Genetics peer commentary Genetics and OUSINS C DY behaviour A SK any psychologist to complete the following phrase: ‘nature–nurture _________’. A ROBERT PLOMIN argues that psychologists should The answer will no doubt be ‘debate’ or ‘controversy’. But the controversy that prepare to maximise the benefits and minimise the swirled around behavioural genetics research in psychology during the 1970s risks that will emerge from DNA research. has largely faded. During the 1980s and, especially, the 1990s, psychology became In my view, this choice represents one as they are to genetic influences. In fact, much more accepting of genetic influence, of the most dramatic shifts in the modern behavioural genetic research provides the as can be seen in the increasing number of history of psychology. Indeed, the wave strongest available evidence for the behavioural genetic articles in mainstream of acceptance of genetic influence in importance of environmental factors. But psychology journals and in research grants. psychology is growing into a tidal wave in some areas of psychology, especially One symbol of this change was the 1992 that threatens to engulf key messages psychopathology, the pendulum centennial conference of the American coming from behavioural genetic research. representing the accepted view may be Psychological Association. In preparation The first message is that genes play swinging too far from environmental for the conference, a committee selected a surprisingly important role throughout determinism to genetic determinism. two themes that best represented the past, psychology. But the second message is present, and future of psychology. One of just as important: individual differences Perspectives the two themes chosen was behavioural in complex psychological traits are due at Behavioural genetics focuses on questions genetics (Plomin & McClearn, 1993). least as much to environmental influences of why individuals within a species differ 134 The Psychologist Vol 14 No 3 March 2001 Genetics peer commentary in behaviour (e.g. why children differ in behavioural genetics and evolutionary increase in risk over the general population. rates of language acquisition), whereas psychology are so different that I think it The results of this small study have been much research in psychology investigates causes confusion to conflate the two fields. replicated in other twin studies (as species-typical behaviour (e.g. the average I should emphasise that perspectives are reviewed by Bailey et al., 1996). age at which children use two-word not right or wrong, just more or less useful sentences). Descriptions and explanations to address particular questions. The twin method Why are identical of species-typical behaviour bear no twins so much more concordant for autism necessary relationship to descriptions and Autism as an example than non-identical twins? The most explanations of individual differences As recently as the 1970s autism was parsimonious explanation is that identical within a species. For example, the fact that thought to be caused by cold, rejecting twins are much more alike genetically. our species begins to use two-word parents. Certainly, parents whose children Another hypothesis puts the blame on sentences at the average age of 18 months are autistic behave differently towards their prenatal factors. Identical twins often share is an evolutionary adaptation ultimately children compared with parents of non- the same chorion (the outermost membrane due to selection of genes, but this does not autistic children, but the direction-of-effects surrounding the foetus during prenatal mean that genetics is responsible for the question looms large: Are the differences in development), which might make them delayed use of two-word sentences by parenting cause or effect? The accident of more similar than non-identical twins (who some children. nature that results in identical never share the same chorion). So far the The fundamental accomplishment (monozygotic) twins or non-identical scanty evidence relevant to this issue is of genetic research in psychology to date (fraternal or dizygotic) twins provides one mixed (Sokol et al., 1995). Another has been to demonstrate the ubiquitous way to address this question. Identical possibility is that twins may not be importance of genetics throughout twins are like clones, genetically identical representative of the non-twin population psychology. As described later, this to each other because they came from the because of adverse intra-uterine evidence consists of twin studies that same fertilised egg. Non-identical twins, on environments caused by sharing a womb compare the similarity of identical and the other hand, developed from two eggs (Phillips, 1993). However, the statistical non-identical twins; and adoption studies that happened to be fertilised at the same distributions for most psychological that consider the resemblance of adopted- time. Like other siblings, they are only half dimensions and disorders for twins and away children to their biological parents. as similar genetically as identical twins. non-twins are generally similar (e.g. These methods and the theory that At the Institute of Psychiatry Michael Christensen et al., 1995). underlies them are called quantitative Rutter and his colleague Susan Folstein A subtle but important factor is that genetics, in contrast to molecular genetic were the first to use the twin method to identical twins might have more similar research, which attempts to identify investigate the causes of autism (Folstein & experiences than do non-identical twins specific genes. Behavioural genetics Rutter, 1977). They reasoned that if autism after they are born. The use of the twin includes both quantitative genetic research is caused by parental treatment, then non- method is based on the assumption that the and, increasingly, molecular genetic identical twins ought to be as similar environments of non-identical twins reared research. Although this brief target article (concordant) for autism as are identical in the same family are approximately as is limited to human behavioural genetics, twins, because both types of twins are similar as the environments of identical more powerful quantitative genetic and reared by the same parents in the same twins reared in the same family. This molecular genetic methods are available for place and the same time and presumably assumption has been tested in several ways analysing animal behaviour (Plomin et al., get similar treatment. But if autism is and appears reasonable for most traits, 2001). influenced by genes, then non-identical although it has not been tested specifically We can see this focus on species-typical twins ought to be less concordant. Folstein in regard to autism (Bouchard & Propping, behaviour in several key areas of and Rutter located 10 autistic children in 1993). Although the possibility remains psychology. For example, experimental Britain who were non-identical twins. In that identical twins may be treated more psychology implicitly studies species- none of these cases was the other twin also alike by their parents because they are typical behaviour, comparing an autistic. This result was not surprising, more similar in appearance and behaviour, experimental group with a control group because autism is rare (an incidence of the twin method provides a rough but representing typical behaviour, with about one in a thousand) and it was already useful screen to unpack the ‘bottom-line’ individual differences considered as an known that only about 3 per cent of the effects of genes and environment (Martin error term in an analysis of variance. non-twin siblings of autistic children are et al., 1997). Similarly, much molecular genetic research autistic. The surprise came in the result for consists of experimentally mutating a gene identical twins. Folstein and Rutter found The adoption method Although there so that it is no longer expressed, and 15 children diagnosed as autistic who were are no adoption studies of autism, the comparing these mutated animals with identical twins. Eight of these children adoption method is another quasi- normal animals; whereas behavioural were in four concordant pairs of twins in experimental design that has a different genetics has focused on naturally occurring which both identical twins were diagnosed set of assumptions and potential problems. genetic variation. Finally, evolutionary as autistic. The incidence of autism in Family members normally share both psychology also focuses on species-typical children who have an autistic sibling, heredity (first-degree relatives correlate behaviour using comparisons between though low in absolute terms, represents .50 genetically) and environment (they species as evidence for evolutionary a risk that is 30 times higher than that for share the same family). Thus, familial adaptations. Although this is a type of children whose siblings are unaffected; the correlations cannot tell us about the relative genetic analysis of behaviour, the incidence in children who have an autistic extent to which genetic and environmental perspectives and empirical foundations for identical twin represents a 500-fold factors contribute to observed resemblance 135 March 2001 The Psychologist Vol 14 No 3 Genetics peer commentary between family members. The adoption I found that adoptive families are Another direction for research concerns method separates the effects of nature and reasonably representative of the population the