Canada- Last month, researchers at McGill University in Canada reconfirmed what we’ll call the porn paradox: When hooked up to instruments measuring sexual arousal, men and women reacted with equal speed when watching pornographic films.

This raises several questions, including why women are so much less likely than men to consider viewing porn a valuable use of our time. But the type of material used in the study also raises broader questions about the effects of porn on both sexes.

Irv Binik, who led the research, said he used special sex films borrowed from the Kinsey Institute in Indiana. One was predetermined to be arousing to men, another to women, though Binik says they both looked the same to him. Both showed heterosexual couplings.

Why do any of us find it arousing to watch films of other people having sex? These people are obviously busy. They’re not inviting you to join them.

For men at least, British zoologist Robin Baker has an interesting hypothesis: The pornography industry owes its billions to an ancient animal instinct still seen in rats and monkeys: If a male of one of those species runs across a pair mating, the onlooker will not only become aroused at the sight but will move sperm into the urethra. Which means, as Baker puts it, “he’s loaded and ready to fire.”

In the evolutionary game, such a reaction could prove advantageous if you’re tough enough to force the other male off and take over, or sneaky enough to wait till he leaves and move in while the female is still in the mood.

Humans tend to have sex in private, so such opportunities don’t present themselves often, but the instinct could remain with us to be exploited by pornographers.

Harvard professor Stephen Pinker offers that some male birds will try to mate with anything resembling a female of the species – a stuffed female or even just the head of a stuffed female.

“The sight of a fertile member of the opposite sex would normally correlate with an opportunity to make babies,” he said. Porn fools your body into reacting instinctively as if the images were real.

Pinker goes on to say men’s taste for pornography goes directly to one of the most salient behavior differences between men and women: Men are much more likely to have sex with a total stranger. And assuming you don’t have many friends who are porn stars, porn shows you images of strangers who haven’t even so much as bought you dinner.

So what’s going on with the women in this McGill study?

Binik, the study’s author, said he didn’t actually show that women like pornography – just that their bodies react to it. He launched this study as part of a wider investigation into why some women experience chronic pain during intercourse. Arousal problems can be a factor.

In the past, measuring arousal involved various probes and meters that many people found intimidating. So he tried an instrument called a thermograph that uses a version of remote sensing to measure blood flow through the body. You still have to take off your clothes, he said, but the machine doesn’t actually touch anything.

For him, the most important result of the study was that the instrument worked. Others before him had seen the same effect, he says, and he doubts the prospect that women secretly like pornography as much as men do.

“In women, you get a lot of disagreement between what the body is saying and what the mind is saying,” he said.

All of which goes to show that, at least for women, arousal doesn’t necessarily imply desire.