The importance of environment in determining an individual’s intelligence can be demonstrated by the case history of the identical twins, peter and Mark X. Being identical, the twins had identical brains at birth, and their growth processes were the same. When the twins were three months old, their parents died and they were placed in separate foster homes. Peter was reared by parents of low intelligence in an isolated community with poor educational opportunities. Mark was reared in the home of well-to-do parents who had been to college. He was read to as a child, sent to good school, and given every opportunity to be stimulated intellectually. This environmental difference continued until twins were in their late teens, when they were given tests to measure their intelligence. Peter’s I.Q. was 85 , well below the level he might have attained if reared under average conditions. Mark’s I.Q. was 125, twenty-five points above the average and forty points higher than his identical brother’s given equal opportunities, the twins, having identical brains, would have tested at roughly the same level.