Biological Approaches

1. Biological Approaches

2. Lecture contents • Genotype and phenotype: Determination and interaction • Group differences. • Heritability indices. • Population variation in IQ: Heritability and the environment. • What can and cannot be inferred from heritability indices.

3. Influential types • Genotypic characteristics are genetic potentialities. • Monogenetic and polygenetic influence. • E.g., brown eyes and height. • Phenotypic traits are manifest characteristics. • I.e., influenced by some mix of genes and environment. • Genetic determination • If gene, then phenotype (irrespective of environment) • Gene-environment interaction • If gene and a particular environment, then a particular phenotype

4. Group Differences

5. Group Differences: The U.S. Immigration Act of 1924 • Lower mean ‘intelligence’ for southern and eastern than for northern Europeans in America. • U.S. Immigration Act of 1924 set rigid quotas to restrict the influx of these “biologically weaker stock.” • ‘Intelligence’ measure scores improved with years of residency • Familiarity with language and with USA cultural knowledge. • No group differences after years of residency. • A seemingly hereditary difference was revealed as due to environmental factors contingently associated with ‘genetic’ ones.

6. Group differences: Do not imply particular causation • Numerous studies have found that average IQ for U.S. blacks is 85-90, i.e., mean 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.

7. Individual Differences

8. Heritability indices • “a heritability index shows the proportional contribution of genetic or hereditary 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 of the characteristic within that population (the individual differences within that population) is attributable to hereditary factors (with the remainder being attributable to the environment).

9. Contributions to heritability indices • Examine individual difference patterns with: • Genetic differences • On average: • Identical twins (MZ) share 100 % of genes • Non-identical twins (DZ) and other siblings share 50% of genes • Parents and offspring share 50% of genes • Environmental differences • On average: • Environments more similar within families than across them

10. Correlations between: • 100% genesIQHI-E • Identical twins reared together .86 .90 .48 • Identical twins reared apart .72 .92 .41 • 50% genes • Non-identical twins reared together .60 .56 .12 • Siblings reared together .47 .52 .20 • Parents and offspring raised by them .42 - .19 • Siblings reared apart .24 - - • Parents and offspring raised by others .24 - - • 0% genes • Adoptive siblings .32 -.07 -.06 • Adoptive parents/offspring .24 - .00

11. Heritability calculation: Rationale • MZ twins raised together have an average correlation of .48 between their extraversion scores (Pervin, 2003, p. 52). • DZ twins raised together have an average correlation of .12 between their extraversion scores (Pervin, 2003, p. 52). • r(MZ) of .48 results from G + E • r(DZ) of .12 results from G/2 + E • If we assume identical environmental influences for both sets of twins • If we assume average of half as much genetic influence on DZ than MZ twins • If we assume that genes have no interactive influence (otherwise 50% genetic similarity is an over-estimation for DZ twins) • r(MZ) -r(DZ) = (G + E) - (G/2 + E) = G/2 • .48 - .12 = .36 = G/2 • Therefore G (for twins reared together) = .72

12. Inheriting differences • “Heritability coefficients are estimates of the influence of genes on individual differenceswithin a population. They are not estimates of the relative importance of genes and the environment.” • Westen (1998, p. 348) • What leads to differences within a specific population (variance) may be different to (or trivial in comparison with) things that lead to differences in other populations (and therefore across populations). • C.F. Restriction of range issues.

13. 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. • “Any change in either hereditary or environmental conditions would [probably] alter the heritability index.” • Anastasi & Urbina (1997, p. 298)

14. 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.

15. 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.

16. Over to you! • “Plomin (1990a, p. 70) describes six different bases for calculating the heritability of IQ. The resulting heritability estimates vary from…30% to 72% of the variance being attributed to genetic variance.” • Pervin (2003, p. 52) • I am happy to think that many human characteristics have a genetic component that set limits on human possibilities. • Few genetic influences operate independently of the environment. • Few genetic influences cannot be modified, especially in expression.