There are two factors which determine an indivi dual’s intelligence. The first is the sort ofbrain he is born with. Human brains differ considerably, some being more capable than others. Butno matter how good a brain he has to begin with, an individual will have a low level ofintelligence unless he has opportunities to learn. So the second factor is what happens to theindividual-the sort of environment in which he is brought up. If an individual is handicapped( 不利的)environmentally, it is likely that his brain will fail to develop and he will never attain the level ofintelligence of which he is capable. 

The importance of environment in determining an individual’s intelligence can bedemonstrated by the case history of the identical twins, Peter and Mark. Being identical, the twinshad identical brains at birth, and their growth processes were the same. When the twins were threemonths old, their parents died and they were placed in separate foster(抚养)homes. Peter was raisedby parents of low intelligence in an isolated community with fewer educational opportunities.Mark was reared in the home of well-to do parents who had been to college. He was read to as achild, sent to good schools, and given even- opportunity to be stimulated intellectually. Thisenvironmental difference continued until the twins were in their late teens, when they were giventests to measure their intelligence. Mark’s I. Q. was 125, twenty-five points higher than theaverage and forty points higher than his identical brother. Given equal opportunities, the twins,having identical brains, would have tested at roughly the same level. 

The main idea of this passage is that_________.