Kinship IQ CorrelationsKINSHIP EXPECTED E-K and Ja E-K and Jb BOUCHARDc BOUCHARDd

Ne rf N r N r MZ TOGETHER 1.00 14 .87 14 .87 34 .86 MZ APART

1.00 4 .75 4 .75 3 .67 4 .7

DZ TOGETHER .50 20 .53 41 .58 DZ APART .50 1 .5 DZ SAME SEX .50 11 .53 11 .56 DZ OPPOSITE SEX

.50 9 .53 9 .49

FULL SIBS TOGETHER

.50 35 .49 36 .55 60 .46 1 .5

FULL SIBS APART

.50 2 .40 3 .47 2 .24 1 .4

PARENT-CHILD TOGETHER

.50 12 .50 12 .50 32 .39

PARENT-CHILD APART

.50 4 .22

HALF SIBS TOGETHER

.25 2 .35

HALF SIBS APART

.25 1 .2

GRANDPARENT-CHILD

.25 3 .27

UNCLE/AUNT-CHILD

.25 1 .34

FIRST COUSINS .125 3 .26 4 .145 SECOND COUSINS

.063 1 .16

FOSTERPARENT-CHILD

0 3 .20 3 .20 6 .18

UNRELATED TOGETHER

0 5 .24 5 .24 6 .31/0 1 .0

UNRELATED APART

0 4 -.01 4 -.01

VIRTUAL TWINS 0 R = .26 BASED ON 90 PAIRS

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

Heritability Estimates From Kinship Correlations

1. h2 = rMZA = .70

2. h2 = 2(rMZT - r DZT) = 2( .86 - .58) = .56

3. h2 = 1 - rUT = 1 - . 31 = .69

4. h2 = 1 - rVT = 1- .26 = .74

5. model-fitting: h2 = .50

Note also: rMZA(entirely genetic) + rUT (entirely environmental)

= .70 + .31 = 1.01

Abilities Twin Correlations

Number of Studies Identical Twins Fraternal Twins h2

verbal comprehension 27 .78 .59 .38

verbal fluency 12 .67 .52 .30

reasoning 16 .74 .50 .48

spatial visualization 31 .64 .41 .46

perceptual speed 15 .70 .47 .46

memory 16 .52 .36 .32

Average Twin Correlations for Specific Cognitive Abilities

Correlations Between Tests’ g-Loadings and their Heritabilities

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

Abilities Heritability Estimate (%)

Bouchard Pedersen

verbal 57 58

spatial 71 46

speed 53 58

memory 43 38

Heritability Estimates for Specific Cognitive Abilities in Two Studies of Twins Reared Apart

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)

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).

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).

Criticisms of MZA Studies

1. Placement Bias

Criticisms of MZA Studies

Assumptions:a) High interrater agreementb) Looks more important than behaviorc) MZAs actually look aliked) Treatment can raise or lower IQ

1. Placement Bias

2. Pygmalion Effect and MZA’s look alike

Criticisms of MZA Studies

1. Placement Bias

2. Pygmalion Effect and MZA’s look alike

3. Pseudoanalyses

a) Highly selective subgroups of datab) Liberal use of significance testsc) Constructive replication is ignoredd) Sample sizes are often very small

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).

Effect of Similar Environments on MZA Correlations

Effect of Being Raised by Relatives on MZA Correlations

Effect of Being Reunited on MZA Correlations

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

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

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

Twin Studies of Infant Cognitive Ability

Age (in months) Twin Correlations

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)

Mental Ability Developmental Profiles in Pairs of MZ and DZ Twins

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

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

Adoption Studies of Infant Cognitive Ability

Twin Studies of Childhood IQ

Study Age Twin Correlations

Identical Twins Fraternal 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

Adoption Studies of Childhood IQCorrelations

Study Age Biological Mother Adoptive 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)

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