Results from a variety of seemingly unrelated research areas indicate that between the ages of 6 and 9, there is a change from a tendency to categorize according to complementary criteria to a tendency to categorize according to similarity criteria. For example, younger children are more likely to categorize complementary items such as a hammer and a nail, while older children are more likely to categorize similar items such as a nail and a pin. Studies of free classification, conceptual styles, word associations, word definitions, and memory clustering indicate such a change. There is also evidence that this change may reverse itself in old age. There are two possible sources for such developmental changes – they may originate primarily from internal, organismic changes or primarily from external, environmental changes. Evidence for both positions was presented and an argument was made in favor of the position that these changes in categorization criteria result primarity from external, environmental changes.