Special issue: Behaviour genetics

I can understand heritability studies on pigs' weight or cows' milk production or on human height, but what does (this cognitive process) represent that one can directly relate to biological processes?

So wrote a mammalian physiologist recently in reviewing a colleague's grant application for a behaviour – genetic study of a cognitive process. Apart from his distaste for any kind of reductionism, his doubts that a psychological process could be subject to inherited variation are obvious. Doubts such as this are not unique among scientists, nor among research and practising psychologists. For some, the unacceptability of the idea stems from adherence to a central plank of the Blank Slate (Pinker, 2002), its insistence that all are born equal, with equal potential, and that differences result almost exclusively from different environments. For others, it is more a matter of concern for what they see as the sociopolitical consequences of acknowledging the influence of genes on behaviour, the fear that ugly “Social Darwinist” doctrines will be bolstered. As one colleague put it when told about our group's discoveries about genetic influence on preschool children's cognitive processes (Byrne et al., 2002), “what if the Right wing gets hold of this?”. If differences are genetic, what is the point of head start programs and the like?

From its earliest days, the study of possible genetic influences on behavioural differences has been the subject of controversy, but now evidence from comparative studies of twins, from pedigree studies, from adoption studies, from animal models, and from studies linking behaviour with processes at the molecular level all converge to demonstrate beyond doubt that genes partly determine human behavioural variation (Plomin & Crabbe, 2000). So it is no longer possible to finesse the concerns of scholars and practitioners by denying a role for genes in behavioural differences. What is required is thoughtful debate founded on proper appreciation of the evidence, what it means and what it does not mean. The aim of this special edition is therefore to present examples of this evidence for the Journal's readership.

As it happens, all of the articles are from Australian laboratories, an indication of the health of the subdiscipline within this country. They illustrate the range of methods and substantive topics that comprise the field of behaviour genetics. As it also happens, in each article there is direct or indirect evidence for genetic influence on one or more aspects of behaviour (and in one article, on physical characteristics as well). But it needs to be remembered that good research tools can discover only what is there, and the fact that a characteristic is investigated in a genetically sensitive design does not mean that genetic influence will be found. In fact, such designs can be used to test specifically environmental hypotheses, such as occurred when Iervolino et al. (2002) investigated the controversial hypothesis of Harris (1998) that adolescents' peers are the primary determinants of delinquency (and found that to be substantially true). Thus, although the term behaviour genetics is used to refer to this subdiscipline, a more accurate if longwinded descriptor could be study of behavioural diversity in genetically sensitive designs.

There is another way in which behaviour genetics can help illuminate environmental factors, and that is through the investigation of identical twins discordant for a characteristic where there is known genetic influence. The paper by Wade, Treloar, Martin, Statham, and Heath illustrates this approach, with interesting findings about possible environmental risk factors for anorexia nervosa from their (necessarily) small sample.

The first paper in the collection, by Wright and Martin, provides an overview of techniques, practices and findings from a large and ongoing series of studies that includes both physical and psychological characteristics of adolescents. It is noteworthy that many of the behavioural variables are as heritable as some of the physical ones, an observation that should give pause to my colleague's reviewer, above. Readers' attention is drawn to the invitation issued by the authors for collaboration in further exploring existing databases or conducting follow-up studies based on them.

Two other papers from the group at the Queensland Institute of Medical Research, by Luciano, Wright, Geffen, Geffen, Smith, and Martin, and by Hansell, Wright, Geffen, Geffen, and Martin, illustrate how very detailed aspects of cognitive functioning can be studied in the classic twin design and, in the case of the second of these two papers, how these can be linked to electrophysiological responses from scalp recordings. One goal of such studies is to determine if more global (and heritable) characteristics such as IQ can be better understood in terms of “simpler” processes such as processing speed and inhibition. Another is to drill down into how genes may influence brain activity, and through that, cognitive functioning. These papers also illustrate the use of multivariate modelling to check whether, for instance, two cognitive functions may share, or partly share, a common genetic basis.

The paper by Hay, Bennett, McStephen, Rooney, and Levy focuses on attention deficit – hyperactivity disorder (ADHD). It demonstrates the use of genetically sensitive designs in a longitudinal setting, allowing researchers to address the question of whether genetic influences remain constant over time. It also shows how a trait often considered categorical (the ADHD cluster in this case) can be treated as a continuous variable in behaviour genetics, an approach that more often reflects reality in psychological research. The paper adds to our understanding of the dynamics of this complex disorder.

Animal models of psychological processes have long been a feature of the discipline, and the paper by Murphy, Newman, Kita, Wilson, Lopaticki, and Morahan demonstrates the extension of that approach to genetic analysis. The use of breeding experiments, with its roots in Mendelian genetics, in combination with modern genome scan techniques, offers a powerful set of tools for genetic dissection of complex behavioural traits, and this paper captures some of the excitement of this kind of science.

Reading disability has been studied extensively from a behaviour genetic standpoint. The paper by Bates, Castles, Coltheart, Gillespie, Wright, and Martin extends the body of knowledge by showing that variation across the normal range is as subject to genetic influence as is frank disability. The paper employs a detailed model of the phenotype, the dual route cascaded model, and in doing so illustrates the importance of a well-specified account of the behaviour in question if progress in unpacking it in terms of genetic and environmental influences is to be made. The paper also shows how longstanding issues in an area, in this case sex differences in reading skill, can be illuminated with a behaviour genetic approach.

What, then, of the worries that beset some people in facing the behaviour genetic agenda? Much could be written, but I will mention just two things. One is that heritability estimates for many aspects of psychological functioning (and dysfunctioning) fall well short of 1.00, leaving room for the environment and therefore for environmental interventions. Finding that genes play a role should be a spur not a deterrent to identifying specific environmental factors that are also at work because it is still those that offer the best hope of prevention and remediation through well-targeted practices. The second is that identifying those characteristics in which genes play a role can shift the burden of responsibility from people previously and unjustly weighed down with it. Too many mothers of dyslexic children have been told that the problems stem from inadequate home literacy practices. A fuller understanding of the aetiology of human differences can only improve our science and our practice.

I wish to thank William Noble, the Journal's editor at the time, for the invitation to put together this Special Edition. I also thank all of the authors who contributed, and the expert reviewers for their contributions of time and knowledge.