There are two factors (因素) which decide an individual's intelligence (智商). The first is the sort of brain he is born with. Human brains differ greatly, some being more capable (有能力) than others. But no matter how good a brain he has to begin with, an individual (个体) 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 brought up. If an individual has less chances environmentally, it is likely that his brain will fail to develop and he will never arrive at the level of intelligence of which he is capable.

    The importance of environment in deciding an individual's intelligence can be shown by the case history of the twins. When they were three months old, their parents died, and they were placed in different homes. Peter was brought up by parents of low intelligence in a separate community with poor educational opportunities. Mark was brought up in the home of well-to-do opportunity to be encouraged intellectually.

    This environmental difference continued until the twins were in their late teens, when they were given tests to measure their intelligence. Mark's IQ was 125, twenty-five points higher than the average and fully forty points higher than his twin brother. Given equal opportunities, the twins, having similar brains, would have tested at roughly the same level.