A fool and his money.
Men tip lap dancers more when the gyrating women are most fertile.

Gary Houlder/Corbis

Something in the Way She Moves?

In a particularly stimulating study, researchers have found that lap dancers--women who work in strip joints and, for cash, gyrate in the laps of seated men--earn more when they are in the fertile phase of their menstrual cycle. The finding suggests that women subtly signal when they are most fertile, although just how they do it is not clear.

Women, unlike many mammals, don't come into heat or estrus, a state of obvious fertility that attracts potential mates. Common wisdom has it that estrus was lost as humans evolved. The notion is that women evolved "concealed ovulation" along with around-the-month sexual receptivity the better to manipulate males by keeping them in the dark, says Geoffrey Miller, an evolutionary psychologist at the University of New Mexico in Albuquerque. But now Miller and colleagues have found evidence that a woman’s state of fertility may not be so secret after all.

The researchers used ads and flyers to sign up 18 lap dancers from local clubs. Each woman was asked to log on to a Web site and report her work hours, tips, and when she was menstruating. Lap dancers generally work 5-hour shifts with 18 or so 3-minute performances per shift. They average about $14 per "dance"--all of which is called a "tip" because it is illegal to pay for sex in New Mexico.

Over a 60-day period, the researchers collected data from 5300 lap dances. They divided the answers according to whether the dancers were in the menstrual phase, the high-fertility estrous phase, or the luteal phase. The result, as they report online this week in the journal Evolution and Human Behavior: Of the 11 women with normal menstrual cycles, those in the estrous phase pulled in about $70 an hour--compared with $50 for those in the luteal phase, and only $35 an hour for those who were menstruating. The other seven women were on birth control pills. They earned less across the board, and there was no peaking at the estrous phase.

The numbers suggest that men can tell when a woman is most fertile, although the message seems to be conveyed by "subtle behavioral signals" that evade conscious detection, the authors say. They add that the study couldn't identify whether it is scent or other physical changes that cue the men in, but they don't think it's anything obvious such as type of dance moves or "conversational content."

Evolutionary psychologist Karl Grammer of the Ludwig Boltzmann Institute for Urban Ethology in Vienna says the result fits with his findings that it's possible to detect ovulation through the effect of raised levels of estrogens on the way women walk and dance. "It is highly possible that estrogen modulates motion abilities," says Grammer, in which case "it seems to be most likely that body motion--and not pheromones--is the information carrier."