Hockenbury Discovering Psychology 5th txtbk

The Nature of Intelligence305To underscore the interplay between heredity and environment, consider thefact that the average height of Americans increased by several inches in the pasthalf-century. The explanation for this increase is that nutritional and health standardshave steadily improved, not that the genetic heritage of Americans has fundamentallychanged. However, heredity does play a role in establishing limits onheight. If you’re born with “short” genes (like your authors), you’re unlikely toreach six foot four, no matter how good your nutrition. (For more on geneticssee Chapter 9.)The roles of heredity and environment in determining intelligence and personalityfactors are much more complex than the simple examples of height or eye color(Plomin, 2003). However narrowly intelligence is defined, the genetic range ofintellectual potential is influenced by many genes, not by one single gene (Plomin& Spinath, 2004). No one knows how many genes might be involved. Given thecomplexity of genetic and environmental influences, how do scientists estimate howmuch of intelligence is due to genetics, how much to environment?Twin StudiesSorting Out the Influence of Genetics Versus EnvironmentOne way this issue has been explored is by comparing the IQ scores of individualswho are genetically related to different degrees. Identical twins share exactly thesame genes, because they developed from a single fertilized egg that split into two.Hence, any dissimilarities between them must be due to environmental factorsrather than hereditary differences. Fraternal twins are like any other pair of siblings,because they develop from two different fertilized eggs.As you can see in Figure 7.8 on the next page, comparing IQ scores in this wayshows the effects of both heredity and environment. Identical twins raised togetherhave very similar IQ scores, whereas fraternal twins raised together have IQs thatare less similar (Plomin & Spinath, 2004).However, notice that identical twins raised in separate homes have IQs thatare slightly less similar, indicating the effect of different environments. And, althoughfraternal twins raised together have less similar IQ scores than do identicaltwins, they show more similarity in IQs than do nontwin siblings. Recallthat the degree of genetic relatedness between fraternal twins and nontwin siblingsis essentially the same. But because fraternal twins are the same age, theirenvironmental experiences are likely to be more similar than are those of siblingswho are of different ages.Thus, both genetic and environmental influences are important. Genetic influenceis shown by the fact that the closer the genetic relationship, the more similarthe IQ scores. Environmental influences are demonstrated by two findings:First, two people who are genetically identical but are raised in different homeshave different IQ scores. And second, two people who are genetically unrelatedbut are raised in the same home have IQs that are much more similar than arethose of two unrelated people fromrandomly selected homes.Using studies based on degree of geneticrelatedness and using sophisticatedstatistical techniques to analyzethe data, researchers have scientificallyestimated heritability—the percentageof variation within a given populationthat is due to heredity. The currentlyaccepted heritability estimate is about50 percent for the general population(see Plomin, 2003; Plomin & Spinath,2004).autismBehavioral syndrome associated with dif -ferences in brain functioning and sensoryresponses, and characterized by impairedsocial interaction, impaired verbal and nonverbalcommunication skills, repetitive orodd motor behaviors, and highly restrictedinterests and routines.Asperger’s syndromeBehavioral syndrome characterized by varyingdegrees of difficulty in social and conversationalskills but normal-to-above-averageintelligence and language development;often accompanied by obsessive preoccupationwith particular topics or routines.mental retardationDisorder characterized by intellectual functionthat is significantly below average,usually defined as a measured IQ of 70 orbelow, and that is caused by brain injury,disease, or a genetic disorder.heritabilityThe percentage of variation within a givenpopulation that is due to heredity.Genetics or Environment? These identicaltwins have a lot in common—including a beautiful smile. Twins areoften used in studies of the relativecontributions that heredity and environmentmake to personality and othercharacteristics.