Many of us have had the experience of disagreeing with friends or family about which celebrity is more attractive. Now, researchers reporting in the Cell Press journal Current Biology on October 1 show in a study of twins that those differences of opinion are mostly the result of personal experiences that are unique to each individual. In other words, even identical twins don't agree.

Of course, some aspects of attractiveness are pretty universal and may even be coded into our genes, the researchers say. For example, people tend to prefer faces that are symmetric. Beyond such limited shared preferences, however, people really do have different "types."

"We estimate that an individual's aesthetic preferences for faces agree about 50 percent, and disagree about 50 percent, with others," write joint leaders of this project, Laura Germine of Massachusetts General Hospital and Harvard University and Jeremy Wilmer of Wellesley College. "This fits with the common intuition that on the one hand, fashion models can make a fortune with their good looks, while on the other hand, friends can endlessly debate about who is attractive and who is not."

Germine and Wilmer say that past research on the way people respond to faces has focused primarily on universal features of attraction. In the new study, however, they and their colleagues wanted to know more about where those disagreements over facial attractiveness come from.

To tackle this question, the researchers first studied the face preferences of over 35,000 volunteers who visited their science website http://www.TestMyBrain.org (http://www.testmybrain.org/setup.php?b=309); they used the insights gained to develop a highly efficient and effective test of the uniqueness of an individual's face preferences. They then tested the preferences of 547 pairs of identical twin and 214 pairs of same-sex, non-identical twins by having them rate the attractiveness of 200 faces.

Comparisons between identical and non-identical twins allowed the researchers to estimate the relative contribution of genes and environments to face preferences. Prior studies of twins and families have shown that virtually every human trait--from personality to ability to interests--is to some large degree genetically passed down from one generation to the next. Indeed, the researchers even found this in an earlier study for another aspect of face processing: the ability to recognize faces.

In contrast, they now show that the origin of the "eye of the beholder" --the uniqueness of an individual's face preferences--is mostly based on experiences, not genes. Those experiences, moreover, are highly specific to each individual.

"The types of environments that are important are not those that are shared by those who grow up in the same family, but are much more subtle and individual, potentially including things such as one's unique, highly personal experiences with friends or peers, as well as social and popular media," Germine says.

In other words, it's not about the school you went to, how much money your parents made, or who lived next door. That pretty face you see apparently has a lot more to do with those experiences that are truly unique to you: the faces you've seen in the media; the unique social interactions you have every day of your life; perhaps even the face of your first boyfriend or girlfriend.

The researchers say that the large impact of personal experience on individual face preferences "provides a novel window into the evolution and architecture of the social brain." They say that future studies could look more closely at which aspects of the environment are really most important in shaping our preferences for certain faces and for understanding where our preferences for other things--like art or music or pets--come from.