Essay Questions
1. What evidence is there that intelligence runs in families?
Key Points:
- Our genotype is the genetic complement, coded in DNA, that we inherit from our parents. No two people have identical genotypes except identical twins.
- The expression of those genes in behavioural traits that we can measure is our phenotype. Phenotypes can vary because of genotypic differences and/or because the environment affects how our genes are expressed.
- IQ test scores are phenotypic measures. Intelligence is one of the most frequently researched traits in behavioural genetics because IQ represents one of the most reliable and important psychological measures.
- Genetic contributions to IQ differences can be estimated by comparing the similarity of IQ in individuals of different degrees of genetic relatedness while also assessing environmental similarities and differences. Heritability is a statistic that represents the proportion of phenotypic variance that is due to genetic differences – that is, the extent to which differences in measured intelligence are due to genetic differences. The maximum heritability is 1.0 (100 per cent of the difference is inherited) and the minimum is zero (none of the difference is due to genetic differences).
- The influence of the environment on phenotypes comes in two main forms. There are differences between families (levels of income, parental rearing style, number of books in the home, etc.) which make children raised in a particular home more similar to each other than to children reared in a different home. This source of differences is often called the effect of the shared environment. The second kind of environmental influence are differences within the same family (in birth order, children’s friends, school teachers, etc.). These effects make children in the same family different from each other and are referred to as non-shared environment effects.
- We can measure the influence of the common, or shared, environment by comparing individuals who are reared together or apart. The effect of non-shared environmental variance can be detected in a number of ways. The most obvious is to measure the extent to which identical twins reared together (i.e. with both genetic and shared environmental variance in common) are different from each other due to the non-shared environmental influences they may experience when growing up (e.g. at school, or from peers).
- Studies on the influence of genetic differences on intelligence are in broad agreement. Intelligence, as measured by IQ tests, has a substantial heritability. Estimates of heritability vary between 80 per cent (Bouchard et al., 1991) and 50 per cent (Plomin 1990). So even the more conservative estimates argue that genetic differences are far from trivial – they are at least as important as environmental differences, and maybe more so.
- The many studies from the Colorado Adoption Project (see Plomin, 1990) suggest that the shared environment is more influential early in development than in later life. For example, the correlation between adopted children and their biologically unrelated siblings (who are usually reared from birth in the same family) averages around 0.2–0.3 before their teenage years. Over the whole lifespan it seems that the most important environmental differences are those that are non-shared and unique to the individual concerned (that is, they are not shared by members of the same family).
- In a review of adoption and twin studies, Scarr (1992) estimated that the contribution of the shared environment to differences in IQ is approximately zero by adulthood. Irrespective of our shared environment, most of us find ways ultimately to realize our genetic potential, depending on the effects of our idiosyncratic life events (i.e. non-shared environment).
- Most recently, great excitement has surrounded the methodology of quantitative trait loci (QTL), which attempts to associate particular genes with specific behaviours. The general consensus is that intelligence must be polygenic, which means that many genes contribute in an additive or dose-related fashion to IQ differences. If this is right, current QTL methods have very little chance of discovering the individual genes that each contribute only a relatively small proportion to the overall genetic effect. Even so, some researchers claim to have discovered a gene that is over-represented in individuals with a very high g (Chorney et al., 1998). While exciting, this methodology is new, and its results should be treated with caution.
- Almost everyone now accepts that there are genetic influences on IQ differences, but the most important recent discoveries concern environmental rather than genetic influences, particularly the finding that it is the non-shared environment that has a lasting effect on individual intellectual differences. The challenge is to move on from the heritability issue to theories of how genetic predispositions may interact and correlate with environmental circumstances to produce the patterns of IQ differences that we find in our society (see Scarr, 1992).
