It's a question that's been pondered by philosophers for thousands of years, but only recently have researchers set out to quantify the answer: Is beauty in the eye of the beholder?
A study out today in Cell Biology offers insight into two aspects of that question: first, it finds that we tend to agree on which faces are attractive about 50 percent of the time. Second, the reasons we disagree can be attributed to our genes about 22 percent of the time - and not at all to growing up in the same household.
"So when you're watching TV and Brad Pitt comes on the screen and one person says, ‘Oh, he's a 7 out of 7, and the other says, no, he's only a 5,' most of that discrepancy is largely a mystery, said study co-author Jeremy Wilmer, Assistant Professor of Psychology at Wellesley College.
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Because the researchers found that even identical twins varied in their ratings of faces, they think that unique individual experiences largely account for our facial preferences.
"So every time we talk to somebody we meet on the street, or see people out in the world, we tend to look at faces – and even identical twins don't share that," Wilmer said. "That's the most plausible explanation."
It could be that thousands of random meetings shape our perceptions, or that intimate relationships make more lasting impressions on our preferences, the researchers said. Other possibilities include slight variations in the womb after the zygote splits (one twin gets more nourishment than the other, for example).
"In principle, it could potentially include unique individual experiences when growing up that aren't shared with twins - e.g. potentially, falling in love with a particular person during adolescence and ‘imprinting' on their face as an ideal," said Geoffrey MIller, an associate professor of psychology at the University of New Mexico who studies mate choice, who wasn't involved in the current study.
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"But ‘nonshared environment' can also include random biological noise - unpredictable aspects of brain development in the womb, in childhood, in adolescence, or adulthood that can't be traced to either genetic influences or family environment."
Although it may seem surprising that growing up with the same parents in the same household has no impact on facial preference, it's not as uncommon as most people think.
"A lot of interesting psychological traits ( such as IQ) show 0 percent shared family environment effect, which is really surprising to most people," said Miller, co-author of the book "Mate."
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What's more unusual, perhaps, is the researchers' finding that genes account for so little of our preferences. Genes influence personality, interests, religion, even our ability to recognize faces ... but very little, apparently, of what we find attractive in faces.
To parse out how and why we differ, the researchers set up an online test (find a sample here) and recruited 35,000 people to rate faces on a scale of 1 to 7. Then, they asked 547 pairs of identical twin and 214 pairs of same-sex, non-identical twins from the Australian Twin Registry to rate 200 images. To ensure accuracy, the researchers asked participants to take the test twice, three weeks apart, checking to make sure ratings remained consistent.
Up next, then, could be a look into what exactly those other influences are. Until then, you'll have to agree to disagree about Brad Pitt.