In the study, 113 participants were asked to rate 74 faces on a scale from one to seven, from very attractive to very unattractive. Among the participants were 20 pairs of spouses, 20 pairs of siblings and 41 pairs of close friends. Each of the pairs completed the test separately, so that they could not influence each other’s ratings. The participants ranged widely in age, but were of a similar background, and were all North American and caucasian. The faces rated were all young and caucasian.

Participants who were part of a pair of close relations were also paired with another individual who they had not met, in order to form a pair of strangers. In analyzing the ratings, the researchers found that while the strangers’ ratings of the faces were often similar, which was consistent with previous findings, the ratings of the spouses, siblings and close friends were markedly more in agreement.

Previous research has shown that while there are cross-cultural standards of beauty, there is greater agreement about facial beauty within cultures. This study narrows the focus of preferences for beauty within even smaller groups: individuals who know each other well and have personal relationships.

The researchers theorized that this greater agreement among close relations could stem from several different causes. Interestingly, the number of years that the pairs of people spent in daily contact was related to the strength of their agreement on facial attractiveness. This could be because those individuals who spent a great deal amount of time together saw many of the same faces on a day-to-day basis.

“Because close relations know and see many of the same people, their visual ‘diet’ of faces has been similar. It’s likely that repeated visual exposure to the same faces could have an effect on their perception of what makes a face attractive,” says Bronstad.

Further research will explore the possibility that attractiveness preferences are genetically determined. However, the siblings’ ratings of the faces were not more closely correlated than those of the spouses or the close friends, which suggests that genetics is not the sole cause of facial attractiveness preferences.