Journal Issue: School Readiness: Closing Racial and Ethnic Gaps Volume 15 Number 1 Spring 2005
The Interplay of Genes and the Environment
To explain this puzzle, James Flynn and I have proposed a formal model in which genes and environment work together, rather than independently, in developing a person's cognitive ability.28 The solution involves three aspects of the process by which individual ability is molded that are overlooked by the logic that implies small environmental effects. We illustrate our argument with a basketball analogy.
How can genes and environment both be powerful in shaping ability? Consider a young man with a small genetic predisposition toward greater height and faster reflexes. When he is young, he is likely to be slightly better than his playmates at basketball. His reflexes will make him generally better at sports, and his height will be a particular advantage when it comes to passing, catching, and rebounding. These advantages by themselves confer only a small edge, but they may be enough to make the game more rewarding for him than for the average person and get him to play more than his friends and to improve his play more over time. After a while, he will be considerably better than the average player his age, making it likely that he will be picked first for teams and perhaps receive more attention from gym teachers. Eventually, he joins a school team where he gets exhaustive practice and professional coaching. His basketball ability is now far superior to that of his old playmates. Through a series of feedback loops, his initial minor physical advantage has been multiplied into a huge overall advantage. In contrast, a child who started life with a predisposition to be pudgy, slow, and small would be very unlikely to enjoy playing basketball, get much practice, or receive coaching. He would therefore be unlikely to improve his skills. Assuming children with a range of experience between these two extremes, scientists would find that a large fraction of the variance of basketball playing ability would be explained by differences in genetic endowment—that basketball ability was highly heritable. And they would be right to do so. But that most certainly would not mean that short kids without lightning reflexes could not improve their basketball skills enormously with practice and coaching.
The basketball analogy so far illustrates two of the considerations that Flynn and I believe are important for understanding the implications of behavioral genetic studies of cognitive ability. First, genes tend to get matched to complimentary environments. When that happens, some of the power of environment is attributed to genes. Only effects of environment shared by all children in the same family and effects of environment uncorrelated with genes get counted as environmental. Second, the effect of genetic differences gets multiplied by positive feedback loops. Small initial differences are multiplied by processes where people's initially varying abilities are matched to complimentary environments that cause their abilities to diverge further.
In theory this same multiplier process could be driven by small environmental differences. But to drive the multiplier to its maximum, the environmental advantage would have to be as constant over time as the genetic difference, because in the absence of the initial advantage there will be a tendency for the whole process to unwind. For example, suppose that midway through high school the basketball enthusiast injures a leg, which makes him less steady and offsets his initial advantage in height and reflexes. Because of all his practice and learning, he will still be a superior player. But his small decrement in performance could mean discouragement, more bench time, or not making the cut for the varsity team. This could lead to a further deterioration of his skills and further discouragement, until he gives up playing on the team entirely. Although each individual's experience will differ, the theory that Flynn and I lay out would have people with average physical potential reverting to average ability over time, on average.
The transitory nature of most environmental effects not driven by genetic differences helps explain why environmental differences do not typically drive large multipliers and produce the same large effects as genetic differences. That same transience helps explain why environment can be potent but still cause a relatively small share of the variance of cognitive ability in adults.29