Do you consider intelligence to have a stronger genetic or environmental basis?

Authors Avatar

Intelligence: Nature or Nurture?                                                                                                                 Joe martin

  Do you consider intelligence to have a stronger genetic or environmental basis?

Since the term nature-nurture was first initiated by Galton (1883, c.f. Plomin, 1988) the question of whether intelligence has a stronger environmental or genetic basis has been the source of much controversy and debate (Bouchard., Lykken., McGue., Segal., & Tallegen, 1990a, 1991., Bouchard & McGue, 1981., Hernstein & Murray, 1994., Plomin, 1988., Scarr 1997).

Traditionally, research into intelligence has been diverged by two opposing positions; Behavioural Genetic Theory and Socialization Theory (Scarr, 1997). Whilst Socialization theory provides useful insights into the qualitative nature of differences in behaviour and intelligence, methods used by such research are criticised as being “antiquated” and “confounded” (Scarr, 1997, p.34) hampered by their inability to include genetically informative designs (Baumrind, 1993., Hoffman, 1991., Scarr, 1997). Behavioural genetic theory on the other hand not only describes the genetic contribution on intelligence (Bouchard & McGue, 1981., Bouchard et al., 1990a, 1991) it also emphasizes the importance of environmental influences which provides a crucial explanation for the major source of variation in behaviour (Eysenck., & Fulkner, 1983., Loehlin & Nicholls, 1976, c.f. Plomin, 1988., Plomin, Loehlin, & Defries, 1985, Plomin & Daniels, 1987., Scarr, 1997).

In light of the pioneering nature of behavioural genetics 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 behavioural genetics to provide a well-versed argument for both environmental and genetic influences on the development of intellectual abilities. In line with behavioural 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 behavioural genetics. Following an overview of traditional twin and adoption studies we shall then aim to examine the implications of the relevant data in line with the current debate.

According to Cooper (1999, p.69) “the basic aim of behavioural genetics, is not to understand the precise manner of which genes influence which behaviour, but determine the extent to which certain behaviours are influenced by our overall genetic make-up, and by various aspects of our environment” [emphasis added]. The underlying ideology of the theory is that individuals in a population differ as a consequence of both genetic and non-genetic factors.

If intelligence is found to have a substantive genetic component this would suggest that “intellectual resemblance can be predicted to the extent that parents, children and siblings are genetically related with little or no similarity in intelligence to people genetically unrelated though reared in the same house” (Scarr, 1997, p.13., Plomin & Daniels, 1987., Plomin,

1988). If on the other hand however intellectual development was solely determined by our environments, it seems unlikely that studying intelligence in terms of the genetic make up will be of use, and that studying the social and cognitive inputs would be more fundamental (Bandura, 1986., Baumrind, 1993., Bouchard & McGue, 1981., Eysenck, 1977, c.f. Plomin & Daniels 1987., Eysenck & Fulkner, 1983., Hoffman, 1991., Steele & Aronson, 1995., Wachs & Gruen, 1982).

To date, the most common research designs employed within the realm of behavioural genetics, the family, twin and adoption studies were developed to avoid the potential problem of conflicting influences in studies of family members who not only share heredity but also similar environments. Accordingly, designs of this nature typically partition environmental variance into two sub-categories; repeatedly referred to as “shared environment” (Plomin, 1988, p.16) and “non-shared environment” (p.17). The distinction between the two is as follows; shared environment- refers to the amount of variance in the environment that makes two family members more similar (Bouchard & McGue, 1981., Wilson, 1983) and non shared environment- is thus environmental variance that make members of a family different from one another (Jensen, 1997., McCall, 1983., Plomin & Daniels, 1987., Rowe, 1987, c.f. Plomin & Daniels, 1987).

Join now!

Twin designs focus on the comparison of the resemblance of same sex fraternal twins and monozygotic twins. As monozygotic twins are twice as similar genetically than fraternal twins the difference between the two can be taken as an adequate estimate of the genetic variance in the population (Plomin & Daniels, 1987., Scarr, 1997., Wilson, 1983). The central problem that thwarts the potential findings of the twin designs, however, concerns the extent to which environmental resemblance covaries with genetic relatedness. Adoption studies are far more powerful in this sense in that it is possible to confidently surmount these confounding interferences, ...

This is a preview of the whole essay