Women's Sexual Satisfaction Linked To The Way Men Smell

WASHINGTON — Male odors that a woman subconsciously recognizes may have a more potent effect on her romantic feelings than good looks, poems or Valentine roses galore.

Scientists say genetic traits that guide the development of the human immune system produce a range of distinctive odors. Without knowing it, women may be attracted to scents they instinctively associate with robust disease resistance.

In a recent study, researchers found that women appeared happiest with their sex lives when their immune system genes were not similar to those of their male partners.

Women whose immune systems were similar to their husband's or boyfriend's were also more likely to have sex with other men — or at least to think about it.

The study found no significant correlation between men's genes and either their enthusiasm for sex with their partners or their interest in other women, said University of New Mexico evolutionary psychologist Steven Gangestad.

The reason women seem less happy sexually when they have a large number of the same immune system genes as their partners may grow out of a woman's desire to have healthy offspring, Gangestad said.

If she passes one set of genes to her children and their father provides different genes, the children may be born with the ability to defend themselves against a wider range of diseases, he said.

Another hypothesis is that the similar immune system genes may be a marker for kinship that discourages inbreeding.

"We're not saying that a woman has a logical thought, such as, 'This would be a good sire for my offspring,' " Gangestad said in an interview. "In fact, my guess is that women can't perceive what it is that's affecting their responses to their partners."

Also, he said, many different factors go into a woman's choice in a partner, and immune system genetics account for "just one little piece" of that larger puzzle.

Studies of the importance of the immune system in mating preferences have concentrated on a strip of genes known as the major histocompatibility complex, or MHC.

MHC genes contain the blueprints that cells use to make proteins that recognize foreign substances, such as pieces of virus, bacteria or pollen, and initiate quick immune responses.

Nature has evolved MHC genes with such astonishing variety that virtually no two individuals have identical sets. On average, heterosexual human couples share only about 20 percent of their MHC genes, scientists say.

It has been known for years that many animals, including rats, mice, some birds and even lizards, detect genetic MHC differences through smell and make mating choices accordingly.

In the past decade, experiments have indicated that people can also smell the differences.

In 1995, European scientists described an experiment in which 49 female college students were asked to rate the "pleasantness" of the smell of unwashed T-shirts that had been worn for two nights by male students.

Genetic analysis of the participants showed the likelihood that a female would give a T-shirt a pleasant score was significantly linked to the degree to which her MHC genes differed from those of the wearer.

The experiment has been repeated in the United States and Brazil, with similar results.

Last October, Gangestad and other researchers at the University of New Mexico reported in the journal Psychological Science on experiments to answer the next logical question: whether MHC variations might have an effect on actual human relationships.

They asked 48 sexually active heterosexual couples to provide information about their relationships. A small scraping of cells was taken from the inside of each participant's mouth for the MHC analysis.

Split up in separate rooms, the men and women answered questions about their relationships. Each rated the partner in terms of such attributes as thoughtfulness, attractiveness, support, intelligence, ambition and the like.

They were also asked to answer questions that probed their enjoyment of sex and were whether they had ever had sex with, or felt strong sexual attraction to, another person during the relationship.

Gangestad said the subject's answers showed no correlation between MHC variation and the nonsexual factors. There also was no relationship between MHC variation and the way males rated their sex lives or whether they had been either unfaithful or tempted.

But women were significantly less likely to grade themselves as sexually happy when they shared MHC profiles with their men, Gangestad said, and the degree of dissatisfaction was proportional to the percentage of shared genes.

Finally, Gangestad said, women who shared genetic similarity with their partners reported significantly greater interest in outside sex — but only during their fertile days. This association remained, even when the data was "corrected" to take into account the fact that a given woman may have expressed casual attitudes about fidelity.

The results appear to be consistent long-held observation that women are more selective than men in choosing partners, Gangestad said, noting that a woman's "investment" in producing offspring is much greater than a man's.

What for him may be just a happy Valentine will be nine months of pregnancy for her, plus several years of child care.

"In an ancestral environment, there were obviously many factors involved in mate selection, so we weren't sure we'd even be able to detect this MHC effect," Gangestad.

"In fact, what we found was that it is a strong factor."