I’d like to start by tipping my hat to my favorite Danish guy reader, who clued me in to a new UK study published in Trends in Ecology and Evolution. It suggests that the Pill is dramatically changing the way women and men experience sexual chemistry. It’s definitely changing how women sniff out potential mates, and it’s probably also changing the pheromones they’re putting out to guys. This is very important says Rachel Herz, PhD, author of The Scent of Desire and a faculty member at Brown University, “My own research says the way a man smells to a woman is the main determinant of sexual attraction.”

Women are naturally attracted to men whose genetic makeup is dissimilar to their own. A man’s natural odor may be the most important information he can give a woman. “Body odor is an external manifestation of the immune system, and the smells we think are attractive come from the people who are most genetically compatible with us.” Much of what we vaguely call “sexual chemistry,” she adds, is likely a direct result of this scent-based compatibility.

Having a genetically different mate increases the chances for a healthy pregnancy and a healthy baby.
Couples with a high degree of gene similarity suffer higher rates of miscarriage and experience longer intervals between pregnancies, indicating more difficulty conceiving. Genes affect important immune responses. By mating with males who have different genes, females give their offspring a better disease-fighting repertoire.

The number of genes couples share corresponds directly with the likelihood that they would cheat on one another.

Women who are genetically similar to their partners report being less satisfied in their sexual relationship with their partner — and seek more new sex partners — than women with genetically dissimilar partners. If a man and woman have 50 percent of their MHC (major histocompatibility complex) alleles in common, the woman has a 50 percent chance of sleeping with another man behind her partner’s back.

In laboratory studies, women who sniff men’s sweaty T-shirts find them more attractive when they come from men whose genes don’t match theirs.

(I hope those women were generously compensated for participating in those studies. Sounds like a lose-lose to me. You’re either smelling a bunch of gross guys you want nothing to do with, or you’re smelling guys who get you horny, only to remember you’re sitting in a fluorescently lit lab.)

Women taking the Pill choose lower testosterone men across the board. In T-shirt-sniffing studies, women taking birth control pills seem to be attracted to the “wrong,” or genetically similar men.

“Women who have their hormone levels smoothed out by the Pill tend to seek men who look like good long-term prospects,” says the new report’s lead author, Alexandra Alvergne, an evolutionary anthropologist at the University of Sheffield.

A woman on a normal menstrual cycle will have a burst of hormones around the time of ovulation that will drive her to lust after the hottest, sexiest guy in the room. When hormones aren’t raging, women want to be with a less testosteronized man, one who is likely to stick around for the long haul.

In addition, when women aren’t ovulating, they tend to gravitate toward photos of men with a more feminine look.

When women become pregnant, they switch to preferring the scent of genetically similar males. Claus Wedekind, PhD, who performed the original T-shirt-sniffing studies, has suggested that birth control pills somehow mimic this process.

What if you marry and then stop taking the Pill?

Many media outlets have dubbed the pill “the divorce pill” in light of these findings. Herz wouldn’t go that far, but she acknowledges that it’s a problem. ”It’s like picking your cousins as marriage partners. It constitutes a biological error.”

(An interesting tidbit: Charles Darwin married his first cousin.)

As a result, explains Charles Wysocki, a psychobiologist at Florida State University, when such a couple decides to have children and the woman stops taking birth control, she may find herself less attracted to her mate for reasons she doesn’t quite understand. “On a subconscious level, her brain is realizing a mistake was made—she married the wrong guy,” he says.

Women’s noses aren’t the only ones affected.

Men are more attracted to women who are not on the Pill.

Men are sensitive to smell as well. The Pill distorts the signals a woman sends, making her seem less appealing to men.

Geoffrey Miller, an evolutionary psychologist at the University of New Mexico and author of The Mating Mind, noticed the pill’s connection to waning male desire while studying a group of exotic dancers—women whose livelihoods depend on how sexually appealing they are to male customers. Non-pill-using dancers made about 50 percent more in tips than dancers on oral contraceptives.

The Pill may also be fostering the hookup culture in ways you haven’t thought about.

“Hunter-gatherers didn’t have to do a lot of kissing, because they could smell each other pretty clearly from a few feet away,” Miller says. “With all the showering, scents, and soap, we have to get our noses and mouths really up close to people to get a good idea of their biochemistry. People are more motivated to do a lot more kissing and petting, to do that assessment before they have sex. In other words, the need to smell our mates—and the comparative difficulty of doing so in today’s environment of perfumes and colognes—may actually be driving the sexual disinhibition of modern society.”

Preference for human body odors is influenced by both gender and sexual orientation. A study published in 2005 in Psychological Science showed that when study participants of different orientations and genders were asked to choose between distinct odors — straight men, gay men, straight women, lesbian women — each picked the odor of a partner of the preferred gender and orientation.

What are the implications for your relationships?

Herz recommends that if you’re looking for a man to be the father of your child, go off the pill before you start your search. Make sure you truly like the way a guy smells before you get serious about him.

If the cow’s already out of the barn, it’s more complicated. “A woman’s perception of her partner’s smell is so intertwined with her emotional reaction to him that it could be difficult for her to assess his scent as if he were a stranger. If she’s in love, he could smell like a garbage can and she’d still be attracted to him,” says Herz.

Interestingly, Fisher believes that ultimately, this all might be for the best. “When we lived in a hunter-gatherer society, there was a village to help raise the children, so picking the most macho man might have been fine. In modern society, it makes much more sense to pick the dad rather than the cad. Then the woman has at least one person to help raise the baby.”

PICK THE DAD RATHER THAN THE CAD.
It’s what I’ve been telling you all along!