A recurrent and often bitter controversy in the study of intelligence is the issue of how heredity and environment contribute to intelligence. Arthur Jensen, a leading figure in the debate, has contributed both data and argument to the "nature" view. Which of the following statements represents an important assumption, rather than an inference or an observation, of Jensen's argument? Circle the letter of the best answer and explain why it is the best answer and why each other answer is not as good.

1. Identical twins have identical genetic endowments.
2. Identical twins should have IQs that are more similar than the IQs of ordinary siblings.
3. The correlation between IQs of twins reared together is .89.
4. Differences between the correlations of IQs for twins reared together versus those of twins reared apart indicate that environment has only a weak effect on intelligence.
5. The environments of twins reared together versus those of twins reared apart are very different.

ANSWER KEY

1. This is an observation. It is a factual statement about the nature of identical twins' heredity.

2. This is an inference. It is a hypothesis about the correlations based on the belief that heredity makes an important and direct contribution to individual differences in intellectual ability.

3. This is an observation taken directly from research on the correlations of IQs in twins reared together.

4. This is an inference. It interprets the finding that these two types of correlation do not differ very much.

5. This is the assumption. According to the text, Jensen and others have not verified this belief about the environments of twins reared together and twins reared apart, but rather take it for granted that these environments differ. In fact, this belief is a point that critics of Jensen's work have challenged.