Discuss the evidence for the genetic and environmental contributions to individual intelligence, and explain what psychologists mean by the heritability of intelligence. Genetic contribution to intelligence does exist, yet it does not necessarily predict or determine one’s intelligence. Findings by researchers suggest that that genetics do influence intelligence, but also that it does not do so reliably or completely. Twin researcher Thomas Bouchard estimated that genetics contributes to “about 70 percent” (p 340) to the intelligence test score of any given individual. He determined this by testing fraternal and identical twins. Identical twin’s scores are almost identical to the scores of the same person taking an intelligence test twice, yet fraternal twin’s scores (who share less genetic make-up than identical twins) are less similar. In addition, the scores of identical twins who were raised separately are still very similar.
In addition to genetics, environment also contributes to individual intelligence. Surprisingly, according to geneticists mental similarities between adopted children and their adoptive parents wane with age until the correlation reaches zero by adulthood. (p 341) Virtual twins show the same correlation. Throughout life genetic influences become more apparent, but environmental influences do not. Adopted children’s test score slowly begin to coincide with their biological parents’ scores.
However, environmental influences are apparent in the intelligence of extremely impoverished children. The results are somewhat generalized, but J. McVicker Hunt determined through very specific studies of impoverished Iranian orphans that lacked care or enrichment had delayed development and lowered intelligence. Among the poor, biological and genetic influences can be almost negated by environmental influences and lead to cognitive underdevelopment.
Heritability can be a tricky concept. “We credit heritability with 50 percent of the...
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