The question as to how much of the IQ is attributable to genetics and how much to experience has been endlessly debated by psychologists. It is far from an academic exercise. If the IQ is entirely inherited, for example, then there is little we can do improve the lot of the poor, or some racial groups who score low on Intelligence Tests. On the other hand if the IQ is malleable then intervention is well worth the effort. In his new book, Intelligence and How to get it: Why Schools and Cultures Count (Nisbett 2009), University of Michigan psychologist Richard Nisbett makes a strong case for the importance of experience in the determination of the IQ.

The strongest evidence for the inheritance of the IQ comes from twin studies which show that even twins reared apart have IQ’s that are even more alike than are those of fraternal twins reared in the same home. Nisbett claims that these findings are the result of the fact that these studies were done with middle class children who one assumes have the optimum stimulation for the development of intelligence. Children growing up in less than optimum environments, however, are not able to realize their full intellectual potential. This fact is supported by my blog on The Deprived Brain which reported the low intelligence of Romanian infants who were reared in kennel-like institutions.

The contribution of genetics to intelligence is most evident at the very bottom and at the very top. Extremely retarded and extremely gifted children are recognizable early in life. But even retarded children can make more progress in an enriched environment than an impoverished one. In the Milwaukee Project for example, two groups of six month old African American children, thought to be at risk for mental retardation, were randomly assigned to two groups. One group received minimal care while the other group received high quality day care and educational enrichment. At age five the children in the control group had IQ’s averaging 83 as compared to the day care group average of 110.

In further support of his argument Nisbett cites studies in which poor children are adopted into upper middle class households. In one such study the IQ’s of the children were raised by 12 to 18 points. In a French study, children adopted into upper middle class homes showed comparable gains in comparison to matched children not adopted into such homes.

At the same time, there are limits to what experience can do. My granddaughter Maya has Down syndrome and has received early intervention from birth. She is three now and is talking, swimming, riding a bike and going to a regular nursery school. Although, thanks to this intervention, she is reaching her full potential, she will never score very high on an intelligence test. Timing is also important, with Maya the intervention came early when sensory stimulation is so important to brain development. Had this intervention only come after the age of four or five, it is not clear that she would have made the same progress.

Nisbett’s argument that the environment can play a huge role in intellectual achievement is important. But it has to be balanced with the equally important understanding that genetics does set the limits on what the environment can do.