Abstract
A new analysis of the original data from the four largest studies (Newman, Freeman and Holzinger, 1937; Shields, 1962; Juel-Nielsen, 1965; Burt, 1955) of the intelligence of monozygotic twins reared apart, totaling 122 twin pairs, leads to conclusions not found in the original studies or in previous reviews of them. Statistical analysis of the twin differences reveals no significant differences among the twin samples in the four studies; all of them can thus be viewed statistically as samples from the same population. They can therefore be pooled for more detailed and powerful statistical treatment.
The 244 individual twins’ IQ’s are normally distributed, with the mean = 96.82, SD = 14.16. The mean absolute difference between twins is 6.60 (SD = 5.20), the largest difference being 24 IQ points. The frequency of large twin differences is no more than would be expected from the normal probability curve. The overall intra-class correlation between twins is.824, which may be interpreted as an upper-bound estimate of the heritability (h2) of IQ in the English, Danish, and North American Caucasian, populations sampled in these studies. The absolute differences between twins (attributable to nongenetic effects and measurement error) closely approximate the chi distribution; this fact indicates that environmental effects are normally distributed. That is, if P = G + E (where P is phenotypic value, G is genotypic value, and E is environmental effect), it can be concluded that for this population P, G, and E, are each normally distributed. There is no evidence of asymmetry or of threshold conditions for the effects of environment on IQ. The lack of a significant correlation (r = -0.15) between twin-pair means and twin-pair differences indicates that magnitude of differential environmental effects is not systematically related to intelligence level of twin pairs.
Keywords
Twin Pair Scatter Diagram Behavior Genetic Twin Sample Twin CorrelationPreview
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