What was the difference between the As and the Cs? Terman ran through every conceivable explanation. He looked at their physical and mental health, their “masculinity-femininity scores,” and their hobbies and vocational interests. He compared the ages when they started walking and talking and what their precise IQ scores were in elementary and high school. In the end, only one thing mattered: family background. The As overwhelmingly came from the middle and the upper class. Their homes were filled with books. Half the fathers of the A group had a college degree or beyond, and this at a time
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