Love is in the Air
> 2/14/2008 11:06:18 AM

Valentine's Day is full of sweet smells— the delicate scent of roses and the rich aroma of chocolates— but the most important smell for romance may be something that you have never heard of: the major histocompatibility complex (MHC). This dense region of genes contains the code for our immune systems, and humans have the ability to detect even small differences with their noses.

Evolutionary psychologists believe that humans seek out mates with very different immune systems to ensure that their children are protected from the greatest number of pathogens. There are thousands of viruses and infectious agents, so each individual parent has only an incomplete defense, and must pick up extra pieces from the mate to fill in the walls.

This evolutionary theory has been confirmed by numerous studies. In a famous experiment by Claus Wedekind, male college students wore T-shirts for two days and then submitted them to a group of women for evaluation. Invariably, the women preferred the shirts of men with the most different MHC, and they assigned more positive qualities to the owner when asked to imagine them. 

It may seem implausible that the magical, romantic phenomenon referred to as "the spark" or "chemistry" can be determined by smell, but its power escapes our notice because the important smells are subtle fragrances that often slip below our conscious perception. While we should not be slaves to our biological impulses, they cannot be heedlessly ignored without risking miscarriage, weak immune systems, and sexless marriages. These dangers are vividly described in Rachel Herz's book, The Scent of Desire.

The likelihood of mismatched immune systems is increased by the many chemical body alterations available to the modern citizen. Deodorants and frequent showers mask our scents; medieval women could instantly judge a potential suitor before getting intimately close. Oral contraceptives may also be confusing matters because they trick the woman's body into thinking it is already pregnant, which switches her preferences. A pregnant woman is programmed to seek out people with similar immune systems, who are more likely to be relatives or potential protectors. This switching is partly responsible for the disrupted sex drive often experienced in pregnancy and when starting or stopping a regimen of the pill. Of course, not using the pill may bring unexpected children along with a greater chance of sensing real chemistry, so the decision is not clear-cut.

What, then, is the advice to take away? Skip the shower and the perfume on the first few dates to make sure that you are biologically compatible. You might fool someone in the beginning, but if the relationship progresses they will eventually wake up next to you and catch a whiff of your unmasked scent, and then basic biology might cut through artifice.

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