If you have ever been attracted by a person's scent, it may be their genes you smell. Researchers believe that women can identify men based on differences as small as a single gene.
In the most recent issue of Nature Genetics scientists look at the link between women's preference for the odors men give off and a group of genes called the Major Histocompatibility Complex (MHC) — which are an important part of the body's immune response.
A group of 49 women were asked to smell 10 boxes which contained pieces of T-shirts that had either been worn by men or contained a familiar household odor such as bleach or clove.
The men who had worn the T-shirts were selected based on their MHC genes, and were told to sleep in the shirt for two consecutive nights while avoiding other scents entirely, such as colognes and even contact with other people.
The women were then asked to rate each scent based on their familiarity, intensity, pleasantness, and spiciness, as well as choose the one odor which they would choose if they had to smell it all the time.
"A clear pattern emerged," said Dr. Carole Ober, co-author of the study and a geneticist at the University of Chicago. "The women did not choose scents of men with genes totally similar to their own, or totally dissimilar to their own. They chose men with an intermediate level of difference."
"The real surprising thing is that there was something special about paternally [from one's father] inherited [genes]," said Wayne Potts, an expert in the area of MHC research at the University of Utah.
Moreover, the women showed no preference for odors from men with the same gene types as their mothers, but did show a preference for odors from men who shared genes they inherited from their fathers.
"Given that you often can't tell who you're father is, it would be important to evolve a way that you can," said Martha McClintock, co-author of the study and a professor of psychology at the University of Chicago.
Experts believe there are two major reasons for preferring a mate with different genes than your own. The first is that parents with more diverse MHC genes will give rise to offspring with better immune systems, and the second is that by avoiding mates who have very similar MHC types, you also avoid inbreeding.
In fact, previous studies have shown that married people tend to have different types of genes than their spouse.
At the same time, however, the propensity to choose someone with slightly similar genes can also avoid the problem of outbreeding — the mixing of genes that are too different.
"The best is in the middle range and no one knew that before," said McClintock. "Everybody always says different is better, well you get different, different, different, and then it becomes a point where it's no longer better."
Potts cautions that choosing an odor from within a box may not represent what actually happens in the real world when women are attracted to their mates.
"It's the wrong emphasis to say that this means that they're going to prefer MHC similar males when it comes to mating," said Potts. "There is other social context."
How Can You Smell Genes?
"That is one of the molecular missing links," said Potts. "We don't know exactly what the odorants are."
Potts explains that one hypothesis is linked to the ability of MHC molecules to stick to very specific pieces of protein in cells. These MHC molecules and the proteins stuck to them are what is believed to be detected as an odor.
"There are thousands of [MHC molecules] on the surface of every cell, [and] you are sloughing millions of cells everyday," said McClintock. "You look sort of like Pigpen with a cloud of cells around you."
Whatever the mechanism, this study disproves the assumption that humans have a poorly developed sense of smell.