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
Group A Group B Length Length Width Width 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?
Heritability & Environmentality • Heritability • 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
Identical twins Fraternal twins 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
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?
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)
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
IQ Scores Year 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?
Summary • Nature vs. nurture debate • 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