Hourglass-shaped women, who have more fat deposited on the buttocks and hips than on the waist, have higher levels of the female hormone estrogen than the male hormone testosterone. This makes them less prone to infertility and adult-onset diabetes, and, therefore, healthier mates.

Chart of thick-waisted (U9 - O9) and hourglass-shaped (U7 - O7) women shown to Matsigenka men.

Though modern culture inspires the search for a mate with social status or financial wealth, our instincts are still inspiring a "survival of the fittest," according to Etcoff’s theory.

But Yu and Shepard challenge this axiom. Every culture tested so far for the hourglass preference "has been exposed to the potentially confounding influence of western media," according to their study.

Men from the isolated Yomybato village in Manu Park, Peru prefer women with "tubular" figures, or a low waist-to-hip ratio, according to Yu and Shepard’s study.