In the nightclub, some people select whom they approach based on the person's body. Others might wait to see how they look on the dance floor.
But, according to a new study, they're both checking out the same thing.
Body symmetry has a major effect on perceived attractiveness, and that symmetry can show itself in features ranging from body size and facial attractiveness to dancing ability, the study concludes. Over time, humans may have evolved in ways to figure out who we choose to pursue as a partner.
"Symmetry reflects good development," said lead study author William Brown, co-director of the Centre for Culture and Evolutionary Psychology at Brunel University in the United Kingdom.
"In animals with two sides that were designed by natural selection to be symmetrical, subtle departures from perfect symmetry may reflect poor development or exposure to stress," he said. "In many species, the degree of departure from perfect symmetry is related to poor health, lower survival and fewer offspring."
Researchers did full-body scans of 40 men and 37 women, using a 3-D optical body scanner. Removing the heads from the images, they had 87 evaluators judge the attractiveness of the bodies.
The investigators found that bodies with higher levels of asymmetry were judged less attractive by the evaluators.
While the asymmetry in the bodies may have been invisible to the naked eye, the researchers speculate that humans may be conditioned to detect these subtleties to determine the fitness of potential mates through more obvious signs -- like dance moves, face and body size.
"More obvious signals are what animals easily detect and are, thus, conspicuous indicators of developmental quality," Brown said. "Thus, it is not necessary for animals to perceive these subtle asymmetries."
He said that asymmetries also tend to have a large impact on a person's attractiveness.
"Previous studies have shown that symmetrical males have more masculine faces," Brown said. "Males with more symmetrical bodies are faster runners, better dancers, have more pleasing body odor, more attractive faces and more attractive voices."
Overall, he explained, men with more symmetrical bodies also had features, like taller stature and broader shoulders, while more symmetrical women had smaller torsos and a "curvy, hourglass-like shape."
These features, markers of genetic fitness, made them more appealing to potential mates.
But whether these findings, published in the Proceedings of the National Academy of Sciences, have any major implications for the dating scene remains to be seen.
"There are contradictory findings, but in my opinion, no more than any other area of scientific study," Brown said. "Nonetheless, in theory at least, symmetrical organisms are predicted to be better able to buffer the stressors of life, and thus be a higher quality mate choice, provided that mate quality helps offspring survival."
In the Brain of the Beholder
What compromises beauty seems to be a topic of great debate, even among those who look at pretty faces for a living.
More than a year ago, ABCNews.com asked photographers what made for the perfect face, without receiving any clear consensus. It seems scientists disagree on that issue just as much.
While Nigel Parry said, "There shouldn't be perfection on both sides ... the symmetry should always be off," fellow photographer Mark Robert Halper said that "we perceive symmetrical faces as prettier."
Neuroscientist Dahlia Zaidel of UCLA said that tying asymmetries to fitness, as this new study does, is to generalize an idea.
"In humans, there are too many natural asymmetries to assume ideal fitness," she said. "Perhaps, though, fitness in humans means normal presence of natural asymmetries."
Zaidel set out to try and resolve the question of facial symmetry's role in attractiveness several years ago, when she digitally cut faces in half and copied and reversed the image to create faces that were perfectly symmetrical.
"When I looked at such perfection, I found that viewers consider such faces to be unattractive," she said. "They were found significantly less beautiful than the original faces that gave rise to them."
Zaidel explained that these results are ingrained in us from birth.
"Even babies originally smile asymmetrically," she said. "From birth, we are used to looking at faces and bodies that are not symmetrical."
In the case of the study, Zaidel said that differences that make someone significantly less fit fall into the realm of deformities.
More research is needed to determine precisely what constitutes a deformity, she said, and "people don't know the answer, but I imagine that there is a continuum."
Zaidel noted that a number of bodily features are asymmetric on almost everyone, including feet, where one shoe invariably fits better than the other, and women's breasts.
She said that women are slightly more asymmetrical than men, which often leads them to move their heads more when they speak.
Men do this less, she said, "and, of course, it drives women crazy," because they may think that the men are not engaged with what they are saying.
Ultimately, Zaidel said, trying to find perfect symmetry in people is futile in the first place.
"Nobody is perfectly, perfectly symmetrical," she said. "Even the most beautiful women, the most handsome men, don't have perfectly symmetrical faces or bodies."