Twin designs focus on the comparison of the resemblance of same sex fraternal twins and monozygotic twins. As monozygotic twins are twice as similar genetically than fraternal twins the difference between the two can be taken as an adequate estimate of the genetic variance in the population (Plomin & Daniels, 1987., Scarr, 1997., Wilson, 1983). The central problem that thwarts the potential findings of the twin designs, however, concerns the extent to which environmental resemblance covaries with genetic relatedness. Adoption studies are far more powerful in this sense in that it is possible to confidently surmount these confounding interferences, which enable a more reliable prediction of genetic variance (Loehlin et al., 1989).
Twin and Adoption studies also allow predictions to be made regarding the environmental contributions in acquired intelligence. Twin designs provide a direct estimate of the non-shared environment. In addition Twin studies by default also provide an estimate of shared environmental influences, which is referred to as the proportion of variance that remains after considering the genetic variance and the non-shared environmental variance (Plomin & Daniels, 1987).
Evidence from adoption studies once again appear to be more fruitful however, as they provide a direct estimate of shared family environmental influences i.e. any individuals in the same family who are similar but are not genetically related must be because of their shared environment. The major finding of adoption studies concerns the extent to which the contribution of shared environment decreases into adulthood (Loehlin, Willerman, & Horn, 1985., Scarr & Weinberg, 1983., Teasdale & Owen, 1984). The answer to this question cannot be reliably forwarded by twin studies however (Plomin, 1988). The major problem with studies of identical and fraternal twin pairs is that data obtained from the major studies only involve children as their major sample (Bouchard & McGue, 1981.,Wilson, 1983).
There are a limited number of longitudinal approaches to twins studies, those that exist however typically yield findings that genetic influences on intelligence increase discretely throughout the lifespan, reaching its end point of 0.8 at age 40 (Pederson & Lichtenstein, 1997, c.f. Cooper, 1999) although Howe (1997, c.f. Cooper, 1999), has thrown caution at the interpretation of twin data, suggesting that studies in which twins have been reared apart are not always what they seem. For instance do twins reared apart actually never see each other? (Howe, 1997, c.f. Cooper, 1999, p.83) if they did this could lead to an underestimated proportion of shared environmental influences and overestimated genetic contribution. It is further added are also unlikely to be representative of the population as monozygotic twins seem to share a greater proportion of their environment with each other than do non-twin siblings, suggested to be caused by a “special twin effect” (Plomin, 1988, p24). Such interpretations in this case could lead to an overestimation of the effects of shared environment and underestimated genetic contribution.
In order to decipher the answer to the nature nurture debate therefore, it is necessary to understand how best to conceive each proportion of variance that contributes to the underlying nature of intelligence. A general consensus within the realm of behavioural genetics seem to suggest the following: Adoption studies are considered the most productive research designs employed in understanding the contribution of the shared environment (Plomin, 1988., Plomin & Daniels, 1987., Loehlin et al., 1989). Twin studies reared together provide evidence for the most direct measures of non-shared environment (Plomin & Daniels, 1987., Wilson, 1983) whilst evidence from a wealth of twin studies reared apart reliably suggest that a substantial amount of variance in measured intelligence has a underlying genetic basis (Bouchard et al., 1990a, 1991).
How can we assess the extent to which individual variability is determined as a result of genetic or non-genetic factors then?
Empirical research within the behavioural genetics realm is vast, in order to keep the discussion succinct it should be noted that only the most stringent twin and adoption studies are employed, it is considered that these characterize the most productive points of the argument. Hest to say that the sizeably excluded research has no implication to our debate however, it is understood that the debate itself has numerous arguments incorporated within it (Eysenck, 1983., Costa & McCrae, 1987., Zuckerman, 1987 c.f. Plomin & Daniels, 1987) not to forget the long standing history of the debate either; of course (Galton, 1883, c.f. Plomin, 1988).
Twin designs provide the most reliable estimate of the genetic component of intelligence. Bouchard (1997) has summarized the entire world literature on studies of twins reared apart, and allows by far the most comprehensive understanding. The picture is quite consistent throughout, converging on the estimation of around 70 per cent past childhood, it must be noted that effects of shared environment in early childhood are far greater than in adulthood (Loehlin et al., 1985., Plomin, 1988., Scarr, 1997) and the data averaged over a number of age ranges, could to an extent underestimate the influence of environmental components. Thompson (1993, c.f. Cooper, 1999) assumes the figure to around 50 per cent in childhood, nevertheless, the results summarized by Bouchard (1997) seem to rest on the undoubted conclusion that at least half the variance in measured intelligence is due to genetic differences.
It is a fundamental point to the argument to note that about half the variance in intelligence is the consequence of genetic differences among us, by virtue of the same conclusion however it is also necessary to consider that about half of the variance is due to non-genetic differences (Plomin, 1988). The question to which non-shared or shared environment account for more variance can be answered using data from adoption studies and studies of twins reared together.
There have been several large-scale studies of adoptees reported in the literature, the Texas Adoption study, the Minnesota Adoption study and the Colorado Adoption study (Cooper, 1999). The findings typically yield consistent patterns. In one study genetically unrelated children adopted into the same homes resulted in an estimation of around 30 per cent of variation in intelligence due to shared environment (Bouchard & McGue, 1981). However, the most important finding uncovered by adoption designs concerns the extent to which shared environmental effects seem to decline after childhood (Loehlin, et al.,1985., Scarr, 1997.,Teasdale & Owen, 1984) such findings are also in common with those of twins reared together (Wilson, 1983). Thus it seems that children will have similar levels of intellectual abilities in adulthood only if they are genetically related.
If the importance of shared environment seems unequivocal in the development of intelligence, what is? We know that genetic heritability of intelligence is a forgone conclusion (Bouchard, 1990a, 1991) which increases throughout the lifespan (Loehlin, et al.,1985., Loehlin et al.,1989, Plomin & Daniels, 1987., Wilson, 1983), however non-genetic variance still explains nearly half of the variance (Plomin, 1988). This variance must therefore be due to non-shared environmental influences on individuals within the same family. The most direct measure of non-shared environment is through studies of twins reared together. Plomin (1988) found that the difference between pairs of identical and fraternal twins reared together typically yield estimates of non-shared environment to be around 10 per cent and by virtue shared environment of around 30 per cent, common to those found in adoption studies (Loehlin et al.,1989, Scarr, 1997).
The conclusion that shared environment has a greater influence on environmental variation has not been agreed however (Plomin & Daniels, 1987). As we have seen the effects of shared environment diminish considerably through out the life span (Loehlin et al.,1985). Furthermore it has been criticised that results concerning the contribution of the shared environment inferred by the study of twins reared together are not generalizable to the population of non-twin siblings. According to Plomin (1988, p.21) “as much as half of the twin estimate of shared environmental influence is due to some idiosyncratic twin effect”.
In consideration of this Plomin (1988) argues that in fact shared environment only submits around 15 per cent toward the environmental variance whereas 25 per cent is a more accurate estimation of the non-shared environmental effects. Thus non-shared environmental variance seems to have been overlooked for much part of the debate, such findings could open fruitful avenues for environmentalists looking to converge methods with behavioural genetics (Plomin & Daniels, 1987)
The importance of twin and adoption designs in the study of behavioural genetics have been widely documented (Bouchard et al., 1990a, 1991., Cooper, 1999., Plomin & Daniels, 1987., Scarr, 1993) the findings of which have been imperative to our understandings of genetic and environmental components of intellectual abilities. A wealth of evidence on one hand suggests, “no other plausible alternative than genetics exist in order to explain the IQ similarities of monozygotic twins reared apart” (Bouchard, 1997, p.153). In further interpretation of the data it can be argued that in most twin studies, selective placement could lead to an underestimated influence of the shared environment, thus twin data should be treated with caution (Howe, 1997., Kamin, 1974, c.f. Cooper, 1999). Nonetheless evidence from a vast degree of family, twin and adoption research seems to consistently show that an underlying genetic contribution is approximately 50-60 per cent of the overall variance (Plomin, 1988., Bouchard et al., 1991) thus by consequence environmental effects must contribute the remaining 40 per cent of the variance (Cooper, 1999). Whether this portion of variance has a stronger shared or non-shared environmental influence is slightly less abated however. Typically findings of adoption research yield an estimate of 30 per cent of the variance attributable to the environment shared between parents, offspring and siblings (Loehlin et al., 1989., Scarr & Weinberg, 1978, c.f. Plomin & Daniels, 1987., Scarr, 1997., Wilson, 1983).
In understanding the literature, it was noted that studies of twins reared together provided the most direct estimate of non-shared environment. Findings of such studies (Bouchard et al., 1990a, 1991., Thompson, 1993, c.f. Cooper, 1999) seem to be consistent with the suggestion that non-shared environment contributes least toward the variation in intellectual abilities, than shared environment. Yet research of this nature still typically only study one child per family (Plomin, 1988) thus non-shared environmental components in all studies are referred to in terms of a between family effect rather than a within family effect, which is where a unique environment is at its most powerful (Plomin & Daniels, 1987).
To push this point further into place, as develop mentalists, we are compelled to emphasize that components of variance change over the lifespan, the area of intelligence to which is no exception (Plomin & Daniels, 1987). Thus it is important to note that estimates of genetic and environmental components of intelligence must depend on the age at which subjects are sampled. Twin and adoption studies focus primarily on child samples with a limited number of exceptions. Those that have developed beyond the normative approach of the study of children have consistently identified that shared environmental influences dramatically decay into late adolescence and adulthood (Loehlin et al., 1985., Plomin & Daniels, 1987., Plomin et al., 1985., Scarr & Weinberg, 1978, c.f. Cooper, 1999).
Given the importance of such findings, Plomin & Daniels (1987) exert the notion that non-shared environmental components are the major source of environmental variation in intelligence “after childhood” (p.6). Such findings have lead to the suggested revision of common developmental stereotypes. It is suggested that “the popular view of genetics fixed at birth and environmental effects as changing has got matters almost backward” in terms of intellectual development (Loehlin et al., 1989, p.1000). Thus it is necessary to understand that the potential underlying variation in intelligence as a component of genetics may in reality be far greater than hypothesized and that focus on environmental variance of the non-shared variety must be an important map to draw (Cooper, 1999., Plomin & Daniels, 1987., Plomin, 1988).
In summary, underlying intellectual abilities seems to be an inherent genetic basis, evidence provided by twin and adoption studies consistently draw the same picture. There is no doubt that intelligence has a far stronger genetic basis than environmental, however it is worth noting that behavioural genetics have a lot to offer by means of truce and reconciliation with environmentalists (Scarr, 1997). Behavioural genetics offer a number of worthwhile approaches which in conjunction with a number of socialization or parental measures can be used to assess non-shared environmental to a more profitable degree. Plomin & Daniels have acknowledged an approach “specific to parental behaviours that appears to be a step in the right direction” (1987, p.54). As yet however no specific causes of non-shared environmental variance have been investigated to a sufficient extent, thus the field shall eagerly await such data, until such times, we rest habitually on findings of both twin and adoption studies, which as noted suggest that “genetic influences in the development of intelligence in even young children is far too substantial to ignore” (Cooper, 1999, p.91).
Furthermore given the overwhelming support for a fundamental genetic influence on a wide range of behaviours i.e. personality and psychopathology (Eysenck & Fulkner, 1983., Loehlin et al., 1985., Plomin & Daniels, 1987) I have no doubt in the concluding that genetic factors are by far the most predominant cause of variation in intelligence within the western society.
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