At the heart of the Cambridge-Somerville Youth Study is a program designed to prevent delinquency targeted on “pre-delinquent,” under-privileged boys. Referred to as “directed friendship,” the preventive intervention involved individual counseling through a wide range of activities and home visits. Counselors talked to the boys, took them on trips and to recreational activities, tutored them in reading and math, supported their participation in the YMCA and in summer camps, and encouraged them to attend church. Boys in the treatment group received the program for a mean average of 5.5 years. Boys in the control group did not receive any special services. The program ended in 1945.

The study began with 650 boys (later reduced to 506), ages 5-13 years (median = 10.5), from Cambridge and Somerville, Massachusetts. Rated as either “average” or “difficult,” the boys were recommended by local schools, welfare agencies, churches, and police. The boys were placed in matched pairs—“diagnostic twins”—and one member of each pair was chosen at random (on a coin toss) to be in the treatment group.

Follow-ups in 1948 and in 1956 indicated that the program had no measurable impact on official offending. In 1975-76 (mean age = 45 years), Joan McCord located records for 480 participants (or 95%) and conducted interviews with (or distributed questionnaires to) 347 of them. Continued data collection up to 1979 located records for a total of 494 participants or 98%.

This 30-year post-intervention follow-up revealed iatrogenic program effects. Comparisons between the treatment and control groups indicated that the treatment group had not fared better on any outcome, and fared worse on a number of key outcomes. The treatment group men were significantly more likely to:

McCord proposed and tested several hypotheses to explain the iatrogenic effects. Perhaps the most important of these was the peer deviancy hypothesis.