Olfaction and Reproductive Behavior

Body Scents

People have mild body scents that are different in men, women, children, teenagers, and grandparents, and individuals also have their own odor signature. Not only can animal species with an exquisite sense of smell (such as dogs and rats) readily distinguish these different odors, so can people. Indeed, for women, the quality of a person's odor is often the most important attribute in a sexual relationship, whereas appearance is most important for men.

The perception of body scents and odors is highly culture bound. In America, strong body odors produced, for example, during exercise are viewed as unpleasant, driving a multibillion-dollar industry to remove or mask them; even mild body scents are taboo. Yet, in the arts and in other cultures, body scents, if not strong body odors, are not only considered natural but celebrated.

In humans, one source of a person's odor signature is the proteins expressed on cell surfaces, termed the human leukocyte antigen, or generally for most species, the major histocompatibility complex (MHC). The MHC has been studied primarily in the context of its immune function, enabling a cell to be recognized as “self” rather than an invading pathogen. By smell alone, humans can detect differences in genes for the MHC. Within inbred groups, and when sexuality is made explicit, people prefer scents from other people with fewer matches to their own MHC genes. In outbred populations, where people typically have no matches, people avoid scents from people with zero matches, preferring those from people with at least a few MHC matches. Interestingly, the odors are not more or less familiar, but instead pleasantness drives the preference. This suggests that processing of MHC information involves olfactory projections to the amygdala, which responds differentially to the emotional value of an odor rather than more circuitous odor recognition, verbal identification, and cognitive processing.

Odor Conditioning

Through conditioning and associative learning, both natural body odors and serendipitous environmental odors play an integral role in sexual behavior of most mammals. This principle has been elegantly demonstrated in rodents. For example, male rats respond to the odor of a female in heat. If he has mated with females anointed with lemon oil, then lemon oil cues sexual arousal. Likewise, when a male fetus in the womb is exposed to lemon oil, then in adulthood he prefers females scented with lemon oil. Conversely, females prefer odors of sexually mature males, and can also learn arbitrary odors as sexual cues. If an odor is associated with negative stressful experiences, then it alone can impair male and female sexual function.

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