From Rita Hayworth and Marilyn Monroe to Kate Upton and Kim Kardashian, the hourglass figure shows little sign of going out of fashion. It’s hardly a surprise when women whose waists are narrower than their hips — who have a “low waist-to-hip ratio” — are consistently judged by men as most attractive.

But why? Are men’s preferences arbitrary, or does the hourglass figure have some underlying significance?

Scientists have theorized that a low waist-to-hip ratio is associated with heightened fertility. That is, women with an hourglass figure are more likely to be able to conceive a child. The idea is that men who are drawn to women with an hourglass figure will have more children. These men would enjoy an evolutionary advantage over their rivals who pair up with women regardless of body shape.

The problem with this theory is that the most attractive body shape might not be the most fertile.

A fertile figure?

William Lassek and Steven Gaulin, anthropologists from UC Santa Barbara, have reviewed the research on body shape, , and fertility. As well as waist-to-hip ratio, they looked at the impact of body mass index (or BMI).

Men tend to prefer women with very low waist-to-hip ratios (whose waists are much narrower than their hips), but these women are actually less likely to conceive than women with a less pronounced hourglass figure. Younger women are more likely to have an hourglass figure, and age is related to fertility, but the most attractive waist-to-hip ratios are generally found in women in their late teens, whereas women’s fertility tends to peak in their mid to late twenties.

Women with lower BMIs tend to be more attractive to men living in developed countries, although a larger body is preferred in populations where food is more scarce (across 58 cultures, around four-fifths of men prefer a woman who is “plump or fat”). Lower BMIs are associated with reduced fertility and skinnier women more often bear infants with a low birth weight.

Lassek and Gaulin dug into the data of a National Health and Examination Survey, a large American study conducted in the early 1970s. They found that women with a higher BMI in their late teens ended up bearing more children than women whose BMI had been lower: heavy women bore more children.

Next, the anthropologists looked at US data on births in 2013: over three million of them. A mother whose pre- BMI was high tended to have a child with a higher birth-weight.

Both findings remained true even after the scientists accounted for the effects of numerous possibly confounding variables, such as age, , race, and behavior.

Future fertility

If an hourglass figure is such a poor indicator of a woman’s fertility, why do men (at least in the West) prefer this body shape? Lassek and Gaulin speculate that it could be because BMI and waist-to-hip ratio are good indicators of a woman’s age. Women with a curvy figure are more likely to be post-pubertal but still relatively young. Lassek and Gaulin say:

Even though these younger women have reduced current fecundability and a decreased likelihood of infant survival, they have maximal long-term reproductive potential.

In other words, younger women with the most attractive body shapes may be less likely to conceive and to bear infants with a healthy body weight, but their whole reproductive lifespan is ahead of them. Men may find these women attractive in the here and now because they are unconsciously motivated by a desire to pair up for the long-term.