Provocative ideas about the nature, development, and effects of children's friendships were included in the lectures of Harry Stack Sullivan, which were edited and published in the 1950s. Sullivan emphasized the love, intimacy, and collaboration found in the close friendships that children form around 8 to 10 years of age. Later research has shown that close friendships have both a positive dimension, with features such as intimacy, and a negative dimension, with features such as rivalry. However, close friendships do not emerge suddenly at 8 to 10 years of age. Rather, the closeness of children's friendships increases gradually during middle childhood and adolescence. Recent studies suggest that having close, high-quality friendships increases children's success in the peer social world. Having high-quality friendships could magnify the positive or negative influence of friends with positive or negative characteristics, but this hypothesis needs to be evaluated more thoroughly in the future.