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Nature, Nurture, and Development: from Evangelism Through Science Toward Policy and Practice

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Nature, Nurture, and Development: from Evangelism Through Science Toward Policy and Practice Child Development, January/February 2002, Volume 73, Number 1, Pages 1–21 Nature, Nurture, and Development: From Evangelism through Science toward Policy and Practice Michael Rutter During the second half of the 20th century there was an immense increase in both empirical findings on, and conceptual understanding of, the effects of nature, nurture, and developmental processes on psychological functioning—both normal and abnormal. Unfortunately, the good science has also been accompanied by ex- cessive polarizing claims and by unwarranted extrapolations. This article provides a summary review of the real gains in knowledge, outlines some of the misleading claims, and notes the potential for research and for science-led improvements in policies and practice. The need to bring about a better interpretation of genetic, psychosocial, and developmental research strategies and theoretical concepts is emphasized. INTRODUCTION that may be occurring. To a considerable extent, it is the failure to do so that has led to many of the polar- Over the last half century, there has been an explosion izing battles and absurd claims. of knowledge on the effects of nature, nurture, and developmental processes. As a result, we have a much improved understanding of many of the mechanisms NATURE: GENETIC RISK AND involved in normal and abnormal development, PROTECTIVE MECHANISMS which carries with it a huge potential for improving children’s lives. Unfortunately, these advances have With regard to the influences that reflect nature, been accompanied by as much misleading scientific quantitative genetics and molecular genetics are dis- evangelism and journalistic hype as by good science cussed separately, because they have rather different and honest reporting. As a consequence, both the pages patterns of strengths and limitations. of scientific journals and the media have been full of the most absurd confrontations and polarizations. These Quantitative Genetics have given rise to an unhelpful level of misunder- standing of the true scientific advances and, more es- Quantitative genetics uses various population de- pecially, about their meaning and the implications for signs (most particularly twin and adoptee studies) to policy and practice. Of course, there have also been quantify the relative strength of genetic and nonge- numerous examples of good reporting by scientists netic factors with respect to population variance; that and by journalists. There is every reason to be in- is, individual differences with respect to some trait or debted to both. The need is to avoid the twin dangers disorder (McGuffin & Rutter, in press; Rutter et al., of destructive cynicism and gullible expectation. 1990; Rutter, Silberg, O’Connor, & Simonoff, 1999a). Nature, nurture, and development are dealt with Nongeneticists tend to assume that the only focus is in this article as separate topics (with the focus being on the heritability quotient, but that is not the case. mainly on their effects on psychopathology). In each The available techniques are able to partition genetic case, the real advances in knowledge are considered influences into those that are additive—that is, due to first, some of the misleading claims are outlined sec- a mixture of many genes without there being any ond, and the potential for research and for improve- requirement for a particular pattern or combination— ments in policies and practice are noted third. Al- and those that are nonadditive—that is, reflect interac- though these are considered as supposedly separate tions among different genes (epistasis) or among dif- influences, the truth is that they are closely inter- ferent alleles of the same gene (dominance). Similarly, twined. The separation is heuristically useful for test- nongenetic influences can be subdivided into so-called ing causal hypotheses, but it is crucial that such hy- shared and nonshared effects—those that tend to potheses deal with the different forms of interplay make siblings similar and those that tend to make 2001 Presidential Address to the Society for Research in © 2002 by the Society for Research in Child Development, Inc. Child Development. All rights reserved. 0009-3920/2002/7301-0001 2 Child Development them unalike, respectively. Gene–environment corre- timate variations (surprising though that may seem), lations and interactions may also be identified. Al- because all indicate substantial genetic effects and the though traditional analyses have, in the past, tended figures are population specific; that is, they apply to assume that genetic and nongenetic influences are only to the particular samples studied at a particular entirely separable, there is no need to make that as- time. Even a heritability as high as 90% does not mean sumption. Quantitative genetic studies have increas- that changed environmental conditions could not ingly tested for, and found, major interplay between make a huge impact. This is not just speculation; the genetic and non-genetic factors, such that the out- findings with respect to height both in childhood and comes cannot sensibly be attributed to just one or the in adult life show the reality (Tizard, 1975). Height is other, because they depend on both (Rutter & Silberg, one of the most strongly genetically influenced of all in press). human characteristics, but it has increased enor- Quantitative genetic designs rely on many as- mously over the course of the 20th century (Kuh, sumptions, and it is crucial that these be put to the test Power, & Rodgers, 1991; van Wieringen, 1986), almost in a rigorous fashion, making explicit just which are certainly due to improved nutrition. Genetic influ- necessary (Rutter, Pickles, Murray, & Eaves, 2001; Rut- ences on the timing of the menarche are also strong ter et al., 1999a). Critics have rightly pointed to prob- but, again, the age of menarche has fallen greatly over lems, but any dispassionate reading of the evidence the last 100 years or so (Tanner, 1962). It is clear that leads to the inescapable conclusion that genetic fac- environmental factors can bring about major changes tors play a substantial role in the origins of individual in features that are strongly genetically influenced. differences with respect to all psychological traits, Despite this reservation, we may conclude that both normal and abnormal (McGuffin & Rutter, in there can be no doubt about the importance of genetic press; Rutter, Silberg, O’Connor, & Simonoff, 1999b). (as well as experiential) influences on individual dif- In a few cases (such as with autism, schizophrenia, bi- ferences, especially with respect to persistent traits polar disorder, and attention deficit disorder with hy- and chronic or recurrent disorders. Genetic influences peractivity), genetic factors account for most of the are particularly strong with respect to several mental variance in populations—over 70%. For the great ma- disorders, where the evidence from other biological jority of psychological characteristics, however, the studies indicates the likelihood that brain abnormali- genetic effects are not as strong. Thus, the heritabili- ties are implicated in the causal processes. This applies, ties of unipolar depression, delinquency, and parent- for example, to autism (Bailey, Phillips, & Rutter, 1996; ing qualities are in the 20% to 40% range. Lord & Bailey, in press) and schizophrenia (Keshavan Two different answers can and should be given to & Murray, 1997), and, in both cases, the quantitative the question of how much faith we can place on these genetic findings point to the likelihood that several estimates. First, there is every reason to believe that interacting genes are involved—that is, synergistic they are close to correct. This conclusion is based on interaction among genes may be implicated. the fact that numerous studies have produced broadly Strengths and achievements. Several other, rather dif- comparable findings; that substantial genetic effects ferent aspects of the genetic findings warrant empha- are evident from twin, adoptee, and family data; and sis. To begin, it is necessary to note the pervasiveness of careful consideration of the findings indicates that genetic influences across all psychological traits, even nongenetic explanations for the pattern of results those involving attitudes or social behavior (Plomin, found lack plausibility. 1994). Thus, genetic effects have been found for fea- The second answer, however, is that the estimates tures as diverse as divorce (Jockin, McGue, & Lykken, must be regarded as very approximate. This is because 1996; McGue & Lykken, 1992), religiosity (Eaves, the precise figures produced by different investiga- D’Onofrio, & Russell, 1999), and various aspects of tions are often rather different, and different methods parenting style (Kendler, 1996a). Critics have been quick often come up with somewhat discrepant findings. to scorn such findings, arguing that it is ridiculous to The use of parent reports and child reports, and of suppose that there could be a gene for divorce or crime twin and adoptee data, have made these points appar- (Rose, 1995, 1998). They are right, of course, that it is ent. In the past, arguments have raged over whether extremely unlikely that such a gene could ever occur. the heritability of, for example, IQ is this figure or that They also point to the indefinite boundaries of the figure, but the important thing to bear in mind is that phenotype, or behavior, being studied. Both argu- it does not really matter. With respect to some traits, ments, however, miss the point completely. The mes- there is real doubt whether the heritability in the pop- sage is that the workings of the mind are based on the ulations studied is 20% or 40% or 60%. There are no functioning of the brain, and that genetic influences theoretical or policy implications from such wide es- apply to individual differences on all somatic features. Rutter 3 It must be anticipated, therefore, that there will be a ge- genetically vulnerable and the most impact on the ge- netic effect on all behaviors. Biologically speaking, this netically susceptible. is exactly what one would expect, and what has been The last example of a specific finding of importance found.
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