Perefect symmetry: Brad Pitt and Geena Davis in Thelma and Louise. Photograph: Ronald Grant Archive
Do you want to know why women have sex with men with tiny little feet? I am stroking a book called Why Women Have Sex. It is by Cindy Meston, a clinical psychologist, and David Buss, an evolutionary psychologist. It is a very thick, bulging book. I've never really wondered Why Women Have Sex. But after years of not asking the question, the answer is splayed before me.
Meston and Buss have interviewed 1,006 women from all over the world about their sexual motivation, and in doing so they have identified 237 different reasons why women have sex. Not 235. Not 236. But 237. And what are they? From the reams of confessions, it emerges that women have sex for physical, emotional and material reasons; to boost their self-esteem, to keep their lovers, or because they are raped or coerced. Love? That's just a song. We are among the bad apes now.
Why, I ask Meston, have people never really talked about this? Alfred Kinsey, the "father" of sexology, asked 7,985 people about their sexual histories in the 1940s and 50s; Masters and Johnson observed people having orgasms for most of the 60s. But they never asked why. Why?
"People just assumed the answer was obvious," Meston says. "To feel good. Nobody has really talked about how women can use sex for all sorts of resources." She rattles off a list and as she says it, I realise I knew it all along: "promotion, money, drugs, bartering, for revenge, to get back at a partner who has cheated on them. To make themselves feel good. To make their partners feel bad." Women, she says, "can use sex at every stage of the relationship, from luring a man into the relationship, to try and keep a man so he is fulfilled and doesn't stray. Duty. Using sex to get rid of him or to make him jealous."
"We never ever expected it to be so diverse," she says. "From the altruistic to the borderline evil." Evil? "Wanting to give someone a sexually transmitted infection," she explains. I turn to the book. I am slightly afraid of it. Who wants to have their romantic fantasies reduced to evolutional processes?
The first question asked is: what thrills women? Or, as the book puts it: "Why do the faces of Antonio Banderas and George Clooney excite so many women?"
We are, apparently, scrabbling around for what biologists call "genetic benefits" and "resource benefits". Genetic benefits are the genes that produce healthy children. Resource benefits are the things that help us protect our healthy children, which is why women sometimes like men with big houses. Jane Eyre, I think, can be read as a love letter to a big house.
"When a woman is sexually attracted to a man because he smells good, she doesn't know why she is sexually attracted to that man," says Buss. "She doesn't know that he might have a MHC gene complex complimentary to hers, or that he smells good because he has symmetrical features."
So Why Women Have Sex is partly a primer for decoding personal ads. Tall, symmetrical face, cartoonish V-shaped body? I have good genes for your brats. Affluent, GSOH – if too fond of acronyms – and kind? I have resource benefits for your brats. I knew this already; that is how Bill Clinton got sex, despite his astonishing resemblance to a moving potato. It also explains why Vladimir Putin has become a sex god and poses topless with his fishing rod.
Then I learn why women marry accountants; it's a trade-off. "Clooneyish" men tend to be unfaithful, because men have a different genetic agenda from women – they want to impregnate lots of healthy women. Meston and Buss call them "risk-taking, womanising 'bad boys'". So, women might use sex to bag a less dazzling but more faithful mate. He will have fewer genetic benefits but more resource benefits that he will make available, because he will not run away. This explains why women marry accountants. Accountants stick around – and sometimes they have tiny little feet!
And so to the main reason women have sex. The idol of "women do it for love, and men for joy" lies broken on the rug like a mutilated sex toy: it's orgasm, orgasm, orgasm. "A lot of women in our studies said they just wanted sex for the pure physical pleasure," Meston says. Meston and Buss garnish this revelation with so much amazing detail that I am distracted. I can't concentrate. Did you know that the World Health Organisation has a Women's Orgasm Committee? That "the G-spot" is named after the German physician Ernst Gräfenberg? That there are 26 definitions of orgasm?
And so, to the second most important reason why women have sex – love. "Romantic love," Meston and Buss write, "is the topic of more than 1,000 songs sold on iTunes." And, if people don't have love, terrible things can happen, in literature and life: "Cleopatra poisoned herself with a snake and Ophelia went mad and drowned." Women say they use sex to express love and to get it, and to try to keep it.
Love: an insurance policy
And what is love? Love is apparently a form of "long-term commitment insurance" that ensures your mate is less likely to leave you, should your legs fall off or your ovaries fall out. Take that, Danielle Steele – you may think you live in 2009 but your genes are still in the stone age, with only chest hair between you and a bloody death. We also get data which confirms that, due to the chemicals your brain produces – dopamine, norepinephrine and phenylethylamine – you are, when you are in love, technically what I have always suspected you to be – mad as Stalin.
And is the world mad? According to surveys, which Meston and Buss helpfully whip out from their inexhaustible box of every survey ever surveyed, 73% of Russian women are in love, and 63% of Japanese women are in love. What percentage of women in north London are in love, they know not. But not as many men are in love. Only 61% of Russian men are in love and only 41% of Japanese men are in love. Which means that 12% of Russian women and 22% of Japanese women are totally wasting their time.
And then there is sex as man-theft. "Sometimes men who are high in mate value are in relationships or many of them simply pursue a short-term sexual strategy and don't want commitment," Buss explains. "There isn't this huge pool of highly desirable men just sitting out there waiting for women." It's true. So how do we liberate desirable men from other women? We "mate poach". And how do we do that? We "compete to embody what men want" – high heels to show off our pelvises, lip-gloss to make men think about vaginas, and we see off our rivals with slander. We spread gossip – "She's easy!" – because that makes the slandered woman less inviting to men as a long-term partner. She may get short-term genetic benefits but she can sing all night for the resource benefits, like a cat sitting out in the rain. Then – then! – the gossiper mates with the man herself.
We also use sex to "mate guard". I love this phrase. It is so evocative an image – I can see a man in a cage, and a woman with a spear and a bottle of baby oil. Women regularly have sex with their mates to stop them seeking it elsewhere. Mate guarding is closely related to "a sense of duty", a popular reason for sex, best expressed by the Meston and Buss interviewee who says: "Most of the time I just lie there and make lists in my head. I grunt once in a while so he knows I'm awake, and then I tell him how great it was when it's over. We are happily married."
Women often mate guard by flaunting healthy sexual relationships. "In a very public display of presumed rivalry," Meston writes, "in 2008 singer and actress Jessica Simpson appeared with her boyfriend, Dallas Cowboys quarterback Tony Romo, wearing a shirt with the tagline Real Girls Eat Meat. Fans interpreted it as a competitive dig at Romo's previous mate, who is a vegetarian."
Meston and Buss also explain why the girls in my class at school went down like dominoes in 1990. One week we were maidens, the following week, we were not. We were, apparently, having sex to see if we liked it, so we could tell other schoolgirls that we had done it and to practise sexual techniques: "As a woman I don't want to be a dead fish," says one female. Another interviewee wanted to practise for her wedding night.
The authors lubricate this with a description of the male genitalia, again food themed. I include it because I am immature. "In Masters & Johnson's  study of over 300 flaccid penises the largest was 5.5 inches long (about the size of a bratwurst sausage); the smallest non-erect penis was 2.25 inches (about the size of a breakfast sausage)."
Ever had sex out of pity and wondered why? "Women," say Meston and Buss, "for the most part, are the ones who give soup to the sick, cookies to the elderly and . . . sex to the forlorn." "Tired, but he wanted it," says one female. Pause for more amazing detail: fat people are more likely to stay in a relationship because no one else wants them.
Women also mate to get the things they think they want – drugs, handbags, jobs, drugs. "The degree to which economics plays out in sexual motivations," Buss says, "surprised me. Not just prostitution. Sex economics plays out even in regular relationships. Women have sex so that the guy would mow the lawn or take out the garbage. You exchange sex for dinner." He quotes some students from the University of Michigan. It is an affluent university, but 9% of students said they had "initiated an attempt to trade sex for some tangible benefit".
Then there is sex to feel better. Women use sex to cure their migraines. This is explained by the release of endormorphins during sex – they are a pain reliever. Sex can even help relieve period pains. (Why are periods called periods? Please, someone tell me. Write in.)
Women also have sex because they are raped, coerced or lied to, although we have defences against deception – men will often copulate on the first date, women on the third, so they will know it is love (madness). Some use sex to tell their partner they don't want them any more – by sleeping with somebody else. Some use it to feel desirable; some to get a new car. There are very few things we will not use sex for. As Meston says, "Women can use sex at every stage of the relationship."
And there you have it – most of the reasons why women have sex, although, as Meston says, "There are probably a few more." Probably. Before I read this book I watched women eating men in ignorance. Now, when I look at them, I can hear David Attenborough talking in my head: "The larger female is closing in on her prey. The smaller female has been ostracised by her rival's machinations, and slinks away." The complex human race has been reduced in my mind to a group of little apes, running around, rutting and squeaking.
I am not sure if I feel empowered or dismayed. I thought that my lover adored me. No – it is because I have a symmetrical face. "I love you so much," he would say, if he could read his evolutionary impulses, "because you have a symmetrical face!" "Oh, how I love the smell of your compatible genes!" I would say back. "Symmetrical face!" "Compatible genes!" "Symmetrical face!" "Compatible genes!" And so we would osculate (kiss). I am really just a monkey trying to survive. I close the book.