Does intelligence have a stronger genetic or environmental basis?

Paul B.

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Does intelligence have a stronger genetic or environmental basis?

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Abstract

In light of the pioneering nature of behavioral genetics and in terms of explanatory power and predictive ability, highlighting constructive directions for future research, the aim of the current essay is to examine a wealth of evidence within the field of behavioral genetics to provide a well-versed argument for both environmental and genetic influences on the development of intellectual abilities. In line with behavioral genetic theory, it is possible to systematically test the degree to which variability is the result of genetic or non-genetic individual differences (Loehlin, Horn & Willerman, 1989., Plomin & Daniels, 1987). By examining the relevant research therefore, it should be possible to determine the extent to which environmental or genetic influences provide the most adequate explanation for a basis of intelligence. In order to understand the empirical findings of research pointing to a genetic or environmental basis of intelligence it is first necessary to briefly identify the basic aims and methods used in the study of behavioral genetics.

Introduction
Understanding the empirical findings of research pointing to a genetic or environmental basis of intelligence
Twin and Adoption studies and predictions to be made regarding the environmental contributions in acquired intelligence
Empirical research within the behavioral genetics
Twin designs: The most reliable estimate of the genetic component of intelligence
Large-scale studies of adoptees: Texas, Minnesota and Colorado
The conclusion that shared environment has a greater influence on environmental variation
Conclusion
Bibliography

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[...] Whether this portion of variance has a stronger shared or non-shared environmental influence is slightly less abated however. Typically findings of adoption research yield an estimate of 30 per cent of the variance attributable to the environment shared between parents, offspring and siblings (Loehlin et al Scarr & Weinberg c.f. Plomin & Daniels Scarr Wilson, 1983). In understanding the literature, it was noted that studies of twins reared together provided the most direct estimate of non-shared environment. Findings of such studies (Bouchard et al., 1990a Thompson c.f. [...]

[...] Jr., (1997). IQ similarity in twins reared apart: Findings and responses to critics. In R. J. Sternberg & E. Grigorenko (1997) Intelligence; Heredity and Environment. Cambridge: Cambridge University Press. Cooper, C. (1999). Intelligence and abilities. London: Routledge. Eysenck, H. J., & Fulkner, D. W. (1983). The components of type A behavior and its genetic determinants. Personality and Individual differences 499-505. Hernstein, R. J., & Murray, C. (1994). The bell curve: Intelligence and class structure in American life. New York: Free Press. [...]

[...] Sternberg & E. Grigorenko (1997) Intelligence; Heredity and Environment. Cambridge: Cambridge University Press. Loehlin, J.C., Willerman, L. & Horn, J. M. (1985). Personality resemblance in adoptive families when the children are late adolescents and adults. Journal of Personality and Social Psychology 376-392. Loehlin, J.C., Horn, J. M., & Willerman, L. (1989). ‘Modelling IQ change: Evidence from the Texas Adoption Project'. Child development 993-1004. McCall, R. B. (1983). Environmental effects on intelligence: The forgotton realm of discontinuous non-shared within family factors. [...]

[...] There is no doubt that intelligence has a far stronger genetic basis than environmental, however it is worth noting that behavioral genetics have a lot to offer by means of truce and reconciliation with environmentalists (Scarr, 1997). Behavioral genetics offer a number of worthwhile approaches which in conjunction with a number of socialization or parental measures can be used to assess non-shared environmental to a more profitable degree. Plomin & Daniels have acknowledged an approach “specific to parental behaviors that appears to be a step in the right direction” (1987, p.54). [...]

[...] Twin studies reared together provide evidence for the most direct measures of non-shared environment (Plomin & Daniels Wilson, 1983) whilst evidence from a wealth of twin studies reared apart reliably suggest that a substantial amount of variance in measured intelligence has a underlying genetic basis (Bouchard et al., 1990a, 1991). How can we assess the extent to which individual variability is determined as a result of genetic or non-genetic factors then? Empirical research within the behavioral genetics realm is vast, in order to keep the discussion succinct it should be noted that only the most stringent twin and adoption studies are employed, it is considered that these characterize the most productive points of the argument. [...]

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