Scientific Method / Science & Exploration

The hourglass figure: built for beauty, but not practicality?

The perfect hourglass figure, with an ideal waist-to-hip ratio, may not be …

Plenty of women want to have a perfect hourglass figure, with a waist that is noticeably smaller than the hips. It's also the object of desire for lots of men—the average waist-to-hip ratio of Playboy centerfolds is an enviably low 0.68. Outside of sexual attraction, there is medical evidence that a small waist-to-hip ratio comes with greater fertility, higher estrogen levels, and reduced risks of cardiovascular diseases, diabetes, and breast cancer. Even a 0.1 increase in that ratio can cause a 30 percent drop in the probability of conception.


While most medical studies conclude that a 0.7 or lower waist-to-hip ratio has a lot of benefits, Elizabeth Cashdan, an anthropology professor at the University of Utah, found it curious that most women around the world have a larger ratio. If a low waist-to-hip ratio is so beneficial for health and fertility, why has selection not made this trait more prevalent? To address this issue, she examined data regarding women from 33 non-Western and four European populations. The results of her study is published in this month's issue of Current Anthropology.

Cashdan found that the average female waist-to-hip ratio is 0.8 or higher, even among the 18-24 year age group. She was able to discount the worldwide trend towards obesity as a cause. Removing overweight women from the calculation still yields an average ratio around 0.8. Plus, ethnic groups in Africa that have no obesity issues also have a ratio greater than 0.7.

What causes this trend? Is there a benefit to having a high waist-to-hip ratio? Cashdan proposes that hormones related to survival, stress, and competition are responsible. Women with larger waist-to-hip ratios have higher levels of cortisol, a hormone that responds to stress, and androgens such as testosterone. Increased androgen levels in women can lead to more muscle mass, physical strength, competitiveness, and assertiveness. Elevated cortisol levels help women mentally and physically respond to stress.

Thus, cortisol and androgens can be viewed as assisting women in competing for resources and coping with difficult circumstances. Unfortunately, this immediate survival fitness does come at a cost. Those hormones cause more fat to be deposited around the waist, where the fat is more metabolically active, and they increase the risk of experiencing health problems.

According to this study, the desire to have a 0.7 or smaller waist-to-hip ratio isn't compatible with the realities of a competitive and harsh environment. Cashadan points out that "rarely can a woman depend solely on an investing man," so, when you factor in the efforts required for independent economic success and political standing, "men's preferences are not the only thing that matters."

Current Anthropology, 2008. DOI: 10.1086/593036

The article has just been released, so the link will not work until it has been published online.

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