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.
Men and women possessing these traits are seen as attractive, Thornhill said, because they advertise reproductive health.
Thornhill also points to the booming nip-‘n'-tuck business—which is very much about improving a person's symmetry—as evidence that people find the quality attractive.
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.
"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 rules of attraction, it turns out, seem sometimes to play out in our subconscious.
In some cases, women in Thornhill's study reported not smelling anything on a shirt, yet still said they were attracted to it.
Estrogen caps bone growth in a woman's lower face and chin, making them relatively small and short, as well as the brow, allowing for her eyes to appear prominent, Thornhill explained.