Imagine you're a heterosexual woman wondering if a guy's in the mood for sex, but your only clue is a glimpse at his groin. The answer should stick right out.
Now imagine the roles are reversed. Not so easy, is it guys?
Canadian researchers have analyzed almost 40 years of sex research and found that while erections and male feelings of sexual desire almost always go hand-in-hand, the link between a female's feelings of desire - and her body's physiological response - is often much more complicated.
The findings, published online yesterday in the Archives of Sexual Behaviour, give a more definitive answer to a question that has interested sex researchers for decades: Do men and women differ in their physical response to sexual stimuli?
"I think it's important for women to realize that their physiological response is different from men," said Meredith Chivers, a psychology professor at Queen's University. "They can have physiological responses and not feel sexually aroused. But that's normal."
The analysis looked at 134 studies, published between 1969 and 2007, involving more than 2,500 women and 1,900 men. Participants were asked how aroused they felt during and after exposure to a variety of stimuli. This subjective measure of arousal was compared with physiological responses: changes in penile erection for men and changes in genital blood flow for women.
The men's subjective ratings more closely matched their physiological measures than the women's; men's brains and bodies were almost always in agreement, while there was more often a reported inconsistency between women's bodies and minds.
For example, Dr. Chivers's previous research suggests women can be physically aroused by a wide variety of sexual imagery, but they don't report feeling any lust or desire - a disconnect in essence between the vagina and brain.
Dr. Chivers said the findings generate questions for future research: Does low concordance influence a person's sexual function? And if it's a problem, how can it be fixed?
The authors offered several possible theories about why men and women differ this way. One involves natural selection: Since men require an erection to have intercourse, it makes sense that a male who always gets an erection will have a biological advantage over one who doesn't achieve one when he feels aroused.
On the other hand, it can be advantageous for females to be choosy about their mates. So having greater independence between what their body signals and what their brain is saying about a partner's staying power might increase the success of their offspring.
Some scientists, however, believe there are better methods to measure a woman's bodily changes. As an alternative to measuring blood flow to the vagina, for example, McGill University researchers are looking at ways to measure clitoral changes.
Alexander McKay, Toronto-based research co-ordinator of the Sex Information and Education Council of Canada, says people shouldn't get too hung up on deciphering subtle body changes in their partners, anyway. "The person who is relying on physiological cues to try and pick up whether a partner is keen ... is a person who is barking up the wrong tree," he said. "In modern culture, a person signals a readiness for sexual activity through a whole range of cues aside from a purely physiological response."