The film Friends with Benefits, an American rom com starring Mila Kunis and Justin Timberlake, starts with the premise that we are now at a point in time where women and men can enjoy casual sex equally. But the film ends up telling us what science already knows: however powerful the sexual and feminist revolutions may have been, the facts remain that statistically speaking girls just can't enjoy casual sex as much as guys.

As part of a research project later published in Human Nature, 1743 men and women who'd had a one-night stand rated their feelings about it the next morning. 80% of men felt great about it but only 54% of women did.

For women, the problem wasn't a sex drive issue: on the contrary, between days 10 to 18 of their hormonal cycle, women reported high sexual desire with a preference for short-term partners.
For women, the problem also wasn't (as is commonly believed), that they were disappointed when they discovered that the casual sex wasn't a prelude to long-term relationships. They knew they were taking part in casual sex but they still reported feeling used, like they'd let themselves down and worried about their reputation.

For women, the problem was the feeling that the man didn't appreciate her and that he didn't express much gratitude for the experience which made her feel like he thought she did this with anyone.

Perhaps the conclusion that men enjoy the morning after more than women do isn't particularly surprising. Maybe it's because men and women can't help but conform to a basic difference in nature, or maybe it's because the feminist revolution still hasn't come far enough. But what is surprising is the psychology, the apparent difference that such a subtle expression of appreciation from the man to the woman would make to the way the woman felt about the experience. It seems that manners really do count-outside and inside the bedroom.

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