This dissertation studies the social structures and dynamics of human networks: how peers at the micro level and physical environments at the macro level interact with the individual preferences and attributes and shape social dynamics. It is composed of three parts. The first essay, "Friendship Choices and Group Effects in Adolescent…

The 1996 National Youth Gang Survey was the largest of its type ever conducted, and the results are representative of the United States as a whole. Almost 5,000 law enforcement agencies were surveyed, and more than 80% of the sample responded. Agencies in all large cities, and a random sample of smaller cities were included, as were suburban…