There are two factors which deter mine an individual's intelligence. The first is the sort of brain he is born【B1】. Human brains differ considerably,【B2】being more capable than others.【B3】no matter how good a brain he has to begin with, an individual will have a low order of intelligence unless he has【B4】to learn. So the second factor is what【B5】to the individual—the sort of environment in which he is brought【B6】. If an individual is handicapped(受阻碍)【B7】, iris likely that his brain will【B8】to develop and he will never【B9】the level of intelligence of which he is【B10】. The importance of environment in determining an individual's intelligence can be【B11】by the case history of the【B12】twins, Peter and John. When the twins were three months old, their parents died, and they were placed in【B13】fostered(寄养的) homes. Peter was【B14】by parents of low intelligence in an【B15】community with poor educational opportunities. John,【B16】, was educated in the home of well-to-do parents who had been to college. This environmental【B17】continued until the twins were【B18】their late teens, when they were given tests to【B19】their intelligence. John's IQ was 125, twenty-five points higher than the【B20】and fully forty points higher than his identical brother. 【B1】