The importance of environment in determining an individual's intelligence can be shown by the case 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 3 months old, their parents died, and they were placed in separate homes. Peter was brought up by parents of low intelligence in an isolated community with poor educational opportunities. Mark was brought up in the home of well-to-do parents who had been to college. He was read to as a child, sent to good schools, and given every opportunity. This difference continued until the twins were in their late teens, when they're given tests to measure their intelligence, Mark's IQ was 125, 25points higher than the a v erage and fully 40 points higher than his identical brother. Given equal opportunities, the twins, having identical brains, would have tested at roughly the same level. The topic of this paragraph is____________