A man can smell when a woman is ovulating - and the proof is in his testosterone, says a new study from Florida State University that had undergraduate men sniffing sweaty T-shirts for course credit.
The study, published last month in the journal Psychological Science, suggests that olfactory cues signalling a woman's ovulation - her most fertile time - can prime men to have sex with them.
Prior studies have shown that smells affect the hormones and subsequent mating habits of animals. Odours emitted by females influence male testosterone levels, particularly during ovulation. Higher testosterone, in turn, makes males more prone to initiate courtship. It has also been associated with competitiveness, dominance and risk-seeking, traits typically valued by women, particularly those at their reproductive peak.
In the latest study, the researchers decided to map the connection between female ovulation and male testosterone by having women wear T-shirts three nights straight during various phases of their menstrual cycles - and getting men to sniff them.
The women - none of whom were on the Pill - were prohibited from wearing perfume or antiperspirant. If they wanted to bathe, they had to use unscented products. They were also asked to ditch smell-inducing foods such as garlic, vinegar and asparagus, as well as cigarettes, alcohol and drugs. The women were even barred from having sex or sharing a bed. (They got course credit and $10 for their trouble.)
During the day, the women popped their shirts inside a sealed freezer bag. After three days, they returned them to the experimenters. A research assistant then sniffed the shirts to confirm that none smelled of extraneous odours. The shirts were next placed in a freezer, until the male volunteers arrived.
Each man was tasked with smelling one T-shirt by sticking his nose in the bag and taking "three large inhalations" three times over the course of 15 minutes. The researchers then collected the men's saliva to test their testosterone levels.
Men who smelled the T-shirts of ovulating women had higher levels of testosterone than men who smelled shirts worn by non-ovulating women and control shirts that weren't worn by anyone.
"Post-smell testosterone" was highest when men smelled a T-shirt worn by a woman on her presumed day of ovulation.
The men were also asked to subjectively rate the T-shirts' smell. Those worn by ovulating women were the "most pleasant smelling" of the lot.
"When we think of why we might be attracted to someone, we often think of things like physical appearance, intelligence, similar interests and humour," said psychological scientist Saul Miller, who co-authored the study with Dr. Jon Maner.
"The research shows us that we don't always know why we are attracted to someone else, and it shows us how attraction can be strongly shaped by very subtle things," Mr. Miller said.
To women aghast at the idea that men can sniff out where they are in their menstrual cycles, Mr. Miller says not to worry.
"I doubt that they consciously know she's ovulating. If we asked men, 'Do you think this woman is ovulating?' most men would probably not know. They only seem to know that the scent of the woman is more pleasant or attractive."
That said, Mr. Miller advises against ditching your favourite perfume for the au naturel alternative.
"We don't know how men rate the smell of women at ovulation compared to the smell of perfume," he said. "It may be that perfume is better-smelling than both ovulating and non-ovulating scents. Given that the perfume industry is alive and well, my guess is that men still find the scent of perfume quite attractive."