Evolutionary curveball for curvy?
Having something less than the classic "hourglass" figure may have its benefits after all.
While women with curvy figures might enjoy more attention from men in Western culture, and find it easier to become pregnant, new research suggests they may also face some evolutionary disadvantages compared to women with thicker waists.
That's because the same hormones that increase fat around the waist can also make women stronger, more assertive, and more resistant to stress, according to a new study published in the December issue of Current Anthropology.
Given those findings, it makes sense that the slim-waisted body has not evolved to become the universal norm, said the study's author, Elizabeth Cashdan, an anthropologist at the University of Utah.
Her study takes aim at a theory popular in evolutionary psychology and medicine: that men universally prefer women with narrow waists and larger hips because their higher levels of estrogen make them more likely to conceive a child, and less vulnerable to chronic diseases. These preferences, the theory goes, have defined women's ideal body shape over time.
The idea took root in the 1990s when psychologists showed men drawings of women's silhouettes and asked them which were most sexy. Researchers found that men gravitated toward images with a waist-to-hip ratio of 0.7 - in other words, with a waist about a third narrower than the hips. Those same hourglass proportions are reportedly shared by stars such as Marilyn Monroe and Jessica Alba, and linked in medical studies with a lower risk of heart disease.
The findings troubled feminists, and drew criticism from anthropologists who said researchers were generalizing about human evolution based on samples of young, mostly white men in industrialized societies. The debate endured for years.
"Many of us anthropologists have been in traditional hunter-gatherer societies and most of the women there don't look like that," said Cashdan. "So the question is, if it is adaptive to have that body shape, what's going on?"
In a review of data gathered from cultures as diverse as East African foragers and Chinese immigrants in Britain, Cashdan found that the average waist-to-hip ratio both within and across populations was higher than 0.7. In more egalitarian societies, where women played a greater role in the economy, they also tended to have thicker waists.
That suggests a genetic trade-off, with nature selecting for factors in addition to fertility and attractiveness. One possibility, Cashdan argues, is that extra doses of the stress hormone cortisol and male hormones known as androgens helped our hunter-gatherer foremothers cope in an environment where they had to sidestep poisonous snakes and went to sleep to the serenade of leopards growling. Those same belly-boosting hormones may even help modern women face stressful situations, she says.
So while men may choose mates based on their figures, Cashdan writes, whether they prefer shapes associated with higher or lower doses of male hormones "should depend on the degree to which they want their mates to be strong, tough, economically successful, and politically competitive."
While the study may boost the self-esteem of women with straighter figures, it does not provide convincing evidence that the benefits of a thicker waist outweigh the costs, say some specialists in the field.
"I sympathize with wanting to find some source of comfort but the medical community is quite strongly united in the view that a high waist-to-hip ratio, and especially a high amount of waist fat, is detrimental," said Dr. William Lassek, an epidemiologist at the University of Pittsburgh. Lassek co-wrote a recent paper indicating that women who store more fat on their hips and thighs than on their waist give birth to smarter babies, since fat in those areas contains fatty acids key to infant brain development.
Evolutionarily speaking, he said, "We believe that the most likely reason men prefer women with more pronounced hips has to do with the lower body fat storing those omega 3 fatty acids."
One of the challenges in analyzing body shape, Lassek said, is that researchers calculate waist size differently. While some measure a woman's waist at its smallest point, others use a specific place, such as halfway between the top of the hip and bottom of the rib cage. That can make it hard to make meaningful comparisons among cultural groups.
And the relationship between stress hormones and body shape is far from straightforward: while stress might change a woman's physique, having a negative body image can itself cause anxiety - and the stress hormones, or other physiological effects, that come with it.
If a bigger belly does make for a more resourceful mate, however, men may be catching on. Terry Pettijohn, a psychology professor at Coastal Carolina University in South Carolina, has studied the measurements of Playboy Playmates of the Year over the past four decades. He concluded that Playboy bunnies chosen during social and economic hard times tended to have more mature features, including higher - and thus less curvy - waist-to-hip ratios. Other studies of popular actresses have shown a similar pattern.
"It's important that people are aware that there is no set standard for what's attractive," said Pettijohn, "and there are a number of different factors that influence what we find attractive at different points in our lives and under different circumstances."
Felicia Mello can be reached at fmello@lmi.net. 