The first phase of an 8-year longitudinal study of the onset and development of delinquency and behavior problems recruited 400 eight- and nine-year-old boys in a densely populated working class urban district. Assessment was made through psychological tests, teachers' and psychiatric social workers' reports, and parent interviews and questionnaires. Results suggested that family income and social level influenced the boys' conduct and performance more than personal and psychological factors did. For example, boys in families rated socially handicapped almost never obtained the best conduct rating; in addition, 76% of these boys tested below average in intelligence as opposed to 42% of the rest of the sample. Information gathered concerned the neighborhood and the subjects studied, conduct and personality at age 8, adversities of home background, and special criminogenic factors. (JD)