What can body odor tells us about sexual attraction and sexual orientation?

Can "BO" (body odor) actually be a good thing?

Sexual attraction is complicated. While we may swoon over self-professed "types" (blue eyes, brown hair, great smile), mate selection relies heavily on a series of conscious and unconscious calculations made deep within our brains.

Take body odor, for instance. Most of us try to cover it up with deodorants and perfumes, but BO might actually help us find our best-fit romantic partners. Considerable evidence suggests that humans produce individually unique body odors, and that we easily discriminate between the body odors of different people. Even newborns can tell the difference between the odor of their own mothers and those of mothers of other infants. Thus, BO may be crucial for forming social relationships, even early in life.

Body odor is largely influenced by Major Histocompatibility Complex (MHC) molecules, which are genetically determined and linked to the immune system. Experiments on non-human animals and human participants have shown that we tend to judge potential sexual partners as more attractive if their MHC composition is substantially different from our own. In other words, opposites really might attract, perhaps because such variation in our immunity makes the population more resistant to disease. Interestingly, MHC has also been linked directly to sexual orientation, suggesting that the differences in MHC that influence sexual orientation may be related to the differences in MHC that regulate odor production.

In the last article of this series, Professor Wirth, also in the Psychology Department, talked about hormones, or the chemical signals that act on cells within an individual. Pheromones, on the other hand, are chemical signals that influence the behavior and physiology of other individuals. Scientists think that pheromones are detected by the olfactory system or by the vomeronasal organ (VNO), which is located in the nasal cavity. Perhaps because this region is vestigial in humans, pheromones may or may not be active in human mating behavior. In lab animals, however, pheromones have profound effects on other animals. For example, when adult male rodents are around, females will reach puberty faster, and in some cases their estrous cycles will synchronize.

Our vestigial VNOs aside, odor may play a large role in human sexual attraction. For example, the chemical 4-16-androstadien-3-one, or ‘and' for short, is an androgen-like chemical found in the underarm sweat of males and females, and we humans are highly sensitive to it. Brain imaging studies show that and exposure activates brain regions that are critical for sexual behavior, such as olfactory areas and regions of the hypothalamus. A recent study showed that lesbian women process ‘and' with their olfactory systems rather than the hypothalamus, which is the opposite of heterosexual women.

Just like heterosexuals then, people who are attracted to the same sex may follow their noses. Consider, for example, a 2005 study published in the prestigious journal Psychological Science, which showed that gay men and lesbian women had different body odor preferences than straight men and women.
Now, this is a study that would be interesting to participate in! For nine days volunteers used only odorless soaps and shampoo, not shaving their armpits, and abstaining from garlic, curry and cumin. They then wore cotton pads wedged into their armpits for several days while going about their typical activities. The pads were cut up and randomly placed in plastic squeeze bottles with flip-top lids for easy access smelling. Shortly thereafter, another set of volunteers, of both sexes and sexual orientations, smelled and rated the odors on intensity and pleasantness on scales of 1 to 10.

Interestingly, homosexual men had different preferences than straight men, straight women and lesbian women. Not only were their body odor preferences different than these other groups, but their own body odor was regarded differently than these other groups as well. Homosexual men were drawn to the odors of other gay men and heterosexual women, while odors from gay men were the least preferred by heterosexual men and women and by lesbian women.

Many scientists, myself included, believe that these findings suggest that neurobiological processes are at work in determining our sexual orientation, just as they are in determining our sexual assignment, although they could certainly interact with environmental influences as well.

In his related article, Professor Fuentes pointed out that homosexual activity exists naturally (i.e. in nature) across a wide array of species. His observations were echoed in the cover article in last Sunday's New York Times Magazine, "The Love that Dares Not Squawk its Name: Inside the Science of Same-Sex Animal Pairings" by Jon Mooallem.

Given the ubiquity of homosexual behavior, across time and across species, and given that research has not found any effect of parenting style or childhood experiences on sexual orientation, what, then, causes people to be gay, straight or bisexual? The answer to that question will surely be complex, but an ever-growing biological database strongly suggests that nature plays a fundamental role in why we are attracted to the people we are and fall in love with the people we do.

Jessica D. Payne is an assistant professor of psychology. She can be reached at jpayne7@nd.edu
The views expressed in this column are those of the author and not necessarily those of The Observer.