Wedekind’s study is often known as the “sweaty t-shirt study”. The researcher (Wedekind) assembled volunteers, 49 women and 44 men selected for their variety of MHC gene types.
The women were asked to record whether or not they were taking oral contraceptives (e.g. the pill) as this would affect the results because it affects levels of hormones.
Evolutionary psychologists argue that our behaviors are the result of natural selection – that is, the behaviors that best served the human gene pool have been passed down from generation to generation. Since mating behaviors are an essential component of how behavior is inherited, it should be no surprise that why we choose our partners is a major focus of evolutionary psychologists.
Wedekind studied what is known as the Major Histocompatibility Complex, a group of genes that play an important role in the immune system. MHC genes make molecules that enable the immune system to recognize pathogens; in general, the more diverse the MHC genes of the parents the stronger the immune system of the offspring. MHC genes are expressed co-dominantly – that is, we inherit the MHC alleles from both of our parents and they are expressed equally. It would be beneficial, therefore, to have evolved systems of recognizing individuals with different MHC genes and preferentially selecting them to breed with to maximize immune responses. Evolutionary psychologists argue that our “smell” is a sign of our MHC. Wedekind wanted to see if women are attracted to a man because of his MHC. He did this in his famous “Smelly T-Shirt Study.”
The aim of the study was to determine whether one's MHC would affect mate choice. The sample was made up of 49 female and 44 male students from the University of Bern, Switzerland. Each participant was “typed” for their MHC, and a wide variance of MHC was included in the sample. It was noted if the women were taking oral contraceptives. The students probably did not know each other as they were from different courses: women from biology and psychology; men from chemistry, physics, and geography.
The men were asked to wear a T-shirt for two nights and to keep the T-shirt in an open plastic bag during the day. They were given perfume-free detergent to wash clothes and bedclothes and perfume-free soap for showering. They were asked not to use any deodorants or perfumes, to refrain from smoking tobacco or drinking alcohol, to avoid all spicy foods, and to not engage in any sexual activity.
Two days later, the women were asked to rank the smell of 7 t-shirts, each in a cardboard box with a “smelling hole.” The women were tested whenever possible in the second week after the beginning of menstruation, as women appear to be most odor-sensitive at this time. The women were also asked to prepare themselves for the experiment by using a nose spray for the 14 days before the experiment to support the regeneration of the nasal mucous membrane if necessary – as well as a preventive measure against colds or flu. Each woman was also given a copy of Süskind's novel Perfume to sensitize their smell perception.
Three of the seven boxes contained T-shirts from men with MHC similar to the woman's own; three contained T-shirts from MHC dissimilar men; and one contained an unworn T-shirt as a control. Alone in a room, every woman scored the odors of the T-shirts for intensity (range 0-10) and for pleasantness and sexiness (range 0 -10, 5 = neutral).
Women scored male body odors as more pleasant when they differed from their own MHC than when they were more similar. This difference in odor assessment was reversed when the women rating the odors were taking oral contraceptives. This suggests that the MHC may influence human mate choice.
To investigate whether MHC (major histocompatibility complex) genes influence human mate choice. The hypothesis was that individuals would be more attracted to the scent of people with dissimilar MHC genes, as this could increase offspring genetic diversity and immune system strength.