In the development of one’s intelligence, environment plays ______.

The limits of a person’s intelligence are fixed at birth, but whether or not he reaches those limits will depend on his environment. It is easy to show that intelligence is, to some extent, something we are born with. The closer the blood relationship between two people, the closer they are likely to be in intelligence. Thus if we take two unrelated people at random from the population, it is likely that their degree of intelligence will be completely different. If on the other hand, we take two identical twins, they will very likely be as intelligent as each other. Relations like brothers and sisters, parents and children, usually have similar intelligence, and this clearly suggests that intelligence depend on birth.
     Imagine now that we take two identical twins and put them in different environments. We might send one, for example, to a university and the other to a factory where the work is boring and fixed. We would soon find differences in intelligence developing, and this indicates that environments as well as birth plays a part. This conclusion is also suggested by the fact that people who live in close contact with each other, but who are not related at all, are likely to have similar degrees of intelligence.