In the case of friends, opposites don't attract. 

By Grace Elkus
Updated July 21, 2015
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That kid you ate lunch with every day in middle school? He likely wasn’t your friend come high school. That’s because the majority of adolescent friendships fizzle after just a year or two, according to new findings from a long-term study. But what’s the reason behind the fleeting friendships?

The study, which was conducted by Florida Atlantic University and published in Psychological Science, supports previous research that shows children seem to flock to friends with similar traits. This study, however, answered the question why, suggesting that dissimilarity is what breaks friendships apart.

“[Dissimilarity] 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,” Brett Laursen, Ph.D., professor and graduate studies coordinator in FAU’s Department of Psychology, said in a statement. “Dissimilarity disrupts relationship bonds.”

Four hundred and ten middle-school students involved in 573 friendships participated in the study. Researchers followed the participants from 7th grade to 12th grade, collecting data annually during a required English class. The researchers sought to determine whether it was dissimilarities or undesirable personality traits (or a combination of the two) that caused adolescent friendships to end.

Fewer than one in 10 friendships lasted into high school, and only 1 percent of friendships continued to the 12th grade. Undesirable attributes had little to do with this—in fact, as long as both friends had comparable levels of the behavior, they weren't necessarily bad for the friendships. Instead, the relationships ended due to key disparities between the two individuals, including differences in sex, differences in the degree the children were liked by other children, differences in physical aggression, and differences in school competence.

Compatibility, therefore, seems to come down to similarity, rather than the presence (or absence) of one particular trait.