Sex on the Brain: A Conversation with Biological Psychologist Marc Breedlove
Timothy Beneke
Eastwood Bay Press, September 22, 2000.
In the fall of 1999, Cal psychologist Marc Breedlove and his team of researchers set up booths at a Bay Area street fair offering lottery tickets to anyone who would, (1) Allow their hands to be photographed on a Xerox machine, and, (2) Fill out an anonymous questionnaire that asked, among other things, the participant's age, sexual orientation and number of older brothers. Breedlove was operating from some intriguing hypotheses. On average, among men, the ring finger is significantly longer than the index finger, especially on the right hand, an apparent effect of the release of testosterone, a "masculine" hormone, in the womb. In women, the two fingers are, on average, almost the same in length. Building on previous research that suggested that lesbians are exposed to greater testosterone in the womb than straight women, Breedlove wanted to see if lesbians had index-to-ring-finger size ratios that were more like men's, which would count as further evidence that they had been exposed to more testosterone in the womb than straight women.
And that was precisely what he found. Results of the experiment, published in the leading science journal "Nature" were widely publicized, and although for a few days newspaper readers were busy looking at their fingers, Breedlove has made it clear that, due to considerable overlap between lesbians and straights, finger length measurements can in no way be used to identify gayness in women.
Breedlove also confirmed another intriguing fact: men who have more than one older brother are significantly more likely to be gay than men with no older brothers. Not only have researchers established that the more older brothers a man has, the more likely he is to be gay, but those rare men who have ten older brothers have a 50-50 chance of being gay. Researchers have struggled to explain this. Breedlove found that the more older brothers men have, the smaller are their index fingers in relation to their ring fingers, which suggests that they received higher than average amounts of prenatal testosterone.
Breedlove's research is part of a move toward greater biological understanding of the roots of human behavior that has blossomed in the last thirty years. Increasingly, certain forms of mental illness like schizophrenia, manic depression, and obsessive compulsiveness are understood less and less as a result of family trauma and more and more in terms of a poorly - or differently -- functioning brain. And proudly held personality traits look more and more to be strongly influenced by genes, as behavioral geneticists inform us that identical twins raised apart are far more alike than fraternal twins raised together. Today, middle aged psychology professors who once dismissed as reactionary the idea that intelligence is strongly influenced by genes now take such ideas for granted.
Many feminists, eager to liberate biological sex from the straightjacket of gender, so we all need no longer feel compelled to act or feel what our culture defines as "masculine" or "feminine" at any given time, have been reluctant to acknowledge any biological basis for gender difference. They point to the undeniably myriad ways cultures create gender differences, and argue that gender differences in behavior are a social construction. From the day they are born, men and women are treated differently, and learn to act differently.
More recently, ambiguous evidence that sexual orientation has biological roots has come to the fore, causing a mix of reactions from gays and lesbians. Some gay men and women simply do not wish to have their sexuality explained. Others, especially gay men, feel strongly that they were just born gay, and welcome biological evidence that supports this intuition. Some fear that biological explanations will be used against them; others hope that society will be more accepting of gays if they learn that they are "just born that way."
What seems clear is that our state of knowledge about the biology of gender differences and sexual orientation is in its infancy, and little can be said with great confidence. In recent years, two mystifications have clouded the public debate on the biological bases of gender and sexual orientation. First, polemicists insinuate that the truth of any scientific claim is somehow connected to the motives of researchers, or the political uses to which research findings may be put. Writers on gender, (this writer included) have in the past dismissed scientific claims and theories, not on their own terms, but because they are afraid of the uses to which they will be put. But the motives for research, and the potential political uses of research tell us nothing about the validity of research.
And second, when neuroanatomical differences between men and women (or gays and straights) are uncovered, the public is quick to believe that biology is the primary cause of difference in gender or sexuality. This fails to understand the neuroplasticity of the brain - the idea that human experience and behavior itself can alter the brain. It could be that the socially mediated different experiences and behaviors of men and women (or gays and straights) cause their brains to be different. Neuroplasticity, the notion that experience itself changes the brain, has yet to be integrated into most public discussions of these issues.
So what, from a scientific standpoint, can we confidently say about genetic and biological influences on the differences between men and women, gays and straights and, for that matter, the transgendered? To address these issues, I met with Marc Breedlove, a professor of biological psychology in the Cal Psychology Department and a leading researcher in these areas. Breedlove received his Ph.D. from UCLA in 1982 in physiological psychology and soon began teaching at Cal. He is the recipient of many awards for his research, the author of 80 scientific articles, and co-author of two textbooks.
At 46, Breedlove is a friendly, unassuming man who comes across as unusually thoughtful in his attempts to interpret scientific findings about gender and sexual orientation. We met for the interview at his office in the Cal Psychology Department.
Timothy Beneke: Since we're going to talk about the origins of sexual orientation, I want to start with a political point. My own view is that all sexuality is worth understanding and explaining. But clearly, it would be homophobic to view "homosexuality" as something "abnormal" or "deviant" and in need of explanation, and heterosexuality as the norm that does not need explaining.
Marc Breedlove: I agree completely. And in fact, we are equally trying to explain both. Gays and lesbians have historically been persecuted and are still persecuted, and we try to be very sensitive to that. If you ask most heterosexuals when they first had sexual feelings toward the opposite sex, and if they remember when they decided to have those feelings, they will tell you that they never made such a choice, it just turned out that way. And if you ask them if it had to do with how they were raised, or how they were formed in utero, most heterosexuals would find nothing threatening about the idea that something that happened in the womb could set off a chain of events that led to them at around 10 years of age to have these funny sexual feelings about the opposite sex. Heterosexuals are in the majority and they have not been persecuted as gays and lesbians have been, so they don't find these ideas threatening. Gays and lesbians are still persecuted, and have good reason to ask questions about the purpose of research about the origins of sexual orientation.
Beneke: So tell us why you did this research comparing ring fingers with index fingers among gays and straights.
Breedlove: I normally do experiments on animals, specifically rats and mice, and in all the cases I know of so far in mammals, we can make an animal's brain and behavior as masculine or feminine as we want, and we do that by manipulating testosterone. If I give the right amount of testosterone at the right time in life, I can make the animal's behavior as masculine as I like. Across a wide range of animal species, the story is that the more masculine the structure of the brain, the more masculine the behavior. And we can make the brain more masculine by administering masculine hormones. This holds true for mice, rats, hamsters, dogs, and monkeys where this has been done. Hormones do not account for all behaviors, but they are very important in sex differences among animals.
So what do I mean by masculine behavior? It can be many things. With human sexuality there is an overwhelming concern about the characteristics of the partner. That is very hard to get at when you work with a species that can't talk. So it is difficult to know about that. You can ask what odors rats find arousing and whether rats either prefer to mount or be mounted, but they don't care that much who their partner is. We also look at things like spatial learning ability; others look at things like preferences for sweets. And for all the differences where researchers find sex differences, testosterone has been able to account for almost all the differences. To give an example, in rhesus monkeys males yawn more than females. They yawn as a social signal to let others know not to mess with them, and that is completely explained by testosterone in adult males. If you take the testosterone away from males, they yawn less; if you give testosterone to females they yawn more.
Beneke: Could this behavior in monkeys be socially mediated? Could it be that the added testosterone affects social status, and the enhanced social status causes the female monkeys to yawn more?
Breedlove: Yes, it could be that the testosterone's effect is mediated by some social factor, or it could be a direct biological effect of the brain. But every sexual difference in behavior we see in mammals, when people ask if testosterone is responsible, it has turned out to be true.
Beneke: That is a remarkable fact.
Breedlove: Yes, natural selection in general does not usually cause a particular principle to be that widespread. The question I have is whether this relates to humans at all. To my mind it is still very much an open question. The question for humans is: do men and women behave differently because society trains them, or do these hormones that masculinize animal brains have similar effects on humans as well? In my heart of hearts I can't help but think that both factors are at work. But it has been remarkably difficult to show that hormones are having any effect on human behavior, because human behavior is so complicated. A frustration of my professional life is that I believe that there is a genetic and hormonal influence on gender differences between men and women, but I can't really prove it. It's not that humans are just like animals, but that just like in animals, hormones play an important role. It is obviously a smaller role in humans, and not nearly as powerful.