There are two factors (determine) an individual's intelligence. The first is thesort of brain he is born . Human brains differ (consider), some beingmore capable than others.
howevergood a brain he has to begin with, an individual will have a low intelligenceunless he has opportunities to learn. So the second factor is what happens tothe individual—the sort of environment he is brought up. If an individualis handicapped(受阻碍)environmentally, itis (like) that his brain will fail to develop and he will never attain thelevel of intelligence of which he is capable.
Theimportance of environment in determining an individual's intelligence can(show) by the case of the twins, Peter and Mark. When the twins were threemonths old, parents died, andthey were placed in separate foster homes. Peter was brought up by parents oflow intelligence with poor (education) opportunities. Mark was educated inthe home of welltodo parents had been to college. This environmentaldifference continued until the twins were in their late teens, when they weregiven tests to measure their intelligence. Mark's I．Q. was 125, twentyfive points higherthan the average and fully forty points higher than his twin brother.
