supportive and opposing papers that follow it, and to a paper by Ketelaar and Ellis (2000), which addresses the question of whether evolutionary explanations are unfalsifiable; we discuss this further in Chapter 11.
Further information about evolutionary developmental psychology can be found in Bjorklund and Pellegrini (2002).
Behaviour genetics: a focus of today's nature/nurture
that this notion runs counter to mainstream developmental theory, Scarr has presented various kinds of evidence that individuals are active shapers of their own environment rather than passive recipients of it (which accords with organismic theories of development, as discussed in Chapters 3 and 4). Overall, Scarr has argued that 'genotype-environment correlations, rather than gene-environment interactions, predominate in the construc- tion of experiences' (1992: 8). Scarr sees the effect of genes becoming stronger as children grow older and become increasingly able to select environments that suit their genetic make-up.
Similar in many ways to Scarr's model is that of Plomin, who refers to his work as environmental genetics (e.g. Hetherington et al, 1994). He has introduced the notion of the 'non-shared environment'. This concept can be exemplified by considering siblings. Behavioural differences between siblings are often very apparent despite their sharing 50 per cent of genetic material and being raised in the same family. Plomin maintains that being raised in the same family does not constitute being raised in an identical environment: the 50 per cent of genetic material that differs between siblings causes children to respond to similar events differently, and also evokes differing responses from parents. This non-shared environment creates different outcomes, while the shared environment is thought to have little effect.
Theories such as these have given genetics a much more predominant place than previously in explaining behavioural differences between indi- viduals. For example, Scarr's theory implies that children could be re- assigned to be raised by different families and they would turn out much the same anyway. Such interpretations lead to profound conclusions. For example, differences in parenting style are seen to matter little - as long as parents are 'good enough' the child will develop as genes dictate.
Furthermore, provided the environment is not very deviant, early enrich- ment programmes for families would be a waste of resources.
Such conclusions have certainly not gone unchallenged. Baumrind, who undertook seminal research on the effects of parenting styles on children's development, strongly took issue with Scarr's notions of average expectable environments and good enough parenting. Baumrind (1993) maintained that the heritability estimates used by Scarr suffer from implausible basic assumptions, underdeveloped constructs, inadequate measures of family environment and unrepresentative populations. For example, the popula- tions in which heritability estimates have been made are extraordinary, mainly being studies of twins and adoptees. Scarr did not specify what constitutes a 'good enough' environment, which appears to be any environ- ment other than abusive. Baumrind cites evidence to support her counter- proposition that 'All nonabusive environments above the poverty line are not equally facilitative of healthy development' (1993: 1299). Scarr accepted that her theory depends on children having a broad range of
environments from which to choose, and excluded individuals with disad- vantaged circumstances or restricted life choices. Baumrind suggested that such 'excluded' individuals are in fact the norm worldwide, the absence of disadvantage not being the same as having a rich environment. She disput- ed Scarr's assumption that the same ontological principles apply within all cultures: 'What is "normal" or "expectable" in one culture frequently is anathema in another' (1993: 1301). For example, as we will discuss in Chapter 9, parents from different cultural backgrounds rear their children very differently because of different cultural values; this necessarily limits the generalizability of heritability indices. Baumrind maintained that nega- tive social or genetic factors can be attenuated by parents, but that parents will not be open to interventions if they accept Scarr's position and believe the situation is genetic and unmodifiable.
More recently, a detailed critique of the interpretations of behaviour genetics research has been produced (Vreeke, 2000). In particular, the assumption that behaviour genetics studies are relevant for questions of development has been questioned. Echoing the critiques of sociobiology, Vreeke observes that the main statistical technique used by behaviour geneticists is analysis of variance, a correlational technique. Although, as every undergraduate psychology student knows, correlation does not imply causation, in the field of behaviour genetics, providing developmental interpretations of analysis of variance is standard practice. If, say, the her- itability of IQ in a population is 80 per cent, this is understood to mean that genetics play a major causal role in intellectual ability (although how this happens is not made explicit). Vreeke argues that there are a number of weaknesses in this logic. For example, as in Baumrind's earlier critique, the nature of the sample is seen as crucial - if the study participants were from a selected background (e.g. college students) one might find a very different heritability estimate than if they were from a sample representa- tive of the broader population. Assuming one does, in fact, succeed in tak- ing such population effects into account, an assumption of analysis of variance is that the variables are additive, whereas the evidence is that developmental processes are interactive, and analysis of variance is arguably not sensitive enough to detect this.
This critique has, in turn, been critiqued by behaviour geneticists who argue, on the basis of Mendel's laws of inheritance, that additivity is the biological reality, as reflected by additive effects of different genes to pro- duce those phenotypes that are determined by multiple genes. However, Vreeke argues that Mendel's laws mention interaction, as well as additivity, between genes, so that taking multiple genes as a model for gene/environ- ment relationships is not a basis for supporting the additivity assumption.
A further argument made in support of additivity is that it is adaptive. Yet, using evolutionary theory in this way can be seen as inappropriate: it relies on a consideration of the outcomes of development (phenotypes) upon
which evolutionary processes operate, and ignores the actual gene/
environment relationships that determine the phenotypes of individuals.
Although various researchers have come to the defence of the additivity principle, this stance flies in the face of evidence from molecular biology and animal research that genotypes are translated into phenotypes by 'complex, dynamic and nonlinear' processes (Vreeke, 2000: 40). Wahlsten (2000) acknowledges the importance of interaction between genes and environment, but does not accept that analysis of variance is necessarily an inappropriate analytic method: rather, the sample sizes must be large enough to enable the interactions to be detected (but, in practice, are often not).
Vreeke maintains that Scarr's and Baumrind's interpretations can be reconciled if we accept that genes and the environment have an interactive relationship: shared environmental effects that are demonstrated in experi- mental social research may not show up in a behaviour genetics study if the research design does not allow for the possibility that individuals with different genotypes may respond differently to the same environment.
Thus 'it cannot be deduced from percentages of explained variance that an intervention cannot be successful. An interactive logic predicts that it is a matter of finding the right key to the right lock, the environment that fits an individual genotype' (Vreeke, 2000: 43).
Others have also criticized behaviour genetics as placing too much importance on genetic influences on behaviour, an extreme view being that the role of genes ceases at conception, with epigenetic processes then tak- ing over, so that phylogeny and genetics add little of significance to an understanding of phenotypes. The neuroscientist Rose maintains that 'heritability estimates are simply meaningless when applied to complex human behavioural traits' leading to 'implausible claims such as a signifi- cant heritability for "religiosity" or "job satisfaction'" (2001: 144).
A newer framework for understanding gene-environment interactions, which gives a more important place to environmental influences on devel- opment is the bioecological model (Bronfenbrenner and Ceci, 1994;
Bronfenbrenner and Morris, 1998). Bronfenbrenner's (1979) earlier eco- logical model was highly influential in drawing attention to the multiple and interacting social and environmental systems influencing children's development (see Chapter 8). The more recent bioecological model takes issue with the view that individual and group differences in developmental outcomes are mainly genetically driven, proposing instead that it is appro- priate and ongoing interactions with the environment that enable genes to exert their potential to a greater or lesser degree (see Chapter 11). It is, thus, these interactions - known as proximal processes - that drive devel- opment, and their quality will affect heritability estimates.