Monday, December 2, 2013

Men's Preference for Younger Women Leads to Menopause

“The appealing thing about younger women is their outlook on life. They tend to be untainted by experiences that have hardened older women. For example, when a woman’s been lied to a lot after years of dating, she always thinks that you’re lying to her, too. And that’s a turn-off. Younger women are less cynical, and that’s a big draw.”
— Alan, 52, New York, NY 

“When a guy reaches a certain age, he worries he will see his youth and vitality wane. A younger woman reaffirms for me that I’ve still got it going on.”
— Mike, 48, Orlando, FL 

“One advantage of dating a younger woman is that you can play ‘cruise director’ — i.e., show her all your secret favorite places that she probably hasn’t experienced yet. They’re easier to impress and very willing to be escorted around. Women like it when you have a plan and it’s well-executed.”
— Bill, 39, New York, NY

From Match.Com

After decades of laboring under other theories that never seemed to add up, a team led by biologist Rama Singh has concluded that what causes menopause in women is men.

Singh, an evolutionary geneticist, backed by computer models developed by colleagues Jonathan Stone and Richard Morton, has determined that menopause is actually an unintended outcome of natural selection -- the result of its effects having become relaxed in older women.

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For thousands of years (at least), men have, on average, mated with younger women, Singh said. That's because, if all else is equal, "those who reproduce earlier, their genes are passed on faster”. Early on, both men and women in the model reproduced until death. But over time, the model found, men's preference for youth reduced older women's odds of reproducing.

Simultaneously, people accumulated random mutations, some of which decreased later-life reproductive ability. But since older women were left out in the cold anyways, those mutations didn't impact their reproductive success, whereas mutations in men that could reduce late-life reproduction were weeded out. (Men who stopped reproducing at some point in life would produce fewer offspring than those who didn't, and the late reproducers would outcompete those who stopped breeding earlier.)

Over 50,000 to 100,000 years, the accumulation of all those mutations could have led to universal menopause, the researchers suggest. Menopause would then be another form of aging akin to graying hair or wrinkles.

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If later childbearing becomes the norm, as current societal trends suggest, women who can reproduce at older ages might gain an evolutionary advantage, and menopause could, in theory be pushed later, Singh said. But it's more plausible that technological changes such as fertility treatments will artificially extend women's ability to reproduce, Singh said.

So, while conventional thinking has held that menopause prevents older women from continuing to reproduce, in fact, the researchers' new theory says it is the lack of reproduction that has given rise to menopause.

Menopause is believed to be unique to humans, but no one had yet been able to offer a satisfactory explanation for why it occurs, Singh says. The prevailing "grandmother theory" holds that women have evolved to become infertile after a certain age to allow them to assist with rearing grandchildren, thus improving the survival of kin. Singh says that does not add up from an evolutionary perspective. "How do you evolve infertility? It is contrary to the whole notion of natural selection. Natural selection selects for fertility, for reproduction -- not for stopping it," he says.

The new theory holds that, over time, competition among men of all ages for younger mates has left older females with much less chance of reproducing. The forces of natural selection, Singh says, are concerned only with the survival of the species through individual fitness, so they protect fertility in women while they are most likely to reproduce. After that period, natural selection ceases to quell the genetic mutations that ultimately bring on menopause, leaving women not only infertile, but also vulnerable to a host of health problems.

"This theory says that natural selection doesn't have to do anything," Singh says. "If women were reproducing all along, and there were no preference against older women, women would be reproducing like men are for their whole lives." The development of menopause, then, was not a change that improved the survival of the species, but one that merely recognized that fertility did not serve any ongoing purpose beyond a certain age.

For the vast majority of other animals, fertility continues until death, Singh explains, but women continue to live past their fertility because men remain fertile throughout their lives, and longevity is not inherited by gender. Singh points out that if women had historically been the ones to select younger mates, the situation would have been reversed, with men losing fertility.

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University of Utah anthropologist Kristen Hawkes, grandmother hypothesis supporter, whose work deals with the evolutionary origins of menopause, points out that great apes, with whom we share a great deal of evolutionary history, exhibit a similar pattern of losing fertility in their mid-40s. The difference is that while the apes' fertility rates seem to be in sync with those of humans, their longevity rates aren't. "They usually die before they get to those post-fertile years," she says. "They get to be old ladies, gray and frail, while they're still cycling."

But Hawkes acknowledges the human male's apparent difference in taste. "I agree the preference men have for young partners is a striking contrast with other primates — especially since it is well-documented that chimpanzee males prefer older females," she says.

There are other experts, who are skeptical of this theory as well. “I know we like to blame men for a lot of things, but I don’t think this can be one of them,” Dr. Rebecca Brightman, a clinical instructor of Obstetrics, Gynecology and Reproductive Science at the Icahn School of Medicine at Mount Sinai Hospital, who wasn’t involved in the study. Brightman said there are some Darwinian aspects of menopause, especially the high risks involved for both mother and child when older women become pregnant. “Pregnancy takes a toll on a woman’s body,” she said. “There’s an evolutionary advantage to being incapable of having children later in life. Mothers want to be around to care for their children.”

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