ABSTRACT
U P t o this point i n our study of individual differences i n ability and personality we have been building up a knowledge of the facts without much attention to causes, and it is time now to round out the subject by looking into this question of causes. Why do people differ, what makes them different? If anyone should ask for the cause of individual differences, i t would be impossible to answer his question, because there is not just one cause. Many causes or factors combine to make people differ. Physiological and social factors were noticed i n the last chapter. We saw how people differ i n personality because one is getting plenty of hormone from his thyroid gland while another is getting very little; and we saw how people develop different traits by filling different roles in the social group. But we can push the question farther back by asking what caused the glands to differ, and by asking what made two persons undertake their different social roles.
