You like what you like (about yourself and others), and what makes you happy is ideal. But let’s not be naïve—there are definitely objective standards of beauty. No, not the standards dictated by magazines and celebrities. We’re talking about standards set by science and evolution.

QUIZ: What’s Your Body Type?

Prehistoric men and women made decisions about their mates without help from the media. They made judgments about partners based largely on appearance.The theory goes that a woman could improve her reproductive success if she chose someone deemed “high quality.” A woman would choose a man with the resources and strength to protect potential family members. Muscles signaled strength, meaning “reproductive hottie.”

When it came to the reproductive value of women, it was harder for a man to judge. There was no outward signal of ovulation and fertility; men had to use indirect cues (physical attractiveness) to judge. The main criterion was distribution of body fat, in scientific terms, a woman’s waist-to-hip (WHR) ratio. In other terms, body shape.

MORE: How Weight and WHR Relate to Health

Waist-to-Hip Ratio

To identify how the Western idea of female attractiveness has changed, researchers studied old and new paintings, actresses, Playboy centerfolds and Miss America contestants over the last century. They found that the waist-to-hip (WHR) ratio for Playboy centerfolds has increased from 0.68 to 0.71. Miss America contest winners have had a decrease, from 0.72 to 0.69. The average body weight of these women has decreased through the years, but they still remain fairly constant when it comes to WHR.

The shape you develop as an adult—we’re all basically the same shape as kids—is due to sex hormones. The big gender difference comes in the form of the classic apple and pear shapes (called gynoid and android).There are a few different variations of these shapes. Testosterone stimulates fat deposits in the belly and inhibits it in the thighs, which is why men are typically apples. Estrogen inhibits fat in the belly and stores it on the thighs, which is why women are typically pears.There’s also an evolutionary reason for this: Fat deposits on the thighs are almost exclusively used during late pregnancy and when a woman is nursing.

RELATED RESEARCH: Large Hips Are Heart Healthy

In eras when men and women all needed fat storage during times of famine, the fat deposits on thighs (as opposed to the belly deposits on men) helped women provide calories not just for themselves but for their offspring as well.An additional evolutionary advantage for women with pear-like body shapes: This shape remains constant whether viewed from the side, front or behind. A woman’s degree of femininity can be assessed easily and accurately by viewing her from behind.

VIDEO: Interpreting your Waist MeasurementResearchers have argued that the front view is more important (assuming that breast size cues reproductive value…though it’s been shown that breasts do not always match the reproductive capability of a woman).This might magnify the sexual attractiveness of women with the classic hourglass figure—shapely breasts and broad hips, set against a narrow waist. In the same vein, the waist is most altered by pregnancy. So, a high waist-to-hip ratio may mimic pregnancy, and thereby make a woman less sexually attractive— aka she’s off limits for men looking to mate.

MORE: 12 Decades of the Ideal Feminine Body