Here are the real reasons why some friendships end.

Conventional wisdom has led us to believe wechoose friends because of who they are. However, what attracts and binds friends, beginning with the shift from acquaintance to friend, is how often two people cross paths with each other. Typically, friends tend to be coworkers, classmates, and people you see on a regular basis. A 2005 studypublished in The Quarterly Journal of Economicsfound geographic proximity and race are greater determinants of social interaction than are common interests, majors, or family background. For example, in a college setting, the black and white student in the same freshman dorm increased the frequency of interaction compared to if they were randomly selected to interact with one another.

The shift from acquaintanceship to friendship is characterized by an increase in reciprocity. In other words, when a person takes the risk to disclose personal information, this serves as a test to see whether the other person reciprocates. Now, while we might know the glue that holds friendships together, what drives friendships apart?

Brett Laursen, professor and graduate studies coordinator in Florida Atlantic University’s Department of Psychology, sought to examine whether adolescent friendships end because of undesirable characteristics of friends, because of differences between friends, or both. A total of 410 adolescents involved in 573 friendships were observed in the six-year-long study. All friendships originated in the seventh grade, and researchers followed the participants from grade seven through grade 12. They collected data annually in the spring semester during a required English class.

The researchers measured the effect of both dissimilarities and undesirable individual attributes in determining whether a friendship would end. The findings revealed fewer than one in four friendships that started in the seventh grade were maintained across the next school year, and fewer than one in 10 friendships that started in the seventh grade survived the transition from middle school to high school. Only one percent of those friendships that started in the seventh grade continued to the 12th grade.

The strongest determinant of friendship termination were differences in sex or other-sex friendships. They were almost four times more likely to dissolve than same-sex friendships. Physical aggression, followed by differences in school competence, and differences in being liked by other children were also strong predictors of ending a friendship.

“We knew from previous studies that children prefer similar others as friends,” said Laursen, in the press release. “Now we know why. Dissimilarity is bad for friendships. It causes conflict, it interferes with cooperative activities and shared pleasures, and it creates circumstances where one friend bears more costs, such as the friend who is less aggressive; or gets more benefits, such as the friend who has lower social status than the other. Dissimilarity disrupts relationship bonds.”

This proves the old adage that birds of a feather really do flock together. A 2010 study published in the journal Human Nature found women who are roughly equal physical attractiveness tend to bond together in same-sex friendships. There was a positive association between the attractiveness ratings of women and their friends.

The downside of dissimilarities among friends can lead them to drift apart and outgrow each other. After all, the glue that holds friendships together is similarities such as common hobbies and interests.

Sources: Cillessen AHN, Hartl AC, Laursen B et al. A Survival Analysis of Adolescent Friendships: The Downside of Dissimilarity. Psychological Science. 2015.

Marmaros D and Sacerdote B. How Do Friendships Form? The Quarterly Journal of Economics. 2005.

Bleske-Rechek A and Lighthall M. Attractiveness and Rivalry in Women’s Friendships with Women.Human Nature. 2010.