Are Your Ears Burning?
It has long been hypothesized that prenatal or perinatal hormone levels influence sexually dimorphic behavior in humans. The evidence from studies of people with Disorders of Sexual Development and in numerous animals studies lends strong evidence that this is true. So it would seem natural that we should ask if sexual orientation and gendered behavior in otherwise phenotypically normal individuals could also have been affected by hormones. The ultimate study would be one that longitudinally follows a large cohort of individuals from conception to adulthood, taking extensive hormonal assays while evaluating gendered behavior and sexual orientation. The problems of doing such a study for transsexuality is obvious… the cohort would have to be in the hundreds of thousands to ensure statistically significant numbers of transsexual individuals were included.
Thus, researchers are interested in finding sexually dimorphic markers that record historical hormone environments. That is to say, something that is an organizational and not an activational effect, preferably one that becomes fixed at the same period in prenatal development as organizational effects in the brain. It must be something that is observable at birth and remains stable long enough to allow us to use it to retrospectively determine ones hormonal environment after we have found our gender atypical population of interest.
I’ve already blogged about one such putative measure, the 2D:4D ratio, which is mildly sexually dimorphic, and in at least some population has a recognizably large enough effect size that we can hope that we can use it. Sadly, the results have been contradictory so far. But despair not, another such sexually dimorphic marker is available, though it requires specialized equipment to measure.
In individuals with normal, unimpaired hearing, a curious effect is found in which our inner ears respond to external sounds with sounds of their own, which is known to be mildly sexually dimorphic, as described in the abstract from a recent paper out of Europe, the first known to explore this effect as a potential probe of the role of androgens in gender dysphoria,
“Click-evoked otoacoustic emissions (CEOAEs) are echo-like sounds that are produced by the inner ear in response to click-stimuli. CEOAEs generally have a higher amplitude in women compared to men and neonates already show a similar sex difference in CEOAEs. Weaker responses in males are proposed to originate from elevated levels of testosterone during perinatal sexual differentiation. Therefore, CEOAEs may be used as a retrospective indicator of someone’s perinatal androgen environment.”
Before we get too excited about this marker, we need to look at the effect size, with is quite small at only d=0.30 in the left ear and was better in the right ear at d=0.60. You may recall that this is of the same order as the 2D:4D finger length ratios at d=0.63. This is large enough to be useful, but only if enough subjects are available to achieve significant statistical power. Sadly, this lack of enough subjects seems to be the case in this study. Too bad, because there is a hint at some exciting results in that looking at a group of gender atypical and gender dysphoric children and teens, the natal males seem to show a shift in the female-like direction, but oddly, the natal females do NOT,
“In the present study, we retrospectively investigated possible organizational effects of prenatal androgens on CEOAEs in relation to gender identity. We found that boyswith GID had sex-atypical (hypomasculinized) emissions. Their mean response amplitudes, though, were not significantly different from either the male or female controls. Thus, boys with GID had an intermediate position between the sexes in terms of CEOAE response amplitudes. By contrast, girls with GID showed emissions in the same range as female controls. Consistent with several earlier studies, sex differences in emission strengths were observed in the control group, with girls having significantly stronger emission amplitudes than boys. Our finding that boys with GID showed stronger, more female-typical emissions compared to control boys suggests that boys with GID might have been exposed to relatively lower amounts of androgens during early development. The effect sizes for the comparison boys with GID versus control boys were similar to those for control girls versus control boys, supporting the notion of a hypomasculinized early sexual differentiation in boys with GID. However, considering the lack of statistically significant differences between the control boys and the boys with GID and the relatively small sample size of subjects with GID, this conclusion may still be premature and our results therefore need to be interpreted with caution. Furthermore, our findings did not support the hypothesis of an increased exposure to androgens in girls with GID during prenatal development. Though speculative, this might reflect that GID in girls does not develop under the influence of prenatal androgens or at least not during the same critical time window as when androgens exert influences over OAEs.”
This result is surprising, in that previous studies involving gay men and women, researchers saw the opposite pattern, in that gay men showed no shift from control men, but lesbians showed a shift from the female to the male response. The most exciting times in science are when you hear, “That’s strange!” This is one of those times. Several possibilities exist. This result could just be spurious, with not enough subjects to have seen the real signal. It could be that there is an additional activational effect that occurs as children mature, that causes a shift for both gay men and gay women toward the masculine response. We may be seeing the effect of heterosexual “tomboys” swamping out the FtM signal. Or, we could be witnessing the first hint that there is a difference between transkids, both MTF and FtM, and conventionally gay men and lesbian women. Time and additional studies will tell.
Sarah M. Burke, Willeke M. Menks, Peggy T. Cohen-Kettenis, Daniel T. Klink, Julie Bakker, “Click-Evoked Otoacoustic Emissions in Children and Adolescents with Gender Identity Disorder” Archives of Sexual Behavior, DOI 10.1007/s10508-014-0278-2
Dennis McFadden, Edward G. Pasanen, “Spontaneous otoacoustic emissions in heterosexuals, homosexuals, and bisexuals” Journal of the Acoustic Society of America, http://dx.doi.org/10.1121/1.426845