Suppose that 100% of the variation in scores on intelligence tests was genetic in a particular population. We could then conclude that

1. (a) efforts to teach children to be more intelligent are a total waste of time;

2. (b) the environment can have no effect on intelligence;

3. (c) to know one's parents would be to know one's IQ;

4. (d) all of the above;

5. (e) none of the above.

The correct answer is (e). Given this fact, why do people care so much about what proportion of the variance in intelligence is heritable? Well, there are a number of reasons, but none of them relate to the topic of this chapter: educating people to optimize their intelligence.

Consider the well-worn example of phenylketonuria. Variation in whether people are susceptible to phenylketonuria is 100% genetic. If a person inherits the right gene, he or she will suffer from the disease – no doubt about it. And the person will be mentally retarded – but only if phenylalanine is not removed from the person's diet. If it is immediately removed upon birth, the person can lead a more or less normal life, aside from watching diet extremely carefully.

The example of phenylketonuria is one of many that make the same point: The question of heritability is separate from the question of mean differences. Even though height, for example, is very highly heritable, with a heritability coefficient exceeding .9, we know that in some populations, especially the Japanese, heights have increased greatly in recent years.