Hockenbury Discovering Psychology 5th txtbk

306 CHAPTER 7 Thinking, Language, and IntelligenceGenetic OverlapRelationship100%Identical twins reared together(.86)100%50%50%Identical twins reared apartFraternal twins reared togetherSiblings reared together(.47)(.60)(.72)Differencein IQscoresshowseffect ofenvironment50%Siblings reared apart(.24)50%Biological parent and child,lived together(.42)50%0%Biological parent and child,lived apartAdoptive parent and child,lived together(.24)(.24)0%Adopted siblings, reared together(.32)Figure 7.8 Genetics, Environment, andIQ Scores This graph shows the averagecorrelations of IQ scores for individualswho are genetically related to differentdegrees. The graph is based on research bypsychologists Thomas Bouchard and MattMcGue, who summarized the results frommore than 100 separate studies on over100,000 pairs of relatives (McGue & others,1993). The data show that both geneticsand environment have an effect on IQscores. The more closely two individualsare related genetically, the more similartheir IQ scores: Identical twins reared togetherare more alike than are fraternaltwins reared together. However, the samedata also show the importance of environmentalinfluences: Identical twins rearedtogether are more alike than are identicaltwins reared apart, and siblings who arereared together are more alike than aresiblings reared in different homes.0 .10 .20 .30 .40 .50 .60 .70 .80 .90 1.00Average correlation of IQ scoresLesssimilarMoresimilarIn other words, approximately 50 percent of the difference in IQ scores within agiven population is due to genetic factors. But there is disagreement even over thisfigure, depending on the statistical techniques and data sources used (Turkheimer& others, 2003).It is important to stress that the 50 percent figure does not apply to a single individual’sIQ score. If Mike’s IQ is 120, it does not mean that 60 IQ points are due toMike’s environment and 60 points are genetically inherited. Instead, the 50 percentheritability estimate means that approximately 50 percent of the difference in IQscores within a specific group of people is due to differences in their genetic makeup.More on this key point shortly.Group Differences in IQ ScoresIf the contributions of heredity and environment are roughly equal, why all the fuss?Much of the controversy over the role of heredity in intelligence is due to attemptsto explain the differences in average IQ scores for different racial groups.In comparing the average IQ for various racial groups, several studies have showndifferences. For example, Japanese and Chinese schoolchildren tend to score aboveEuropean American children on intelligence and achievement tests, especially inmath (Lynn, 1987; Stevenson & Lee, 1990). Are Japanese and Chinese children geneticallymore intelligent than European American children?Consider this finding: In early childhood, there are no significant differences inIQ among European American, Japanese, and Chinese schoolchildren. The scoresof the three groups are essentially the same (Stevenson & Stigler, 1992). The gapbegins to appear only after the children start school, and it increases with every yearof school attended. By middle school, Asian students tend to score much higherthan American students on both math and reading tests.