Arthur Jensen is both famous and infamous for his claim that the main causes of individual differences in intelligence are genetic. One of his stronger arguments is based on the positive correlation between degree of genetic relatedness and intelligence (e.g., intelligence is more highly correlated between identical twins than it is between fraternal twins). Which of the following statements is an assumption one must make, rather than an inference or an observation, in order to accept the evidence from studies such as twin studies as evidence for a genetic cause of individual differences in intelligence? 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 exactly the same genetic makeup.
2. The environments of all pairs of siblings in the studies that Jensen reviewed are identical.
3. Genetic differences between individuals cause intellectual differences.
4. Modifying environments will have little effect on intelligence.
5. The correlation of IQs averages .82 for identical twins, but .50 for fraternal twins.

ANSWER KEY

1. This is not the best answer. This statement is an observation, being a statement of "the way things are" for identical twins (assuming no genetic accidents that may have altered one or both twins' genetic makeup).

2. This is the best answer. Jensen does not consider the possibility that one or more genetic-environment interaction creates substantially different environments for fraternal twins from those for identical twins, and hence the possibility for different environmental effects on intelligence. (See chapter 2 for the discussion of different heredity-environment interactions.)

3. This is not the best answer. The statement is an inference. The work upon which Jensen relied is essentially correlational, and hence it is not direct evidence of a cause-effect relationship. Thus Jensen interprets the facts as such.

4. This is not the best answer. This is an inference, based on Jensen's belief that environment contributes little to individual differences in intelligence.

5. This is not the best answer. It is an observation, being namely the result of examining various studies of the correlations between identical and fraternal twins and averaging their results.