The most compelling explanation comes from a study of twins separated at birth and given up for adoption. When these children were tested in early childhood, their IQs tended to match the IQs of their adoptive parents. The subjects were more like the people they lived with than the people with whom they shared genes. But by the time the children in the study had become teenagers, they more closely resembled their identical twins than they did their adoptive parents. The researchers' explanation was that environmental influences of parents temporarily lifted the children's IQs but that this influence didn't last. As the children aged, the expression of their intelligence seemed less dependent on what was going on around them day to day (the conversations people had, the activities they were encouraged to engage in, the kinds of schools they went to). Instead, it seemed that biology was taking over, and their IQs inched toward a level that much more closely resembled the long-lost siblings with the exact same genetics, even when the two had been raised in very different circumstances. Maude probably had an IQ more similar to the IQs of her biological parents than to those of her adoptive parents, but the disparity only showed up as she entered adolescence.
The research on IQ tells a pretty unambiguous story: children's IQs and the intelligence IQ tests measure are stable and heavily determined by a child's genes. And yet, as we have seen, there are factors that can either temporarily disguise a child's true intellectual ability or in some way inflate or depress it. If you think your child is more intelligent than tests or academic performance suggest, what might explain the discrepancy?
What Gets in the Way
Many years ago, when I was teaching at a fairly progressive independent school in New York City, there was a four-year-old boy, Alec, who came in for the usual interview and admissions assessment— a combination of a shortened IQ test and some openended play in the admissions office. Alec had been touted as very bright, and his parents were sure this was the right school for him. And yet he did miserably in all aspects of the admission process.
He did not get a high score on the test, he was uncommunicative in the interview, and his playing seemed immature, lacking the kind of complexity that is often seen as an expression of intelligence. Alec was denied admission. However, a teacher at the school later learned from a friend of the family that when Alec had gotten home that day, he had complained bitterly to his mother about his sore feet. It turned out his mother had mistakenly put him in a pair of shoes that were two sizes too small. The poor little boy had spent the whole time at the new school thinking about his aching feet, not old enough to realize that he could let someone know something was wrong. His performance in that interview was not a good measure of his real abilities. Anyone's intellect can be temporarily masked by some glitch of circumstance, and in the long run, such momentary setbacks won't alter the path of a child's intellectual development. A very smart teenager who does poorly on the SAT because he has a headache will do well the next time he takes it. And yet some glitches can have a long-lasting impact.