Wednesday, January 13, 2010

The heritability of reading ability

A twin study on the influence of genes and environment on reading ability is going to be misunderstood.
(In the baseline test) For word and letter identification, genetics explained about one-third of the test results, while environment explained two-thirds. For vocabulary and sound awareness, it was equally split between genetics and environment. For the speed tests, it was three-quarters genetic.
But when the researchers measured growth in reading skills, environment became much more important, Petrill said.
Heritability is something people in child development often misunderstand. This does not mean that 1/3 of human reading ability is genetic and 2/3 environmental. It means that 1/3 of the difference in reading ability between two sets of kids was due to genetics and 2/3 due to environment.

To explain the difference, let me talk about the genetics of IQ. I've posted about it before. For now, let's just stipulate that there is a range of human cognitive intelligence, whether or not it is measured by an IQ test. Some people are smarter than others, and there is a maximum smartness any particular person can achieve. A kid raised in a rich environment will become as smart as she is biologically capable of becoming. A kid raised in a poor environment (meaning low stimulation, not low income, though they may be the same) is less likely to reach her own physical smartness limit.

Now compare a bunch of kids from rich environments to each other. If they were all raised in environments that do not limit them, you should get a heritability of 100%. If the environments were identical, then any differences in their intelligence is genetic. If you compare one group of kids from a rich environment and a group from a poor one, you would expect a small genetic heritability and a large environmental component.

As a liberal society, and in particular as child development people, our goal is to remove the environmental component and make genetics the only difference by providing a rich environment. This is what it means to educate each kid to her cognitive potential: to remove environmental impediments to reaching that potential.

So what we learn from this study is that environmental impediments are important in several areas related to reading, different amounts for different skills, and they become increasingly important as the kids age. The study tells us more about societal impediments than about the genetics of reading.

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