Sept. 19 2012 12:00 AM

Dennis McFadden, of the University of Texas at Austin, is the third speaker in MSU’s semester-long series “Whom You Love: the biology of sexual orientation,” which aims to demonstrate that homosexuality is a natural occurrence in humans. His lecture, 4 p.m. Monday, is in Wells Hall room 115B and is free and open to the public. 

McFadden is Ashbel Smith Professor Emeritus in UT’s Department of Psychology and Center for Perceptual Systems. He is an auditory psychophysicist, specializing in measuring sounds that come out of the ears, known as otoacoustic emissions. These sounds are affected by the prenatal exposure to hormones experienced by all developing fetuses, and they differ depending upon a person´s sexual orientation. His lecture is called “Physiological Evidence about the Origins of Sexual Orientation.” 

What ground will your lecture cover? 

I plan to talk about how difficult it is to distinguish between inborn and acquired differences between the sexual preferences of men and women. I also hope to briefly discuss a few measurable, physiological differences between straights and gays that do appear to be inborn, such as birth weight, finger-length ratios and the fraternal birth order effect. Then I will explain our auditory findings about sexual orientation.

What types of physiological differences have you discovered? 

We found that both otoacoustic emissions (OAEs) and brain waves known as auditory evoked potentials (AEPs) can differ in people of differing sexual orientations. Otoacoustic emissions are sounds given off by the inner ear when it is stimulated by a sound, causing the outer hair cells to vibrate. This produces a nearly inaudible sound that echoes back into the middle ear that can be measured with a small microphone. Auditory evoked potentials are very small — but measurable —brain waves recorded from electrodes on the scalp in response to an auditory stimulus. Both of these measures exhibit sex differences in newborns, suggesting that they are affected by events occurring during prenatal development.

What do these findings tell you about a person’s sexuality? 

Our findings for both OAEs and AEPs reveal inborn physiological differences that are attributable to the degree of exposure fetuses received to certain types of hormones during prenatal development. The differences are group effects and cannot be used to predict any individual person´s sexuality, but they are informative about the biological causes of non-heterosexuality. Although the link between exposure to androgen (the hormone we study) and sexual orientation is still speculative, the circumstantial evidence is substantial and, to me, pretty darn convincing. 

The OAEs and AEPs in the undamaged ears of young adults also exist in newborns, implying that the measurements we find in young adults will be good indicators of the measurements they had at birth. If there are sex differences at birth, then the cause obviously could not be lifestyle differences, and must be attributable to something that happened during prenatal development. The most likely event is the process of exposure to androgens prenatally.

Exposure to high levels of androgens leads to a weakening of the cochlear mechanisms, responsible for OAEs. The link between OAEs, AEPs, and androgen exposure is a logical one, not an experimental one. Because the degree of androgen exposure is known to be responsible for dozens of differences in body, brain and behavior between the two sexes, a reasonable working hypothesis is that androgen exposure also is responsible for the sex differences in OAEs and AEPs seen in newborns.

So now let´s apply this logic to the differences we observe between straights and gays. We are measuring young adults, but we have good reason to believe that their OAEs and AEPs are reasonably accurate representations of their OAEs and AEPs at birth.

What inspired you to follow this line of research?

As Art Carney used to say on “The Honeymooners” about how he ended up working in the sewers, I just kind of fell into it. I followed my experimental nose — anyone would have done the same given the hints that existed in the research literature. My experimental nose also led me to study the OAEs of spotted hyenas, rhesus monkeys, and sheep, because we found some animals that had been treated with androgenic or anti-androgenic agents during prenatal development, and basically all of the important comparisons with those animals supported the prenatal-androgen-exposure interpretation we had about OAEs and sexual orientation.

What do you think the speaker series could do for the social perception of homosexuality?

A person’s identity is largely determined both by the genes we received from our parents and from the conditions of the prenatal environment we were exposed to. My hope is that once reasonable people understand this, they will see that discriminating against homosexuals makes about as much sense as discriminating against left-handers. Both conditions likely result from the degree of exposure to certain hormones during our development, and no conscious choice was involved — just as no conscious choice was made by people who are heterosexual or right-handed. People simply are who they are. 

For more information on this series, go to