This study was designed to investigate friend influence on academic achievement and task avoidance during middle childhood in a sample of 794 participants in 397 stable same-sex friendship dyads (205 girl dyads and 192 boy dyads) from four municipalities in Finland: two in Central Finland, one in Western Finland, and one in Eastern Finland. Longitudinal data were collected during the spring of 3rd grade and 4th grade and reports were available from both members of each friendship dyad. The Actor-Partner Interdependence Model (APIM; Kenny, Kashy & Cook, 2006) was used with a single sample of participants to estimate friend influence on academic achievement and task avoidance between two types of friendship dyads: (1) dyads that were distinguishable as a function of relative math achievement and relative peer acceptance and (2) dyads that were indistinguishable as a function of relative math achievement and relative peer acceptance. The results demonstrate that when friends are distinguished by math achievement the high achiever influences the low achiever's math achievement, but not the reverse. When friends are distinguishable by peer acceptance the high accepted partner influences the low accepted partner's math achievement, but not the reverse. When friends are indistinguishable on the basis of math achievement and peer acceptance there is mutual influence on math achievement. There was no evidence of friend influence on task avoidance. There was no evidence of friend influence from an individual's own task avoidance predicting changes in friend math achievement, except among dyads that could not be distinguished on the basis of math achievement. Math achievement predicted within-individual changes in task avoidance for all friendship dyads, except those that could not be distinguished by relative math achievement. The findings suggest that friends influence math achievement during middle childhood. Furthermore, when friends are distinguished, relative math achievement and peer acceptance determines who is influencing whom within a friendship dyad. The use of the APIM for distinguishable and indistinguishable dyads on a single sample of participants illustrates that it is not sufficient to ignore differentiating features between friends, or to discard friendships that are more similar. Implications for teaching strategies and classroom interventions are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]