For decades, anthropologists and biologists have been puzzling over one of nature’s great mysteries: Why do women go into menopause?

At first blush, menopause doesn’t make much evolutionary sense. According to one widely accepted theory, the point of evolution is successful reproduction. After that, nature doesn’t have much interest in keeping us alive, and so we die. Why, then, keep women kicking for decades after they can no longer make babies?

Rama Singh, a professor in the department of biology at McMaster University in Hamilton, Ontario, thinks he’s figured it out. Menopause, the Canadian researcher argues, is men’s fault.

In a paper published today in PLOS Computational Biology, Singh and two colleagues detail elaborate evolutionary computer models demonstrating that “male mating preference for younger females” leads to the development of a long menopausal period.

“I am saying what women have been saying all their lives,” Singh told NBCNews.com. “Men are to blame.”

Singh’s argument relies on one key fact: “Sex is fun,” he says.

In humans, sex isn’t just about making babies, it’s also about pleasure and bonding. Over time, he contends, men found that having that sex with younger women was more desirable than sex with older women, without regard to making babies.

As men began to prize younger women as sex partners, gene variants that led to infertility with advancing age were not eliminated. Menopause became built in, even as women lived longer and longer.