The Scent of Sex

Her nose decides whether he's gonna score!

Posted Feb 03, 2009

This is the month when we turn our hearts and minds to love. February 14, our calendar's date for romance, became connected with St. Valentine in the high middle ages, and has evolved to be celebrated with love notes, mass consumption of chocolate, and first or special dates. Valentine's Day was originally marked by a mutual exchange of love notes between men and women, but the US Greeting Card Association estimates that today 85% of Valentine's are bought by women. The fact that women do the majority of the "sending" to their would-be or current lovers reflects the biological orientation of our species-- women are the choosers of their suitors.

A number of surveys examining how we rate the appeal of the opposite sex, have found that men place the highest importance on how physically attractive a woman is, while women find men of high status, power, resources and money most attractive. The contrast of "looks" with "status" between men and women makes evolutionary sense. Physical attractiveness in women is indicative of potential fertility and social status in men is indicative of being able to help a woman rear her children.

Though a warm house, hired help, and plenty of nutritious food will certainly support a woman's ability to raise a child, the most important factor in the evolutionary game is that the child herself be healthy enough to thrive and be reproductively successful. So for women, finding a man who can provide material benefits is good, but most importantly he should be able to sire physically healthy children. But what determines whether a particular man can produce healthy children? The answer lies in his immune system.

Our immune system determines what diseases we can defend against and how well, and what diseases we may carry as recessive traits. But even more important than the general robustness of a man's immune system is the complementarity of his immune system with the immune system of the particular woman he is having sex with. That is, how the number of possible pathogens the two of you can fight against and how unlikely you are to double up on nasty recessive disease traits measure up. So the goal for a woman is to find a healthy man who has a different immune system from her own. But how can a woman tell what a man's immune system is like?

Our immune systems are coded for by a cluster of genes called the major histocompatibilty complex (MHC), and everyone, except if you have an identical twin, has a unique set of MHC genes. Your unique string of MHC genes are the genotype for your immune system, and your phenotype, the external manifestation of the genes for your immune system, is your body-odor! And your odorprint is as unique as your fingerprint.

In the now famous "T-shirt" experiments it was shown that specific women chose as most sexy and pleasant smelling T-shirts belonging to men who had immune systems that were different from their own. Because we all possess different MHC genes (and body-odor), for every woman a different set of men will be delicious smelling and others won't be. There's no Brad Pitt of body odor! A woman's nose not only responds to a man's body-odor in terms of his biological suitability, women actually find how a man smells to be the most important factor in their sexual attraction.

In two large studies we conducted to examine how important various physical and social status factors were for men and women when choosing a sexual partner, we discovered that above all other physical characteristics, women ranked a man's scent as the most important feature for determining whether she would be sexually interested in him.* How a man smelled was also more much important than any social status factor. And of all physical characteristics women preferred a man to be "better than average" in his body-odor than anything else. Women also found men who smelled great due to the fragrance they wore irresistible. In the words of one respondent: "If I'm with a guy who smells really good, nothing else about him seems to matter." So listen up men, it's real chemistry between you and your love interest and her nose is going to decide whether she'll let you be her Valentine tonight.


*Men found how a woman looked to be the most important factor. For more information see references below.
Herz, R.S. & Cahill, E. D. (1997). Differential use of sensory information in sexual behavior as a function of gender. Human Nature, 8, 275-286.
Herz, R.S., & Inzlicht, M. (2002). Gender differences in response to physical and social signals involved in human mate selection: The importance of smell for women. Evolution and Human Behavior, 23, 359-364.

Rachel Herz is the author of The Scent of Desire and on the faculty at Brown University.

For more information, click: Rachel Herz