A common bit of pop evolutionary psychology is that men want quantity in their sexual partners, because that is how they maximise their chances of reproducing, while women want quality, because that is how they maximise the chances of their off-spring surviving and reproducing in turn. But some problems crop up when you do the numbers.

If we consider only heterosexual relationships, and if we simplify just a little for the sake of avoiding a pornographic level of detail, then every relationship has to involve one male and one female. In turn this means that, if you sum up the number of relationships had by women, and sum up the number of relationships had by men, then the sums must come out the same. Given that the sexually active male and female populations are also roughly equal in number, the average number of relationships for men and woman must also be roughly equal.

So how do we reconcile the idea that men want quantity, while women want quality, with the fact that the average quantity is the same for both? I can think of two possibilities:

(1) Rape. Men want quantity, women want quality, men get what they want because they are physically stronger. In other words it is true that most women want a relatively small number of quality relationships, but what they get is a larger number of forced relationships. No doubt this explanation would fit in nicely with the views of certain feminists, but I don't think it fits very well with the facts (at least not in civilized societies).

(2) Sluts. Most women want quality but some are total sluts who will fuck anything that moves. The very high number of relationships involving these women balances out the relatively low number for most women and so pulls up the average. Most women want, and have, fewer than the average number of relationships, while some want, and have, far more than the average. I don't know how well this explanation fits the facts, but even if it fit the facts very well, it would still leave us with the problem of explaining why most women want quality but some women want quantity, quantity, quantity.

Another possibility is that both men and women want multiple sexual partners, but they want different sorts of sexual relationships. There are good evolutionary reasons to think that women would want multiple sexual partners. If a woman has just one partner, and he is firing blanks, then she is missing reproductive opportunities. A woman may not have the looks, or social standing, to form a long term relationship with a genetic "good catch" but she might have the looks, or enough alcohol handy, to get a bit on the side. Finally, if there are a few men around who suspect that they may have fathered her children, then she may also get more support in raising them, or have a back-up if her main-man does a runner, dies, whatever.

Still it will be a priority for most women to get at least one man who is willing to stick around and invest considerable resources in supporting them through child birth and child rearing. Women will want at least one stable, long-term relationship, even if they also want many fleeting relationships. Men, on the other hand, might be just as happy with only fleeting relationships.