So far, we have considered how our species thinks and communicates. But we differ from one another in these abilities. School boards, courts, and scientists debate the use and fairness of tests that assess people’s mental abilities and assign them a score. In psychology, no controversy has been more heated than the question of whether there exists in each person a general intellectual capacity that can be measured and quantified as a number.

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In this module, we consider some findings from a century of research, as psychologists have searched for answers to these questions and more: