After a late night of getting hot and heavy, my friend Calypso quietly got dressed and snuck out of the apartment of the guy she had met the previous night, only to run into three of his fratty housemates watching TV and drinking bottles of Busch Lite with breakfast. Cringing, she imagined the YEAH MAN!s and high fives that would be exchanged between her boy and his friends as soon as she left. She, on the other hand, told only me and another friend, with a giggle and an eye roll, after making us promise that we wouldn’t tell anybody because she didn’t want it to “get around.”

Calypso’s situation illustrates what is probably the most oft-cited male/female double standard; guys are encouraged to fool around with anyone and everyone—the more the better—while when girls are promiscuous, they are assumed to have self-esteem or “daddy” issues. Girls may fawn over players, pimps, and bad boys, but a girl who has more than one sexual partner will more likely than not be called a slut.

As much as girls bitch and moan about double standards, it turns out that this one is more than just simply another manifestation of gender inequality; it actually is rooted in evolutionary psychology. On our small, gossipy campus, women must be careful to be choosy or otherwise risk being known as “easy.” But before the days of birth control, women had to be highly selective with the men that they chose to sleep with because, unlike men, they are far more limited in the number of children they can bear and raise in a lifetime. Men can theoretically father a nearly unlimited number of children. In order for women to ensure their own reproductive success and increase their Darwinian fitness (measured by the ability to pass your genes on to the next generation), their best bet was to have sex only with men who are financially secure or have other traits predictive of future success, such as intelligence, ambition, and stability. In contrast, the easiest way for a man to ensure his reproductive success and increase his fitness was to inseminate as many women as possible. And while women are always sure that their babies are indeed their babies, in the days before paternity tests, men did not always enjoy the same peace of mind. Thus, men evolved to favor women who were perceived to be chaste as opposed to those known to be sexually promiscuous.

Taken together, the different ways in which men and women ensure reproductive success and the issue of paternal uncertainty have caused men to prefer chaste women while simultaneously trying to spread their sperm as widely as possible, while women have evolved to be choosy and only mate with men who are able to provide the resources to support them and their future children. And although the advent of paternity tests and contraceptives (hallelujah!) have made both of these factors irrelevant, men and women are still hardwired the same as always.

In 1989, a team of researchers led by David Buss set out to test this theory. They surveyed over 10,000 men and women from 37 cultures across the globe. In the vast majority of countries, it was important to men that women have “no previous experience in sexual intercourse,” whereas men who were “ambitious and industrious” and who had “good financial prospects” were ranked more highly by women. Additionally, men consistently care more about the physical appearance of a prospective partner than do women, tending to prefer women with smooth skin, fuller lips, shiny hair, and other features which suggest that a woman is youthful and, thus, fertile. Buss also found that on average men prefer women who are 2.7 years younger, and women prefer men who are 3.4 years older than them, which again shows that men want young partners who are more likely to be fertile, while women prefer older partners who are more likely to have financial resources.

In the end, Calypso’s story ended up making the rounds; we do go to Tufts, after all. In response to her desperate pleas for advice on how to combat the wild rumors that were spreading (hot sauce where?!), I told her that her best bet was to spread the word that paternity tests and birth control had made the evolutionary origins of the dreaded double-standard irrelevant. After all, a woman can now sleep with a man without considering whether or not he has the resources to raise her children should he impregnate her. And if a guy has doubts as to whether or not he is really the baby’s daddy, he can order a simple paternity test. Unfortunately, Calypso wasn’t amused, and my friends and I had to cut another fraternity from our social scene.