Intelligence runs in families. But why? Are our intellectual abilities mostly inherited? Or are they molded by our environment?

Few issues arouse such passion or have such serious political implications. Consider: If we mainly inherit our differing mental abilities, and if success reflects those abilities, then people’s socioeconomic standing will correspond to their inborn differences. This could lead to those on top believing their intellectual birthright justifies their social position.

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But if mental abilities are primarily nurtured by our environments, then children from disadvantaged environments can expect to lead disadvantaged lives. In this case, people’s standing will result from their unequal opportunities.

For now, as best we can, let’s set aside such political implications and examine some of the evidence.