In several twin studies, MZ twins brought up in different families, would seem to be of particular value in deciding on the relative importance of genetic factors and of the environment in determining intelligence. Those arguing that genetic factors are of most importance would expect such twins to resemble each other closely in intelligence. In contrast, those favouring an environmentalist position would argue that placing twins in different environments should ensure that they are not similar in intelligence.
The findings from MZ twins brought up apart seem on the face of it to provide convincing evidence for the importance of genetic factors. However, many of the MZ twins brought up apart were, in fact, brought up in different branches of the same family. Other MZ twins were actually brought up together for several years before being separated. As a result, some of the similarity in IQ of MZ twins brought up apart is due to environmental rather than genetic factors.
In “The Bell Curve” (1994), Richard Herrnstein and Charles Murray claim that separated-twin studies represent the "purest" of the direct measures of heritability and that intelligence is strongly heritable with a heritability estimate of 0.60 + 2, within whites. They also go on to state that social intervention can do very little to raise IQ. This claim is based on data from the National Longitudinal Survey of Youth (NLSY). This survey was an ongoing federal project testing over 10,000 youths in the 1980’s.
Another method of assessing the role of genetic and environmental factors in intelligence is by means of adoption studies. The measured intelligence of adopted children might depend more on genetic factors (the intelligence inherited from the biological parents) or depends more on environmental factors (related to the intelligence of the adopted parents).
Horn (1983) discussed the findings from the Texas Adoption Project, which involved almost 500 adopted children. The correlation between the adopted children and their biological mothers for intelligence was +0.28, indicating that there is only a moderate degree of similarity in intelligence. The correlation between the adopted children and their adoptive mothers was even lower at +0.15. both of these correlations are so low that it is hard to make any definite statements about the roles played by heredity and environment, though it does suggest a greater role for heredity.
A further study into adopted children by Loehlin, Horn and Willerman (1989) found that there were some differences in the findings when the adopted children were tested again 10 years later. Now the children had increased correlation with their biological mothers but less with their adoptive mothers. Shared family environment between the adopted children and their adopted mothers was reduced in importance, whereas genetic factors had a greater influence on the adopted children’s intelligence than had been the case 10 years earlier.
Capron and Duyne (1989) reported a very impressive adoption study. They made use of four very different groups of adopted children. These groups involved four possible combinations of biological parents of high or low socio-economic status and adoptive parents of high or low socio-economic status. They found the effects of socio-economic status of the biological and of the adoptive parents were much the same. This suggested that genetic and environmental factors were of about equal importance in determining the intelligence of adopted children.
A way to study the effects of environmental factors on the development of measured intelligence is to consider enrichment programmes such as Operation Headstart. In the 1960’s there was a political move in the United States to set up an intervention programme that would help disadvantaged children. It was argued that such children lacked some of the early benefits enjoyed by more, middle-class children in terms of health and intellectual stimulation, and were therefore disadvantaged even before they started school. Lazar and Darlington (1982) reported that the Headstart children were less likely to be placed in special classes, were more likely to go to college and, in terms of social benefits, were less likely to need welfare assistance or become delinquent. Therefore, IQ can be affected by environmental factors.
There has been great political controversy about the fact that the mean difference in IQ between white people and black people in America favours white people. Most psychologists have assumed that the difference between white people and black people is due to the environmental deprivation suffered by black people. However, Jensen (1969) and Eysenk (1981) argued that genetic differences might be involved.
In 1988 Rushton published research that also claimed to demonstrate significant differences in IQ between white and black children. The data was based on IQ tests that, although they purported to be “culture free”, actually rewarded those children who were taught basic arithmetic in school. The white children assessed were U.S school children whereas the black children were from Africa and did not have the same access to school concepts. This is therefore not surprising that the white U.S school children did considerably better in the tests.
Mackintosh (1986) compared white and West Indian children in England. Some of the children were matched for father’s job, numbers of brothers and sisters, family income, and other measures relevant to deprivation, whereas the others were not matched. In one study a 9-point difference between unmatched groups, and only 2.6-point difference in the matched groups. Therefore showing very small differences in intelligence between the two groups when they were equated for the level of deprivation.
In conclusion, there is no solid evidence that it is either the role of genetics or the role of the environment that is the sole influence on the development of intelligence. The twin studies showed that in the case of Monozygotic twins and Dizygotic twins, heredity is of major significance in determining intelligence. However, this assumed the degree of environmental similarity experienced by MZ twins is the same as that of DZ twins. Also, Loehlin and Nichols brought to attention that similarity of treatment has an effect on the similarity of intelligence in the form of IQ.
The “Bell Curve” research also recognised that genetics play an important role as in the separated twin studies there was strong evidence of links between intelligence and heritability in whites. Also, it was claimed that social intervention can do very little to raise IQ.
Adoption studies have shown that ten years after a study, genetic factors have a greater influence on adopted children’s intelligence than had been at the original study, therefore implying that the role of genetics on intelligence develops over time. On another adoption study by Capron and Duyne (1989) it was found that genetic and environmental factors play their part in about equal importance on the development of intelligence.
Other studies such as “Operation Headstart” and cross-cultural studies have also shown that although genetics do play an important role in the development of intelligence, there is an equally important role of environmental factors.
Therefore, the in-ability to recognise which factor, nature or nurture, is responsible for any specific trait, but knowing that one or both of the two is responsible, will keep the argument of genetics vs. environment in the forefront of our search for answers for a long, long time.
