intelligence of heritability, The

Canadian Psychology, Jul 1994 by Wahlsten, Douglas

Abstract

The influences of heredity and environment on behaviour are sometimes quantified as a heritability ratio, which assigns a percentage of variation in test scores to variation in the genotypes of individuals. There are compelling reasons, both biological and statistical, to doubt the validity of the common practice of partitioning variance in this manner. This paper outlines the conceptual foundations and explains the weaknesses of heritability analysis, reviews evidence of heredity - environment interaction during development, and argues for an alternative research strategy to detect and understand the functions of specific genes relevant for individual differences in behaviour.

The importance of heredity for human intelligence and other mental attributes is a perennial topic of debate in psychology with a history extending to Rousseau and earlier. Since 1970 these questions have been addressed by a specialization in psychology calling itself behaviour genetics and having its own professional society and journal. The dominant school of thought in this subdiscipline asserts that almost every human characteristic is determined by both the genetic inheritance and the life experience of the individual, and it seeks to estimate the relative strengths of these two factors. The index commonly used to summarize the results for a specific behaviour is the heritability coefficient, which is the most salient feature of the academic discipline of behaviour genetics. Studies of twins and adopted children have claimed substantial heritability of everything from time spent watching television to religious conservatism and what brand of beer you prefer (Plomin et al., 1990; Rosen, 1987; Waller et al., 1990). At the 1991 Behavior Genetics Association meeting in St. Louis, among the 64 presentations involving studies of human beings, 45 involved heritability analysis. Recent feature articles in Science touted heritablity analysis as the state of the art in the discipline (Bouchard et al., 1990; Plomin, 1990). Many articles have been devoted to the likely value of the heritability of intelligence in particular. The present article focusses on the concept of heritability itself and questions its intelligence.

Heritability in the broad sense (h'Symbol not transcribed'B'Symbol not transcribed'2) is said to estimate the proportion of variance in a measure of behaviour or other phenotype (V'Symbol not transcribed'Y) in a breeding population that is attributable to genetic variation (h'Symbol not transcribed'B'Symbol not transcribed'2 = V'Symbol not transcribed'G/V'Symbol not transcribed'Y). The estimation of this parameter involves a model based on the inheritance of genes via the principles of Mendel, which are well established, plus assumptions about how genetic effects are related to environment and behaviour, which are still contentious.

Genetic Facts

Before presenting the main argument, a few genetic facts should be considered. A gene is a segment of a DNA molecule occurring at a specific locus or place along the DNA, and it codes for the structure of a polypeptide molecule that may function as a protein, enzyme or hormone. A person's genotype is the pair of genes he or she has at the locus, one coming from each parent. There are perhaps 50,000 (possibly as many as 100,000) different genes in the 23 human chromosomes, each coding for a specific polypeptide (McKusick, 1991). Of these, about 30,000 are expressed as distinct proteins in the brain and 20,000 are specific to the brain (Sutcliffe, 1988), which gives some idea of the complexity confronting those who hope to understand the relation of heredity to brain development and behaviour. There are also 37 genes in the mitochondria in the cell cytoplasm deriving mainly from the mother (Wallace, 1991), although evidence from mice indicates a few mitochondria may be paternal (Gyllensten et al., 1991). As of October, 1990, there had been 1,909 chromosomal genes, perhaps 2% of the total number, identified and mapped to an approximate location on a chromosome (Stephens et al., 1990), and one year later the count was 2,144 (Chipperfield et al., 1991). The basic principles of genetic inheritance and gene action are well documented, but the era of discovering new genes has barely begun. In some instances people carry slightly different forms or alleles of a gene at a particular locus, such that each form codes for a slightly different structure of the protein molecule. When this occurs, the locus is said to cause a protein polymorphism in the population and can give rise to noteworthy differences between individuals in their blood types, immune system functions, brain structures or behaviours. Less than 5% of the human genetic loci are believed to cause substantial protein polymorphism. If 20,000 genes are unique to the brain, perhaps 19,000 will be effectively the same in almost all people, whereas only 1,000 will give rise to individual differences. From a genetic perspective, what people have in common vastly exceeds what makes us different; genetic differences between human beings and our close relative the chimpanzee are far greater than the modest variation among people of different ancestries (Vigilant et al., 1991). The methods of Mendelian genetics are sensitive only to the small minority of genes which are polymorphic and tend to make us different; they allow no meaningful statements about the role of heredity in general. Sometimes a defect in a single gene may have obvious consequences, such as in colour blindness, phenylketonuria or cystic fibrosis. On the other hand, some genes have rather slight effects that require large samples