Is beauty in the eye of the beholder? A new project aims to get to the bottom of this age-old adage. A collaboration of scientists from Boston, Massachusetts are working to uncover why we find certain people attractive- and others repulsive.

Much research has been done into what factors people find attractive, for instance, a study proved that people prefer symmetrical faces. However, few studies have focused on the reasons behind the attraction.

Certain elements of attraction are the result of physiological adaptation. That is to say, certain physical traits ignite the instinctive need of every living being to reproduce. These features are generally universal and are coded into our genes.

Yet that is only half of the story.

To determine other factors of desirability, the scientists launched an test on the website TestMyBrain.org. Over 35,000 volunteers assessed the site and rated different faces based on attractiveness. Among the participants were 547 pairs of identical twins and 214 pairs of same-sex, non-identical twins.

Each volunteer looked at photos of both men and women with only the faces showing- no hair, no clothes, no background. The scientists then ranked the variations in preference to create an individual preference profile for each volunteer. The researchers found that most people agree someone is attractive 50 percent of the time.

Next, the team set out to determine if the results were based on nature or nurture. This is where the twins came in.

The researchers compared the individual preference profiles of each pair of pair of twins to determine the rate of similarity. Finally, they compared the overall rate of similarity between the identical twin pairs and the fraternal twin pairs.

If the identical twin pairs had preferences that were more similar than those of the fraternal twin pairs, it would suggest that genes play a bigger role, said Laura Germine of Massachusetts General Hospital and Harvard University. But if the identical twins’ preferences were not more similar to each other, it would suggest the environment plays a larger role.

Standard calculations from this data show that a person’s individual environment accounts for 78 percent of the differences in the perception of attractiveness- even if they are twins.

“If you were to rate faces [for attractiveness] and I were to rate the same faces, we would agree about 50% of the time,” said co-author Jeremy Wilmer of Wellesley College.

“In our case, we found that even though identical twins share all of their genes and their family environment they were really, really different from each other in their facial aesthetic preferences.”

The scientists believe that personal experience is the cause behind this variation in allure. The ‘type’ of person someone is attracted to will tend to have similar facial qualities – eye shape, nose size, etc. According to the researchers, these differing preferences “provide a novel window into the evolution and architecture of the social brain.”

“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,” said Germine.

The study provides unique insights into the “evolution and architecture” of the social brain. The next step for the researchers is to try and find links between certain experiences and certain facial preferences.

The report was published in the journal Current Biology.