Genetics vs. Environment Their Role in Intelligence

Nature vs. Nurture in IQ “Is a person’s intelligence due more to genes or to environment?” Both genes & intelligence crucial for any trait Environmental factors determines which genes get switched on. Example of nutrition effecting you reaching your genetic potential in height. 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.

Heredity and Environment Heritability The variation within a group caused by genetic factors Thought to be about 50% Environment Degree to which variation is due to environmental rather than genetic differences Environmental Factors can be early intervention, schooling and family life

Twin Studies & Family Influence Thomas Bouchard & the Minnesota Twins studies 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 MSClip art 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

Twin Studies figure scanned in from Gray's text, pg. 370, figure 10.7

Data Analysis The data show that both genetics and environment have an effect on IQ scores. The more closely two individuals are related genetically, the more similar their IQ scores: Identical twins reared together are more alike than are fraternal twins reared together. Also show the importance of environmental influences: Identical twins reared together are more alike than are identical twins reared apart siblings who are reared together are more alike than are siblings reared in different homes.

Both genetics and environment have an effect on IQ scores

Adoption Studies Children's verbal ability scores became modestly more like their biological parents' scores. (Adapted from Plomin & DeFries, 1998.)

Group Differences in Intelligence Test Scores

Group Differences in Testing A number of studies show scoring differences between different racial, ethnic, and gender groups. Are these differences due to nature or to nurture? Studies suggest environment is playing a heavy role. Heredity and environment interact to produce intelligence in individuals.

Why do Japanese and Chinese students perform better than Americans on intelligence and achievement tests? They score the same before school but once school starts they begin to show more gains. This due to: Spending more time in school More time doing homework Pressure and support from parents to achieve academically

Group Differences Australian aborigine & white scores on IQ tests Play “Intelligence and Culture” (4:02) Module #4 from The Brain: Teaching Modules (2nd edition). Are the differences in scores the result of environment or genes? Which theory of intelligence does this example prove? Gardner’s Multiple Intelligences or Spearman’s g factor?

Racial Difference in IQ Racial difference in average IQ among different racial groups can be measured More variation in IQ scores within a particular group than between groups Stereotype Threat – When members of a particular group fear they are being thought of or evaluated according to a negative stereotype, they fear that they may confirm the stereotype leading to self-doubt and anxiety resulting in poor performance. (see Box 7.4 on pg. 311)

Effects of Discrimination on Intelligence Burakumin people of Japan are severely discriminated against. Despite being genetically similar to Japanese people… Their IQ scores are 10 to 15 points lower than “normal” Japanese people.

Seed Analogy: Argument for Environment

Within and Between Group Differences Each corn 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) Unless the environments of two racial groups are totally identical, it is impossible to estimate the overall genetic differences between the two groups.

Are IQ Tests Culturally Biased? If IQ test reflects white, middle-class cultural knowledge and values it could result in poor scores by minorities. Test will favor the people from the culture in which it was developed. Factors like test-taking behavior, motivation, attitudes towards the test and previous experiences with tests can affect performance. Try some culturally biased tests yourself: Robert Williams IQ Test & the Dove Counterbalance Test Chitling Intelligence Test