A. separateD. unlessG. birthJ. clearly M. intelligence
B. stimulatedE. whenH. improved K. demonstrated N. capable
C. smartF. fullyI. environment L. similar O. college 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 (1) than others. But no matter how good a brain he has to begin with, an individual will have a low order of intelligence (2) he has opportunities to learn. So the second factor is what happens to the individual—the sort of (3) in which he is reared. The importance of environment in determining an individual’s intelligence can be (4) by the case history of the identical twins, Peter and Mark. Being identical, the twins had identical brains at (5) , and their growth processes were the same. When the twins were three months old, their parents died, and they were placed in (6) 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 (7) . He was read to as a child, sent to good schools, and given every opportunity to be (8) intellectually. This environmental difference continued until the twins were in their late teens, when they were given tests to measure their (9) . Mark’sI.Q. was 125, twenty-five points higher than the average and (10) forty points higher than his identical brother. Given equal opportunities, the twins, having identical brains, would have tested at roughly the same level.