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The scientific rules for the game of love
Researchers study the sights, smells and brain scans behind attraction
 | Which face is more attractive? Researchers at the University of St. Andrews created composite images of women with high levels of estrogen, at left, and low estrogen levels, at right. They found that men tended to rate women with higher levels of estrogen as more attractive, healthier and more feminine-looking than those with lower levels. |
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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, a process with 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 facial symmetry for 15 years. He scanned faces and bodies into computers to determine symmetry ratios. Both men and women rated symmetrical members of the opposite sex more attractive, and they also inferred them as being in good health. The differences can be just a few percent — perceivable, though not necessarily consciously 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 choosy. They are being sexually competed for. They have to be wooed and all that."
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