But other researchers believe that initial attraction isn’t about symmetry at all. Jason Weeden, an evolutionary psychologist at the University of Pennsylvania, found that men are more attracted to a specific body-mass index (BMI) and waist-to-hip ratio (WHR), both of which signal fertility.

BMI is calculated by weight in kilograms divided by the square of height in meters.

A healthy BMI is between 20 and 25, said Weeden.

“Women tend to be more attractive around 20…men tend to be more attractive around 25,” he said.

WHR is calculated by measuring waist circumference divided by hip circumference.

For women, the normal range of WHR is from 0.7 to 0.9, but 0.7 is considered most attractive in Western women, said Weeden. Both Marilyn Monroe and Kate Moss have a WHR of 0.7, despite their drastically different sizes. Proportion, apparently, is more critical than BMI in attraction.

While weight and body proportions are not the same thing as symmetry, all three indicate good physical and reproductive health. 

As women fall below the normal BMI range they are considered less attractive and are also in danger of infertility.

“They lose their reproductive capacity – it’s called amenorrhea,” said Dr. Kerry Page, an assistant clinical professor in the Department of Medicine at Columbia. “These women stop menstruation, so they are not fertile.”

Likewise, a BMI above 27 in women can cause serious health problems, such as Type 2 diabetes, high blood pressure and heart disease.

“There are reproductive consequences of high BMI,” said Page. Obesity can lead to polycystic ovarian syndrome, when an ovary has multiple cysts, which can result in infertility, she said.

Like BMI, WHR not only determines how attractive a woman is, it also indicates good reproductive health.

“What a low waist-to-hip ratio tells you is that the woman has a very feminine profile,” said Weeden, “one that’s conducive to pregnancy.”

The feminine profile is what makes fertile women curvy in the first place. The balance of estrogen in