The Nature-Nurture Debate

1. The Nature-Nurture Debate Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

2. Lecture contents • Group differences in IQ. • Population variation in IQ: Heritability and the environment. • What can and cannot be inferred from heritablity indices. • Culture and intelligence testing. • Modifying IQ scores. • My tentative conclusions. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

3. The U.S. Immigration Act of 1924 • Rigid quotas to restrict the influx of “biologically weaker stock” from southern and eastern Europe. • The proof? Members of these groups had lower Army Intelligence Scores than obtained by Northern European-Americans. • These scores turned out to correlate with length of time in the States, improving with English language proficiency and AUS cultural knowledge. • After years of residency, European are of origin had no discernible effect. • A seemingly hereditary difference was revealed as due to environmental factors related to ‘genetic’ ones. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

4. Group differences • Numerous studies have found that average IQ for U.S. blacks is 85-90, i.e., 10 to 15 points lower than for U.S. whites. • The dispute revolves around why this is so and whether much can be done about it. • The difference in average racial IQs decreases proportionate to the extent that environments are relevantly matched (e.g., on SES dimensions). • Also, average IQ for U.S. blacks adopted into white middle-class households is 110 (25 points higher than average blacks, 10 points higher than average whites). • Thus, differences in group means seem at least largely to be due to different environments. • The rest of the lecture concerns variation in individual scores within given populations. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

5. A distinction and an interaction • Genotypic traits are genetic potentialities. • Phenotypic traits are manifest characteristics. • All phenotypic traits are often claimed to be the result of an interaction between the genotype and the environment: “There can be no organism without a genotype, and this genotype cannot express itself independently of the environment” (Gleitman et al., 1999, p. 655) • Organisms are dependent on the genotype, but are all traits so dependent once the organism exists? Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

6. Heritability indices “a heritability index shows the proportional contribution of genetic or heriditary factors to the total variance of a particular trait in a given population under specific conditions” Anastasi & Urbina (1997, p. 297) E.g., A heritability index of .70 means that 70% of the variance is attributable to hereditary factors (with the remainder being attributable to the environment). Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

7. How heritability indices are calculated • Primarily, by examining resemblances among genetically related people. E.g., • Twin studies 1) Compare monozygotic and dizygotic twins. 2) Compare siblings (especially monozygotic twins reared together and apart. - Adoption studies • Compare adopted children with both biological and adopted relatives. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

8. Correlations between IQ scores for family members Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

9. Difficulties with heritability indices • Unassessed contributions of environmental factors. • Monozygotic (identical) twins may share more similar environments than do dyzygotic (fraternal) twins. • Distal environments can be similar and close ones rather different. • There may be genetic influences on environments. • Issues of generalisability from twins to non-twins. • E.g., Twins more likely to have prenatal trauma leading to severe mental retardation (and inclusion of such people raises heritability indices). Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

10. Inheriting differences Heritability coefficients are estimates of genes on individual differenceswithin a population. They are not estimates of the relative importance of genes and the environment. Westen (1998, p. 348) • They indicate determinants of variances (differences) not means (similarities). Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

11. Inheriting differences from differences Heritability estimates are highly dependent on the sample. Westen (1998, p. 348) • Heritability indices are frequently obtained from relatively homogeneous environments but relatively genetically diverse people. • Including a random sample of people from various environments would typically lower heritability indices. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

12. Inheriting differences from specific gene-situation interactions Any change in either hereditary or environmental conditions would [be liable to] alter the heritability index. Anastasi & Urbina (1997, p. 298) • E.g., making either the gene pool or the environment more or less homogeneous in ways that affect the characteristic of interest (e.g., intelligence). Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

13. Average inheritance effects Almost irrespective of the size of a heritability index in the population, a particular individual’s characteristics may be almost entirely determined by their genes, their environment, or any combination of the two. • E.g., intelligence is largely hereditary, but any particular instance of extraordinarily high or low (or even average) intelligence may result from extraordinary genetic conditions, extraordinary environmental conditions, or both. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

14. Indicating actualities, not potentials Even if the heritability index of a trait in a given population is 100%, it does not follow that the contribution of environment to that trait is unimportant. Anastasi & Urbina (1997, p. 298) • It simply follows that the environment is having no differential effect across individuals in that population. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

15. Cultural bias in IQ tests • Linguistic familiarity. • Vocabulary, dialect. • Content familiarity. • Information, concepts, tasks • Medium familiarity. • Paper and pencil, interview • Test motivation. “Every test tends to favour persons from the culture in which it was developed” Anastasi & Urbina (1997, p. 342) Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

16. Modifying IQ scores • IQ is not fixed: it is amenable to modification. • Educationally impoverished environments depress IQ. • Interventions (and other factors) that richen such environments increase IQ. • Worldwide improvements in IQ scores over time: 3 IQ points every 10 years (‘The Flynn effect’). Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

17. My (tentative) conclusions • Substantially overlapping variation in IQ scores across groups makes differential treatment of members of those groups an example of prejudice. • Whatever the difference in the means and the reasons for such difference. • Evidence for genetic differences in average IQ across groups is poor. • (I have encountered no relevant evidence about possible current differences in IQ heritability across groups.) • Evidence for environmental differences in average IQ across groups is strong. • Evidence for (current) genetic differences in IQ scores within groups is strong. • Evidence for the ability to influence IQ scores environmentally is strong. • All the above holds for ‘intelligence’ as well as for IQ, only more so. • Overall conclusion: From social science and societal perspectives, it makes sense to look for both genetic and environmental determinants of intellectual and other important functioning. • There is no compelling evidence that any such functioning is genetically ‘determined’. • As a consequence of this, it is better to recognize inhibitors and enhancers, and then learn how to potentially overcome the former and exploit the latter. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate

18. Points to ponder • Environmental improvements can increase average IQ scores. Would you predict that they also affect heritability indices for IQ with the relevant populations. If not, why not and, if so, why (and how)? • How convincing is Gleitman’s argument (p. 662) that studies such as Quay (1971) demonstrate that cultural bias in IQ tests is “evidently” not responsible for average racial difference in IQ scores? Hint: content (esp. information) bias. Approaches to Psychology: Lecture 26, The Nature-Nurture Debate