The waist:hip ratio (WHR)—waist width divided by hip width. WHR is recognized as a key indicator of female attractiveness reflecting fat distribution. Typical healthy ranges are 0.67-0.80 for premenopausal women.
Around the globe, across cultures, and throughout history, one physical feature has stood out as consistently correlated with attractiveness in females: waist-hip-ratio (WHR). The desirability of a low WHR often cuts across other preferences in body size and shape both among individuals and in cultural predominance. Interestingly, the preference for a low waist-hip-ratio (WHR) is not purely sexual. When studies have asked participants to rate female bodies on attractiveness, both heterosexual women and homosexual men mostly mirror their heterosexual male counterparts in identifying a low WHR as attractive, even when they are divergent in how they rate other physical features. All of this speaks to some kind of ingrained preference or instinct, but why would that be?
Many authors claim natural selection created men’s preferences to optimize mate choice. Combined with the conviction that female body shape signals health and fertility, this amounts to claiming that beauty resides in the genes of the beholder. Karl Grammer and colleagues boldly stated this notion in 2003: “…particular proportions of waists and hips are only considered to be beautiful because our ancestors with such preferences left more healthy offspring than the individuals in the population without the preferences.”
Most theories have focused on WHR as an indicator of health and fertility. Indeed, in both males and females, a low WHR does correlate reasonably well with cardiovascular health and longevity, while a high WHR puts us at greater risk for diabetes and even autoimmune diseases and cancer. For both men and women, WHR is a better measure of health than weight alone, BMI, or physical fitness.
A crucial part of the attractiveness theory is that the low WHR has to be indicative of something related to fertility, or else it wouldn’t have any evolutionary value. The key feature in a potential mate is biological fitness, that is, the potential to give birth to many healthy and successful offspring.
Led by Piotr Sorokowski at the University of Wroclaw in Poland, the researchers measured the WHR of nearly 1,000 women and plotted those measures against reproductive history. In their sample were women ranging in age from 13 to 95. They utilized both age and body-mass index (BMI) as control variables as they attempted to focus solely on fertility and WHR.
The results are dramatic. Both BMI and age separately correlated with the number of children that the women had. No surprise there. However, even when age and BMI are normalized, the researchers observed a subtle, but direct linear relationship between waist-hip ratio and number of children.
To understand the data, we must invert the relationship. Rather than waist-hip ratio being purely predictive of future fertility, it may also be reflective of past fertility. In other words, other things being equal, a lower WHR in these women may indicate that they have not had many (or any) children. This could signal to a potential mate that a female’s days of reproductive investment are ahead of her, not behind her.
Another way to say this is that women transition from low to high WHR as they give birth to more children, a conclusion supported by this study and others.
Thus, the attractiveness of a low WHR could have evolved as a means of helping men detect and favor those with greater future reproductive potential, not just fertility in general. As Kasia Pisanski, the vocal experiment who discovered the connection between vocal features and WHR, told me, “I find [that argument] convincing, that a high WHR could indicate that a woman already has children (men don’t want to invest resources in another man’s children).”
This interpretation especially makes sense if we consider that a low WHR does indeed correlate well with potential fertility in nulliparous (childless) women.
So, rather than contradicting the prevailing thinking about why we are attracted to a low WHR, this result strengthens and extends the attractiveness theory. In general, this attraction could have been favored by evolution because a low WHR is a good indicator of the kind of hormonal balance that supports health and reproductive potential. It is also a reliable indicator that a woman has not already had a large number of children.
Waist-hip-ratio is a fascinating phenomenon in evolutionary psychology. If this physical feature truly correlates with future reproductive potential, it makes sense that we would have evolved to find it attractive.
Researchers claimed evolution favored “pendulous” breasts — as scientists dryly describe them — because they gave babies something to cling to like handle bars.
Others suggested permanent breasts evolved as an energy reserve, lumps of fat to be tapped when food is scarce. From this perspective, women’s breasts may have originally functioned liked camel’s humps. Whereas in cold climates, animals maintain fat throughout their bodies for energy storage and insulation, desert-dwelling camels sequester fat in their humps to avoid overheating. Since our ancestors evolved in equatorial Africa, perhaps females stored fat similarly, in their breasts, butts and hips. Males wouldn’t have the same pressure to do so, since they don’t nurse or carry children in the womb.
Whether or not breasts first served as energy banks or handle bars for mothers and infants, they eventually caught the attention of males. This is why most researchers assume sexual selection has been at work: They argue that over the course of human evolution, permanent breasts helped females attract mates. Like the ostentatious feathers of male peacocks, breasts served as a message to the opposite sex, “Hey, look at these! Reproduce with me!”
And in the case of peacock feathers, and potentially breasts, there’s truth in advertising. The more spectacular the feathers, the healthier the bird, and therefore this trait is what evolutionary biologists call an honest signal: a reliable indicator of an animal’s quality as a mate or parent. It’s a feature potential suitors can assess before deciding, “Oh yeah, I want to make babies with this individual.” As for breasts, some hypotheses in the sexual selection camp contend the fat mounds are an honest signal of a woman’s mate-worthiness.
If the honest signal hypothesis is true, then some breast qualities, like size or shape, should correspond to a woman’s motherly potential.
In Western populations, some studies have shown that most men prefer women with narrow waists and large breasts.
The researchers measured two female sex hormones — progesterone and a form of estrogen (E2) — from spit samples, collected from 119 healthy Polish women every morning for the duration of one menstrual cycle, or roughly one month. Not considering waists, breast size alone related only to E2 levels. Because estrogen plays a crucial role in girls developing their womanly figures, this may suggest that breasts are just a side effect of gaining healthy voluptuous fat overall.
But the study also found that women with large breasts and narrow waists had higher values for both hormones — and the combo of these hormones predicts pregnancy success. This led the researchers to conclude that at least among these Polish women, “the cultural icon of Barbie as a symbol of female beauty seems to have some biological grounding.” Keep in mind though, successful pregnancy is not the same thing as raising a healthy child to adulthood. The hormone measures do not indicate a participant’s overall quality as a mother, just her chances of getting knocked up and giving birth.
Men’s preferences seem influenced by number of factors, including: culture (says one study, “Brazilians preferred larger breasts and buttocks than Czechs”), socio-economic status (larger breasts for lower status men), sexual habits, (larger for men in short-term, non-committal relationships), body image, (larger for men who rated themselves more attractive), sexist attitudes (larger for men hostile toward women) and even hunger (hungry British men liked bigger breasts than fed participants).