Matching methods such as nearest neighbor propensity score matching are increasingly popular techniques for controlling confounding in nonexperimental studies. However, simple k:1 matching methods, which select k well-matched comparison individuals for each treated individual, are sometimes criticized for being overly restrictive and discarding…

Close relationships among children and adolescents are ordinarily considered to encompass friendliness and fun. Recent studies, however, reveal that many friendships have dark sides consisting of competitiveness, hostility, and conflict. Relationships based on aversion and antipathy may turn out to have major developmental implications, but more…