Evolutionary scientists don't believe in free will when it comes to how humans choose a mate. They side more with the idea of sexual chemistry.
MHC (major histocompatibility locus) genes produce a distinct odor in all of us that serves as a kind of genetic fingerprint. The idea that the smell of these genes could influence behavior and mate selection started in 1974 with biologist Lewis Thomas, who tested the theory with mice. When his laboratory mice reached puberty, they demonstrated a clear preference for mating with mice that had different MHC scent signatures than their own. By mating with mice that had different MHC genes, these mice produced offspring that had stronger immune systems able to combat a wider variety of diseases.
A similar preference for mates with different MHC scents was discovered in humans during the original "Sweaty t-shirt" experiment in 1996. Claus Wedekind, a zoologist at Bern University in Switzerland, originally wanted to test his theory that MHC genes were important in mate selection in fish. However, he decided to experiment on humans as they are
able to express their aversions and preferences much more clearly.
Wedekind selected 44 men and 49 women who had a variety of MHC gene types. He gave the men clean t-shirts which they were to wear two nights in a row. Wedekind also gave the men odor-free soap and aftershave and instructed them to remain as "odor neutral" as possible.
Each shirt was then placed in a cardboard box lined in plastic and equipped with a sniffing hole. The women came in individually to sniff the boxes at the midpoint in their menstrual cycles, as this is the time women's noses are the most sensitive. Each woman was presented with seven boxes. Three contained T-shirts from men with MHC similar to their own; three contained T-shirts from men with different MHC genes; and one contained an unworn T-shirt as a control. The women then were asked to describe each odor in terms of it being pleasant or offensive.
The women strongly preferred the scents of those T-shirts worn by men with differing MHC genes. They even described these scents as reminding them of boyfriends past and present. If the T-shirt belonged to a man whose immune system was similar to hers, the woman often describe the odor as like that of her father or brother. Some of the women were surprised by how beautiful or dreadful they found the various odors.
But what smell does a woman prefer after she is already pregnant? When mice are pregnant their odor preferences tend to revert to familiar odors of mice with MHC genes similar to their own. By nesting with related mice, the mothers get help nursing as well as protection from foreign males. While Wedekind didn't include any pregnant women in his study, he did examine women who were on hormonal birth control which mimics pregnancy by raising the estrogen levels. These women on the pill preferred the odor of males with similar MHC genes.
Selecting MHC-dissimilar mates serves three basic functions: it boosts fertility, results in offspring with stronger immune systems, and reduces the risk of genetic disease. While humans are not thought to make mate selection entirely on odor preferences, Wedekind believes the mechanism is still strong. This experiment at least proves that pheromones and scents do play a role in sexual attraction. It also may provide insight into why products marketed to men claiming to contain pheromones that appeal to women may have mixed results. Women are drawn to different types of scents based on their own immune system. This study may also serve as a warning for women on birth control. If these women meet and marry mates while on hormonal birth control, they may find that they have a completely different reaction to their mates when they get off the pill in attempt to conceive.