girls with and without CAH.
Finally, several studies have employed a hormone transfer paradigm, where comparisons
are made between co-twins from same-sex dizygotic (DZ) twin pairs and opposite-sex
DZ twin pairs, to investigate potential links between fetal testosterone exposure
and 2D:4D (Cohen-Bendahan, 2005; Medland, Loehlin, & Martin, 2008; van Anders, Vernon,
& Wilbur, 2006; Voracek & Dressler, 2007). While the first study did not find any
effects (Cohen-Bendahan, 2005), two subsequent studies did find evidence linking fetal
androgen to variance in 2D:4D (van Anders et al., 2006; Voracek & Dressler, 2007).
Given the conflicting evidence, and the fact that the two studies reporting effects
employed very small samples (ranging between eight and ten twin pairs), Medland et
al. conducted a large-scale investigation including 118 pairs of same-sex female DZ
twins and 106 opposite-sex DZ twins. Despite the exceptionally large sample size,
they did not find evidence that fetal hormone transfer between opposite-sex twins
affected 2D:4D measured in adolescence. Taken together, these studies suggest that
either hormone transfer does not occur or that fetal androgen exposure is not directly
related to variance in 2D:4D. It is possible that positive findings for the putative
link are tapping latent effects of heritability in digit ratio (Gobrogge, Breedlove,
& Klump, 2008; Medland & Loehlin, 2008; Paul, Kato, Cherkas, Andrew, & Spector, 2006;
Voracek & Dressler, 2007). In sum, trying to find reliable evidence directly linking
fetal androgen to 2D:4D finger–length ratio is like searching for the Higgs Boson
(NB: Scientists most definitely have found the Higgs Boson, but it wasn’t what they
were looking for!).
And so, in consideration of the lack of clear support for 2D:4D finger–length ratio
as primarily dependent on fetal androgen exposure, interpretation of the vast array
of findings linking the marker to sexual orientation (or other sexually dimorphic
outcomes) seems a near impossible task. There are plenty of studies finding directly
conflicting results (e.g., Van Honk et al., 2011; Voracek & Dressler, 2006), others
find effects in different hands (e.g., Atkinson, Smulders, & Wallenberg, 2017; Voracek,
Pietschnig, Nader, & Stieger, 2011), and still others find variable effects for men
versus women (e.g., Voracek et al., 2011; Wallien, Zucker, Steensma, & Cohen-Kettenis,
2008). Though various meta-analyses may take such factors into consideration (e.g.,
Grimbos et al., 2010), conclusions relying on 2D:4D as a proxy for fetal androgen
exposure are simply based on a questionable premise. Nevertheless, Breedlove’s conclusion,
in large part based on findings from the Grimbos et al. meta-analysis, that fetal
androgen exposure relates to sexual orientation in females, but not males, is probably
at least partially true. In support of this conclusion, Li et al. (2017) found a link
between childhood sex-typed behavior and later sexual orientation in both sexes, while
the link between sex-typed behavior and fetal testosterone levels was found for girls
only (Hines et al., 2002). Given the comparatively robust and consistent evidence
from longitudinal studies and studies of outcomes in women with CAH and CAIS, my view
is that we can safely assume a role for early androgens in the development of sexual
attractions in women. Selective findings from the 2D:4D literature do not strengthen
that argument.
Mini-Puberty: Effects of Perinatal Androgen Exposure
As Breedlove concluded, explaining homosexuality in men is a difficult task. By now,
the majority of scientists studying the topic likely agree that homosexuality is definitely
not a choice and probably not due to socioenvironmental factors. At the same time,
there appear to be no physical indicators of disrupted fetal sexual differentiation
in homosexual men that would fit with the basic premise of the hormone theory of sex
development. However, it is possible that alterations in the androgen surge that occurs
in the early postnatal period, also called mini-puberty, could have effects that are
not immediately or physically obvious. Based on the finding that penile growth in
the first three months of life correlates with a concomitant surge in serum testosterone
levels (Boas et al., 2006), Pasterski et al. (2015) considered the possibility that
penile growth may act as a proxy for neonatal androgen exposure and that change measurements
may be related to later neurobehavioral outcomes. In a longitudinal study of 81 typically
developing boys, we found that the strength of the early postnatal androgen surge,
from birth to approximately three months of age, predicted masculine behavior at 4 years
old. By controlling for effects of prenatal androgen exposure using measurements of
penile length and anogenital distance (AGD; sexually dimorphic and roughly twice as
long in males compared to females) at birth, we showed that penile growth in the first
three months of life, but not thereafter, accounted for significant variance in later
sex-typed behavior. In the overall regression analysis, which controlled for various
factors, penile length at birth was not related to sex-typed behavior. This suggests
that disruption to male mini-puberty could have implications for future sex-related
outcomes that are masked by a typical appearance at birth. Further, this provides
support for the hypothesis that early (postnatal) hormone exposure influences aspects
of sex-typed development in men, in a similar fashion to prenatal hormone exposure
that is presumed to affect women.
Though an ever-growing body of literature aims to elucidate mechanisms underlying
human sexual orientation, there is potential for the “wealth” of knowledge to obfuscate
genuine discoveries. Sifting through mountains of research that may or may not seem
compatible in order to integrate evidence relevant to particular theoretical frameworks
is a difficult task. However, if we are to understand the true nature of increasingly
complex human experiences, we must be willing to modify our interpretations of scientific
findings with an open mind aimed at collective intellectual growth. I agree with parts
of Breedlove’s interpretation, but not others. Hopefully, perspectives presented in
this Commentary can be integrated into new thinking about the original article.