A second study of familial resemblance in intelligence: Environmental and genetic implications of parent-child and sibling correlations in the total sample.

Abstract

This chapter describes a continuation of a previous study (12) that appeared in the Twenty-Seventh Yearbook of this Society. The earlier report was based on intelligence measurements of husband, wife, and two or more children in each of 105 families. The present study is based on 997 cases in 269 family groups, including the previous 105 families, and 164 additional families of two or mare members.Studies of familial resemblance impose no direct control on either heredity or environment, and for this reason cannot yield any direct answer to the problem of the relative contribution of heredity and environment to individual differences. 2 In spite of this, such studies 1 Assistance in the preparation of these materials was furnished by the personnel of Works Progress Administration Official Projects Nos. 65-3-5403 and 65-3-5406. In connection with the field work, acknowledgment is due to the Social Science Research Council of Columbia University for a grant in aid and to Professor A. T. Poffenberger as representative of the Council. 3 We do not wish to overemphasize the difference between the biometric and experimental approaches in this field, particularly where human traits are concerned. The biometric studies are never entirely without the equivalent of some experimental control, and the experimental studies never completely achieve the desired control of all relevant factors. Thus, the present biometric study, having as its locale a comparatively homogeneous rural environment, achieves some control of interfamilial environmental variations. The studies by Pearson (18), Elderton (6), and Wilcocks ( 22) have exploited this type of experimental feature of purely biometric studies; unfortunately, these investigators overlooked the importance of the ratio of intrafamilial to interfamilial environmental variation (10), and the possibility of a significant positive correlation between the two.

An Investigation of Similarities in Parent‐child Test Scores for Evidence of Hereditary Components

“…There does not seem to be any trend towards correlations with the mothers' scores being any higher or lower than the fathers' scores for either sons or daughters. This seems to refute any conjecture about the environmental power of the early rearing of the mother over that of the father (Conrad & Jones, 1940). …”

“…This correlational method is unsatisfactory for providing evidence of genetic influences, especially in behavioral traits, since it is nearly impossible to separate the effects of environment from those of heredity. Contrary to this opinion, Conrad and Jones (1940) maintained that if mother-son and mother-daughter correlations are approximately equal to father-son and father-daughter correlations, it is evidence for a genetic influence on the trait. They reason that if environmental effects predominate, the mother-child coefficients should be higher than those of father-child because of her greater influence during the formative years.…”

“…Few studies examined sex differences and only one tested the hypothesis that there might be other than an autosomal mode of inheritance. In fact, only two studies involved a discussion of genetic theory at all (Conrad & Jones, 1940;Woodburn, 1954). A few of the studies did use foster or adopted children as controls, however.…”

“…In a study involving adopted children as controls, Leahy (1935) A replication of Jones' study (Conrad & Jones, 1940) More than one child of the same family was included in the study. No correction for age had been made although age differences were reported.…”

A Constitutional Correlate of Early Intellective Functioning

“…Other investigators have demonstrated that monozygotic twins are more similar in IQ than dizygotes, and that the correlation between the IQ of parent and child is moderately high (r = .50 to .60) (3,4,9). These data:…”