The Rules of Attraction

To figure out how we pick mates, scientists have measured every shape and angle of the human face, studied the symmetry of dancers, crafted formulas from the measurements of Playboy models, and had both men and women rank attractiveness based on smelling armpit sweat. After all this and more, the rules of attraction for the human species are still not clearly understood. How it all factors into true love is even more mysterious.

But a short list of scientific rules for the game of love is emerging. Some are as clearly defined as the prominent, feminine eyes of a supermodel or the desirable hips of a well-built man. Other rules work at the subconscious level, motivating us to action for evolutionary reasons that are tucked inside clouds of infatuation.

In the end, lasting love depends at least as much on behavior as biology. But the first moves are made before you're even born.

Symmetry equals sex

Starting at conception, the human body develops by neatly splitting cells. If every division were to go perfectly, the result would be a baby whose left and right sides are mirror images. But nature doesn't work that way. Genetic mutations and environmental pressures skew symmetry, and the results have lifelong implications.

Good symmetry shows that an individual has the genetic goods to survive development, is healthy, and is a good and fertile choice for mating.

"It makes sense to use symmetry variation in mate choice," said evolutionary biologist Randy Thornhill of the University of New Mexico. "If you choose a perfectly symmetrical partner and reproduce with them, your offspring will have a better chance of being symmetric and able to deal with perturbations."

Thornhill has been studying symmetry for 15 years and scanned faces and bodies into computers to determine symmetry ratios. Both men and women rated symmetrical members of the opposite sex as more attractive and in better health than their less symmetrical counterparts. The differences can be just a few percent—perceivable though not necessarily noticeable.

By questioning the study participants, Thornhill also found that men with higher degrees of symmetry enjoy more sexual partners than men of lower symmetry.

"Women's sex-partner numbers are dependent on things other than attractiveness," Thornhill told LiveScience. "Because of the way that the sexual system in humans works, women are choosey. They are being sexually competed for. They have to be wooed and all that."

Body shape is of course important, too. And scientists have some numbers to prove it. Psychologist Devendra Singh of the University of Texas studied people's waist-to-hip ratio (WHR).

Women with a WHR of 0.7—indicating a waist significantly narrower than the hips—are most desirable to men.

And an analysis of hourglass figures of Playboy models and Miss America contestants showed that the majority of these women boast a WHR of 0.7 or lower.

In general, a range of 0.67 to 1.18 in females is attractive to men, Singh concluded in a 2004 study, while a 0.8 to 1.0 WHR in men is attractive to women, although having broad shoulders is more of a turn-on.

What exactly is encoded in the hip ratio? A big fat clue to whether the person will have enough energy to care for offspring.

Where fat is deposited on the body is determined by sex hormones; testosterone in men and estrogen in women. If a woman produces the proper amount and mixture of estrogen, then her WHR will naturally fall into the desired range. The same goes for a male's testosterone.

People in the ideal hip-ratio range, regardless of weight, are less susceptible to disease such as cardiovascular disorders, cancer, and diabetes, studies have shown. Women in this range also have less difficulty conceiving.

"The idea is that beauty is conveying information about health and fertility, and we admire that," Singh said in a telephone interview.