Topic sentence________________________________. 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. So the second factor is what happens to the individual——the sort of environment in which he is reared. If an individual is handicapped environmentally, it is likely that this brain will fail to develop and he will never attain the level of intelligence of which he is capable.