Friendship changes differentially predict neural correlates of decision-making for friends across adolescence.

Adolescents' peer world is highly dynamic with constant dissolution of old friendships and formation of new ones. Though many of adolescents' risky decisions involve their peers, little is known about how adolescents' ever-changing friendships shape their ability to make these peer-involving risky decisions, particularly adaptive ones, and whether this association shifts over time. In a 5-wave longitudinal fMRI study, 173 adolescents (at wave 1: Mage = 12.8, SDage = 0.52; range = 11.9-14.5) made risky choices to win money for their best friend. We assessed whether participants nominated the same or different best friend as their previous participation year (a total of 340 data points of friendship maintenance / change). In early adolescence, adolescents with the same best friend took more adaptive risks for that best friend than those with a different best friend. In late adolescence, however, adolescents with a different best friend took more adaptive risks for the new best friend than those with the same best friend. Further, the amygdala was differentially sensitive to friendship maintenance / change during these peer-involving adaptive risks across time. This study has implications for how stable and flexible peer landscapes differentially modulate social motivation and social decision-making over the course of adolescence.