A、Children reared under average conditions possess average intelligence. B、Lack of opportunity prevents the growth of intelligen

There are two factors which determine an individual’ s intelligence. The first is the sort of brain he is born with. Human brains differ considerably, some being more capable than others. But no matter how good a brain he has to begin with an individual will have a low order of intelligence unless he has opportunities to learn. Sc, the second factor is what happens to the individual -- the sort of environment in which he is.
    The importance of environment in determining an individual’s intelligence can be demonstrated by the case history of the identical twins, Peter and Mark. 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 homes. Peter was reared by parents of low intelligence in an isolated community with poor educational opportunities.
    Mark, on the other hand, was raised in the home of rich parents who had been to collage. He was read to as a child, sent to a good school, and given every opportunity to be stimulated intellectually. This environmental difference continued until the twins were in their late teens, when they were given tests to measure their intelligence. Peter’s score was 85,well below the level he might have attained if reared under average conditions. Mark’s score was 125, twenty-five points above the average and fully forty points higher than his identical brother. Given equal opportunities, the twins would have tested at roughly the same level.