When I read that a pair of professors at the University of Texas, Austin, had conducted a study into what motivates people to have sex, my first reaction was that academics get grants to research the blindingly obvious.
Researchers Cindy Meston and David Buss claim to have uncovered a staggering 237 reasons for human copulation, the baby dance, the horizontal jog -- call it what you will -- which they divide into nine categories: pleasure; love; conquest; jealousy; duty; adventure; barter; ego boosting and, finally, the darker motives of deception and coercion.
There are almost certainly as many reasons women have sex as there are women in the world, so Meston, one of the world's leading researchers on women's sexuality, and Buss, evolutionary psychology professor at the University of Texas, probably don't have the whole picture, but chances are, their findings are pretty relevant to us.
Looking at the sexual histories of a thousand heterosexual women of all ages and backgrounds from around the world, as well as conducting a study of 400 Americans, both male and female, Meston and Buss have concluded that we women have sex because we enjoy it; because we are in love; because we like the thrill of the chase; because of rivalry or jealousy; because we don't want to upset our partners by saying 'no'; because we enjoy novelty; because we want a favour; because we want to feel good about ourselves, and possibly because we've been lied to or manipulated.
Attraction, sexual desire and pleasure feature in seven of the top 10 reasons for women, while love and affection make up the rest. Things get interesting when you compare women's answers to men's -- eight of these are the same for both sexes.
BATTLE OF THE SEXES?
The gender wars appear to be much exaggerated. If physical attraction is as important to women as men, and if emotions play as big a role for both men as women, then what the hell are we all fighting about?
We live in a world where we are told that men are from Mars and women are from Venus. Self-help books, movies and magazines all tell us that men and women think, act and feel completely differently.
Men are supposedly emotional cripples, while we women are sensitive, passive souls who have been dreaming of Prince Charming and a big white wedding ever since we were old enough to pull a pillowcase over our heads and pretend it was a veil.
The truth, of course, is far more complicated than that. Of course we are somewhat different, and the reasons men and women give reflect this. And surveys, even anonymous ones, by their nature compel us to give answers we think are expected of us. If men think they really ought to be more interested in casual sex, their answers echo this; if women believe they should be good girls, then they tweak their answers to fit this too. After all, nobody wants to come off looking less than manly, or as a slut, even in anonymous surveys.
Taking all this into account, according to 'Why Women Have Sex' and much other research too, it does seem that both genders are pretty much in agreement when it comes to sex. So where does this idea of the sexual divide come from?
STUD VERSUS loose woman
I hate to say it, but we are living in a world where the old sexual double standard still exits -- men are valorised for having multiple sex partners while women are condemned for the same behaviour.
Jodie Marsh and Jordon are perceived as loose women; Callum Best -- God love us! -- is a stud, at least in his own mind. Then there's Warren Beatty, Hugh Hefner, Jack Nicholson and porn star Ron Jeremy. They have all allegedly bedded several thousand women. That makes them heroes; men want to be them.
Lindsay Lohan, Madonna and Pamela Anderson have apparently each racked up a significant number of notches too, but they're not regarded as role models -- they're a bad influence on impressionable young girls.
Be it a hangover from the bad old days of patriarchy, sleeping around will still have a woman's reputation tarred and feathered. In fact, "slut-shaming" is a common strategy women use when competing for the same man.
Since society may punish a woman for having casual sex, why do we do it?
THE PLEASURE PRINCIPLE
Unsurprisingly, physical pleasure is the most frequently cited reason given as to why women -- and men -- have sex. We want sex to be enjoyable, as ideally it should be. As one woman, who probably speaks for most of us, told Meston and Buss: "I cannot imagine going into a sexual situation without expecting a pleasurable experience, it wouldn't make sense to me."
Mary (36), a single solicitor from Clare living in Dublin, regularly has casual sex for enjoyment. "I am single and I have needs, so I have absolutely no problem going to a bar and picking someone up for a one-night stand.
"Whenever I feel like it, which is usually about once a month, I go out with the intention of bringing someone home for sex. I am not looking to meet a potential partner on these particular nights, just someone to go to bed with. I wouldn't exactly advertise this fact at work on Monday morning, but I am not ashamed of it.
"The perception is that single women in their 30s are dying to settle down and have babies, but some of us are happy with our busy careers and disposable income, and just want men occasionally for sex.
"It is exciting getting ready to go out on these nights when I have just one thing on my mind and I find it empowering and arousing. Even if the guy in question isn't great in bed, the thrill of the unknown more than compensates," she adds.
For other women, familiarity breeds contentment. Heather (35), an office worker from Dublin who has been happily married for 10 years, puts it like this: "Even now, after all these years, I think my husband is the sexiest man I know. I love having sex with him and I love him and he knows how to please me. I sometimes think it would be exciting to be with someone else, but I don't think it would be as good because there wouldn't be any love or special connection."
GIRL VERSUS GIRL
The power that comes from sexual conquest is not just a man's prerogative. In the early days of sex studies, competition among women was an academic blind spot because, compared to the more obvious male rivalry, researchers didn't notice that women have a few tricks of their own.
As every woman knows, sexual competition can be a fierce battle fought under the most ladylike of demeanours. "I know this is a terrible thing to confess, but I have to be the sexiest, most attractive and popular girl when I'm out with my friends," says Fiona, a 23-year-old graphic designer from Cork.
"I don't like it when guys flirt with my friends. I guess it makes me feel insecure, so I spend ages getting ready and always wear short skirts."
If a man is famous or popular, we have two very good reasons to want to have sex with him -- bragging rights and the chase.
Here's how one woman in the book explains it: "I had sex with a person who was very well-known and popular. I didn't do it because I wanted to have a relationship with him; I wanted to be the one that stole him away from the other girls.
"I couldn't wait until the next day when the other girls knew I had been with him. It made me feel great."
But a man doesn't have to be George Clooney or Robert Pattinson to cause a bun-fight among women.
Sinead (32), an accountant and respectably married mother of two from Kerry, tells me about her colourful past: "We used to have this competition where we'd choose a guy in a pub and see which one of us could get him. We had ground rules, but if I was really keen on the guy I'd break them, mostly by whispering in his ear what I wanted to do to him later. Worked every time."
KEEPING HIM HAPPY
Getting a man and keeping him are two different things. This means that we need to protect our interests -- 'hands off ladies, this one's mine!'.
If we're worried our partners might stray, one way of trying to prevent it is 'mate-guarding'. If you're a fan of 'Sex and the City', you may remember an episode where Samantha was convinced her boyfriend Richard would cheat on her a second time. Samantha had sex with him repeatedly hoping that he'd be sexually satisfied and way too tired to get it on with another woman -- that's mate-guarding.
Love and wifely duty causes mate-guarding, too. Sometimes a woman will have sex with her husband or boyfriend when she is not really in the mood, simply because she cares about him -- or because he nags.
As a 32-year-old married woman tells Meston and Buss: "Sometimes, as a woman you do not feel like having sex, either being too tired or too busy. But, being in a married relationship, you must put the other person's needs in front of your own at times. At times, I have given in to sex just to keep the peace in the household.
"Mind you, I'm always extremely glad that we did have sex after the act."
Sometimes women use sex to trade up -- to get a better partner than the one you already have or prove to ourselves that even if one man doesn't want us, another one will. As the saying goes, the best way to get over one man is to get under another.
Sex with someone new can be a much-needed boost to the ego.
Aine, a 40-year-old nurse from Dublin, describes how her separation sent her into the arms of another man: "I had no idea my husband was so unhappy. Everything seemed fine, and one day he announced he no longer loved me.
"About a month after he moved out, I went on a date with this man I knew through work and we ended up in bed that night. I told myself nothing was going to happen, but I still wore good lingerie just in case, so I suppose I was lying to myself. It was a rebound fling but having sex made me feel better, like I was still an attractive woman."
For some women, financial benefits are all part of the sexual give and take. Think of the sugar daddy phenomenon -- sleep with some men and expensive gifts will follow.
The casting couch still exists in Hollywood: Charlize Theron, Gwyneth Paltrow and Megan Fox all claim they were offered roles in return for sexual favours, a barter they refused.
Madonna famously stated that she lost her virginity as a career move, but for most women sleeping their way to the top isn't as easy as you'd think. A 2006 poll found that only 12pc of women who'd had it on with the boss snagged a promotion as a result.
Sex with the boss can have the opposite effect, as a friend of mine found out after a Christmas party fling. "I didn't do it to get a promotion or favours, but because I really fancied him. Maybe it was because he was the boss -- it felt incredible to be able to seduce him, even if it was for one night.
"Everything was fine until I told a friend and she blabbed and the whole office found out. After that he started acting really cold towards me, really picking on me. I suppose he didn't want to look like I was getting preferential treatment. It was awful. I really considered quitting, but then he left and the gossip died down. Never again!"
They say all is fair in love and war, but is it really? Sexual deception is a painful experience and we rarely let it happen to us again -- once bitten, twice shy.
A 27-year-old woman relates this story: "When I was in college there was this one guy I really liked. He told me he was not like other guys, how pretty and smart I was and how lucky he would be if we were together. I completely bought his lies. We had sex and the next day he did not call me. Then I found out he told all his friends how easy I was. I felt completely degraded."
Marianne, a 33-year-old sales manager in Dublin originally from America, found out the hard way that the man of her dreams was still married. "I know getting divorced in Ireland is long and difficult. He was still living at home with his wife and he told me that if he moved out he would lose his claim on it.
"We had been together about six months when his wife found a text from me. She called and cried down the phone. She was pregnant and accused me of trying to break up her marriage. I felt awful. I never planned on being the other woman."
TO PASS THE TIME
Sometimes women have sex as a cure for boredom and some evidence suggests we have more sex in a recession. Naomi (35), a PR officer from Galway, reckons that this is one of the benefits of having a boyfriend.
"I wouldn't have sex with a random stranger because I was bored, but with someone you're seeing regularly, well, it does whittle away the time."
Dublin-born Nic, who is 26 and engaged, agrees: "While I wouldn't say I've had sex due to boredom, if I'm at a loose end it's just the best way to fill the time!"
We've looked at some of the 237 reasons but, of course, there are lots and lots more. Women also have sex because they get caught up in the moment; because they are lonely or sad; because they are drunk; because they want intimacy; because they have been coerced or emotionally blackmailed; because they want to celebrate an occasion; because they want to feel used; because they want to feel special; because it can help cure a headache; because they want to fall pregnant; because they get turned on by a conversation or situation; because they want to end a relationship or start a new one.
Or because it seems like the logical thing to do; because they are feeling romantic; because they are trying to cheer up someone, and sometimes just because an opportunity presents itself. And yes -- men have sex for these reasons too.
Most of you probably recognise a good few of the reasons we've looked at -- they may be part of your own sexual history, or the histories of your friends.
But the question remains: why are we so reluctant to treat men and women as equally motivated by sex when we have sex for much the same reasons? Why do we regard men as sexual predators and women their prey?
The sexual double standard means many women spend their lives worrying about their reputations. Society has forced us to be a little circumspect. A lot of the time, it doesn't change our behaviour, just what we allow others to know.
Whether we are single women, young women, older women, divorced women, women on the pull or women in search of true love, we know we're supposed to be good and we know we're not to have casual sex or rack up multiple partners, but many of us do. The truth is, there are very few bad girls and very few good ones. Most of us fall somewhere in the middle -- just like men.
'Why Women Have Sex, Understanding Sexual Motivation from Adventure to Revenge' by Cindy Meston and David Buss is published by Vintage