1. Kinship IQ Correlations

2. References a) Erlenmeyer-Kimling, L., & Jarvik, L.F. (1963). Genetics and intelligence: A review. Science, 142, 1477-1478. b) Jarvik, L.F., & Erlenmeyer-Kimling, L. (1967). Survey of familial correlations in measured intellectual functions. In J. Zubin & G.A. Jervis (Eds.), Psychopathology of Mental Development. New York: Grune and Stratton. c) Bouchard, T.J. & McGue, M. (1981). Familial studies of intelligence: A review. Science, 212, 1055-1059. d) Bouchard, T.J. (1993). The genetic architecture of human intelligence. In P.A. Vernon (Ed.), Biological Approaches to the Study of Human Intelligence. New Jersey: Ablex. e) N = number of studies f) r = median correlation from each set of N studies

3. Heritability Estimates From Kinship Correlations 1. h 2 = r MZA =.70 2. h 2 = 2(r MZT - r DZT ) = 2(.86 -.58) =.56 3. h 2 = 1 - r UT = 1 -. 31 =.69 4. h 2 = 1 - r VT = 1-.26 =.74 5. model-fitting: h 2 =.50 Note also: r MZA (entirely genetic) + r UT (entirely environmental) =.70 +.31 = 1.01

4. AbilitiesTwinCorrelations Number of StudiesIdentical TwinsFraternal Twinsh2h2 verbal comprehension27.78.59.38 verbal fluency12.67.52.30 reasoning16.74.50.48 spatial visualization31.64.41.46 perceptual speed15.70.47.46 memory16.52.36.32 Average Twin Correlations for Specific Cognitive Abilities

5. Correlations Between Tests’ g-Loadings and their Heritabilities

6. Studies of Twins Reared Apart Bouchard: Minnesota study –74 pairs of MZA’s –54 pairs of DZA’s Pedersen: Swedish study –46 pairs of MZA’s –100 pairs of DZA’s

7. AbilitiesHeritabilityEstimate (%) BouchardPedersen verbal5758 spatial7146 speed5358 memory4338 Heritability Estimates for Specific Cognitive Abilities in Two Studies of Twins Reared Apart

8. Kamin To the degree that the case for a genetic influence on IQ scores rests on the celebrated studies of separated twins, we can justifiably conclude that there is no reason to reject the hypothesis that IQ is simply not heritable. (e.g., Kamin, 1974, p. 67; cf. also Kamin in Eysenck & Kamin, 1981, p. 154; Lewontin, Rose & Kamin, 1984, pp. 106-110)

9. Farber My own evaluation, particularly of the allegedly scientific analyses of the IQ data, is more caustic. Suffice it to say that it seems that there has been a great deal of action with numbers but not much progress-- or sometimes not even much common sense. (Farber, 1981, p. 22).

10. Taylor In sum, given the available methods and data, there once again appears to be no compelling reason to postulate the existence of any genes “for” intelligence. (Taylor, 1980, p. 111).

11. Criticisms of MZA Studies 1.Placement Bias

12. Criticisms of MZA Studies Assumptions: a) High interrater agreement b) Looks more important than behavior c) MZAs actually look alike d) Treatment can raise or lower IQ 1.Placement Bias 2. Pygmalion Effect and MZA’s look alike

13. Criticisms of MZA Studies 1.Placement Bias 2. Pygmalion Effect and MZA’s look alike 3. Pseudoanalyses a) Highly selective subgroups of data b) Liberal use of significance tests c) Constructive replication is ignored d) Sample sizes are often very small

14. Taylor The similarity in educational, socioeconomic, and interpersonal environments, referred to here as social environment, is a central reason why monozygotic twins regarded in the professional literature as separately raised reveal similar IQ scores. MZ twin pairs who have had similar social environment (such as similar schooling) have similar IQs, and twin pairs who have relatively different social environments (especially different schooling) have different IQs. (p. 92).

15. Effect of Similar Environments on MZA Correlations

16. Effect of Being Raised by Relatives on MZA Correlations

17. Effect of Being Reunited on MZA Correlations

18. If partial reunions can cause MZA’s to correlate.84, then why do: Non-twin siblings raised together only correlate.46? DZ twins raised together correlate.58? Unrelated adoptees raised together correlate.31? Virtual twins raised together correlate.26? Effect of Being Reunited on MZA Correlations

19. Developmental BG Studies of Mental Ability Louisville Twin Study: –500 pairs of twins tested from 3 months to 15 years Colorado Adoption Project: –245 adoptive families + 245 matched control families –adoptees (from infancy to 16 years), both adoptive parents, all birth mothers, and 25% of birth fathers Colorado Infant Twin Project –200 pairs of twins tested at 7, 8, and 9 months

20. Developmental BG Studies of Mental Ability MacArthur Longitudinal Twin Study: –200 pairs of twins tested at 14, 20, 24, and 36 months Western Ontario Twin Project –200 pairs of twins tested between 3 months to 6 years –150 additional 4- to 6-year-old twins tested in Vancouver

21. Twin Studies of Infant Cognitive Ability Age (in months)TwinCorrelations Identical Twins Fraternal Twins 3.66.67 6.75.72 9.67.51 12.68.63 18.82.63 24.85.65 30.88.79 36.88.79 Wilson (1983)

22. Mental Ability Developmental Profiles in Pairs of MZ and DZ Twins

23. MZ, DZ, and Sib Correlations for Mental Ability at 3 Age-Ranges

24. Correlations of Interest in the Colorado Adoption Project Non-adoptive Parent/Own Offspring: –familiality: upper limit estimate of genetic and shared environmental influences Biological Parent/Adopted-away Offspring: –direct estimate of genetic effects Adoptive Parent/Adopted Offspring: –direct estimate of environmental effects

25. Adoption Studies of Infant Cognitive Ability

26. Twin Studies of Childhood IQ StudyAgeTwinCorrelations Identical TwinsFraternal Twins Wilson (1983)3.88.79 4.83.71 5.85.66 6.86.59 7.84.59 8.83.66 9.83.65 Segal (1985)5-13.85.42

27. Adoption Studies of Childhood IQ Correlations StudyAgeBiological MotherAdoptive Parent Burks (1928)5-14.13 Freeman, Holzinger, & Mitchell (1928)2-22.32 Leahy (1935)5-4.14 Snygg (1938)3-5+.11 Skodak & Skeels (1949)4.28 7.38 Fisch, Deinard, & Chang (1976)4.07 7.08 Scarr & Weinberg (1977)4-16+.19 Horn (1983)5-7.36 5-9.15 Plomin & Defries (1985)3.18.15 4.22.15 Phillips & Fuller (1989)7.37-.05 (M).11 (F)

28. Molecular Genetic Studies of IQ Intelligence influenced by many genes, each exerting only a small influence QTL designs need to be very powerful to detect small genetic effects To-date, only a few QTL studies of IQ and limited (if any) replication Plomin’s IQ QTL Project