1 Nature vs. Nurture in IQ zAre differences between people due to environmental or genetic differences? zMisunderstanding the question y“Is a person’s intelligence due more to genes or to environment?” yno genes = no intelligence yboth genes & intelligence crucial for any trait

2 Nature vs. Nurture in IQ zRectangle analogy yis a rectangle’s area due more to its width or its length? yare differences in area among rectangles due more to differences in their length or in their width? Group A Group B Length Width Length Width

3 Heritability & Environmentality zHeritability ydegree to which variation in trait stems from genetic, rather than environmental, differences among individuals zEnvironmentality ydegree to which variation is due to environmental rather than genetic differences

4 The Heritability Coefficient zSingle number, ranging from 0 to 1.0 zRepresents amount of trait due to genetic differences z0 means no variance due to genetics z1 means all variance due to genetics z.30 means 30% is due to genetic differences, 70% due to environmental

5 Twin Studies & Family Influence zIf trait genetic: yclosely related more similar than less closely related zMany close relatives share environments too zTypes of studies to separate effects ymonozygotic twins reared together ymonozygotic twins reared apart ysiblings/dizygotic reared together ysiblings/dizygotic reared apart yadoptive siblings reared together

6 Family Influence on IQ zTransient influence of family in which you are raised yadoptive siblings as children and adults yidentical vs. fraternal twins reared together Fraternal twins Identical twins

7 IQ Differences among Racial/Cultural Groups zFind differences among racial & cultural groups on IQ yAmerican Blacks score about 15 points lower than Whites zWHY? ygenetics? xWitty & Jenkins (1935) xNo support for genetic differences yenvironment?

8 IQ Differences among Racial/Cultural Groups zEach wheat field planted from same package of genetically diverse seeds zOne field is quite fertile, the other is not zWithin each field, differences due to genetics zBetween each field, differences due to environment (fertility)

9 Why differences between Blacks & Whites? zSocial designation influences yautonomous minorities xdeliberately separate themselves yimmigrant minorities xcame to country to better lives xsee selves as better than those left behind yinvoluntary minorities xdid not choose minority status xroutinely judged as inferior by dominant majority xcross-cultural findings on IQ scores

10 Historical increase in IQ zImproved performance on IQ tests over the years zBig increases in tests assessing fluid intelligence, smallest in factual knowledge zIncreased schooling only accounts for small portion of this increase zDue to changes in the culture? Year IQ Scores

11 Summary z Nature vs. nurture debate y heritability vs. environmentality y studies of twins & family influence z Racial & cultural differences in IQ y why we can’t interpret differences as due to genetics y what we can attribute the differences to z Historical increase in IQ scores