Nature vs. Nurture in IQ Are differences between people due to environmental or genetic differences? Misunderstanding the question “Is a person’s intelligence due more to genes or to environment?” no genes = no intelligence both genes & intelligence crucial for any trait Clearly, IQ is not the only psychological construct for which this debate applies, and instructors may wish to take a moment to name the other areas (i.e., personality, mental disorders, etc.) for which heritability is hotly debated.

Nature vs. Nurture in IQ Rectangle analogy is a rectangle’s area due more to its width or its length? are differences in area among rectangles due more to differences in their length or in their width? Group A Group B Length Width Clearly, IQ is not the only psychological construct for which this debate applies, and instructors may wish to take a moment to name the other areas (i.e., personality, mental disorders, etc.) for which heritability is hotly debated.

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

The Heritability Coefficient Single number, ranging from 0 to 1.0 Represents amount of trait due to genetic differences 0 means no variance due to genetics 1 means all variance due to genetics .30 means 30% is due to genetic differences, 70% due to environmental

Twin Studies & Family Influence If trait genetic: closely related more similar than less closely related Many close relatives share environments too Types of studies to separate effects monozygotic twins reared together monozygotic twins reared apart siblings/dizygotic reared together siblings/dizygotic reared apart adoptive siblings reared together figure taken from CD-ROM, Myers text, pg. 353, figure 11.7 Discussion here about the higher correlation between identical twins’ IQ scores than fraternal twins’ and whether siblings were reared together or apart and the genetic role then in intelligence

Family Influence on IQ Transient influence of family in which you are raised adoptive siblings as children and adults identical vs. fraternal twins reared together Fraternal twins Identical twins figure scanned in from Gray's text, pg. 370, figure 10.7

IQ Differences among Racial/Cultural Groups Find differences among racial & cultural groups on IQ American Blacks score about 15 points lower than Whites WHY? genetics? Witty & Jenkins (1935) No support for genetic differences environment? Gray discusses the Witty & Jenkins study of high-IQ Black children in Chicago on page 372-373 -- it makes a good discussion topic

IQ Differences among Racial/Cultural Groups Each wheat field planted from same package of genetically diverse seeds One field is quite fertile, the other is not Within each field, differences due to genetics Between each field, differences due to environment (fertility) figure taken from CD-ROM, Gray's text, pg. 373, figure 10.8

Why differences between Blacks & Whites? Social designation influences autonomous minorities deliberately separate themselves immigrant minorities came to country to better lives see selves as better than those left behind involuntary minorities did not choose minority status routinely judged as inferior by dominant majority cross-cultural findings on IQ scores Gray reports that involuntary minorities consistently score lower on IQ tests than the dominant majority

Historical increase in IQ Improved performance on IQ tests over the years Big increases in tests assessing fluid intelligence, smallest in factual knowledge Increased schooling only accounts for small portion of this increase Due to changes in the culture? Year IQ Scores figure taken from CD-ROM, Myers text, pg. 345, figure 11.5

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