Continuing our IQ theme, the New York Times has a fascinating article on the contributions of genetics and environment to IQ and argues that the effect of the environment becomes much more crucial for those from low-income backgrounds.
[Psychologist Eric Turkheimer] has a reputation as a methodologist’s methodologist. In combing through the research, he noticed that the twins being studied had middle-class backgrounds. The explanation was simple – poor people don’t volunteer for research projects – but he wondered whether this omission mattered.
Together with several colleagues, Turkheimer searched for data on twins from a wider range of families. He found what he needed in a sample from the 1970’s of more than 50,000 American infants, many from poor families, who had taken I.Q. tests at age 7. In a widely-discussed 2003 article [pdf], he found that, as anticipated, virtually all the variation in I.Q. scores for twins in the sample with wealthy parents can be attributed to genetics. The big surprise is among the poorest families. Contrary to what you might expect, for those children, the I.Q.’s of identical twins vary just as much as the I.Q.’s of fraternal twins. The impact of growing up impoverished overwhelms these children’s genetic capacities. In other words, home life is the critical factor for youngsters at the bottom of the economic barrel. “If you have a chaotic environment, kids’ genetic potential doesn’t have a chance to be expressed,” Turkheimer explains. “Well-off families can provide the mental stimulation needed for genes to build the brain circuitry for intelligence.”
This interaction between economic background and mental functioning has now been replicated in a number of studies looking at everything from mental illness to criminal behaviour (e.g. see Christian’s previous post).
It seems that the simple summing of genetic and environmental effects is no longer a valid way of understanding how we develop, as the duration, quality and frequency of different life experiences seem to have unique influences on the expression of our inherited traits.