Birds of a Feather: Intensification Effects of Teen Best Friendships on Psychological Functioning Student Author: Meghan Costello Faculty Advisor: Joe Allen The importance of the best friend in the high schooler’s life is widely accepted but the best friend relationship can work in both positive and negative directions with regard to psychological dysfunction. The present study seeks to understand the mechanisms at play in toxic relationships. This work draws from two waves of data, collected one year apart from 184 15-to-16-year olds, to analyze the relationship between social interaction styles within a best friendship and subsequent changes in self-reports of depression and aggression. It was hypothesized that teens with higher baseline scores of depression and aggression would display an increase in these psychopathologies in more intense friendships. Hierarchical regression analysis indicated that different levels of friendship intensity predict changes in relative depression and aggression in adolescents such that more intense friendships amplify pre-existing tendencies and less intense friendships do not, moderated by baseline expression of depressive or aggressive tendencies.