Can you remember who your best friend was in seventh grade? If you are having difficulty, it could be because relationships at that age are often short-lived. Half don't last a year. The friendships that do last can be predicted based on demographic and behavioral similarities, according to new research from psychologist Brett Laursen of Florida Atlantic University.

“There is a lot of change during middle school, and that change makes it hard to maintain friendships,” Laursen says. As kids move from one academic track to another, join or leave sports teams, or take up new extracurricular hobbies, the opportunities to interact with friends wax and wane. Middle school is also a time when growing personal autonomy first allows children the chance to pick their friends and invest—or not—in those relationships.

Laursen tracked 573 seventh-grade two-person friendships until they ended or until 12th grade. A few important behavioral traits emerged as predictors of friendships that lasted more than a year: popularity, aggression and academic success. The more similar two friends were in these traits, the longer a relationship lasted. The results mirror a previous study by Laursen in which he found he could predict which people would become friends based on the similarity of their behavior during seventh grade.

Friends of the opposite sex were least likely to last. “They're completely doomed,” Laursen says, in part because of pressure from other friends. Adolescents tend to sort themselves based on age, race and gender, so being friends with a member of the opposite sex limits the size of one's larger group of “running buddies” and taxes the relationship.

Laursen says this quick turnover in friendships is nothing to be worried about unless a child has trouble making friends. Adults who want to help those children might emphasize that finding peers who are similar in personality and academic interests is key to creating lasting relationships. “But for most kids, the end of a friendship means a new one is starting,” Laursen says.