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The best-known Studies into MZs reared separately, Newman et al (1937), Burt (1955, 1958, 1966), Shields (1962) and Juel-Nielsen (1965), show that they are still more alike than DZs reared together suggesting a strong genetic link. Twin studies have, however, attracted a great deal of criticisms. This criticism related to the fact that the MZs classified as separated were in some instances not separated until the age of seven, eight or nine and in other cases were raised in related branches of the parents families and even attended the same schools.

Taking these criticisms into account and with the improvements of research techniques we still find impressive correlations in the study of twins. McGurk’s second proposition states that, as genetic inheritance is a constant, there should be a high degree of continuity in IQ throughout an individual’s lifespan. To determine whether or not an individuals IQ has remained constant over time there is a need to calculate the stability coefficient. Honzik et al (1948) found in their studies of over 250 Californian children aged between 2 and 18, testing them at regular intervals, a strong correlation and very little fluctuation.

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However different research has produced a very different set of results, the Fels Longitudinal Study of Development (McCall, 1973) studied 140 middle-class children ranging in age from 2 1/2 to 17 years old. This study showed an average change in IQ of 28 points and even in the case of the most ‘stable’ individuals a change, on average, of ten points was noticed; about 15% of the participants shifted 50 points or more, in either direction, and there was one increase of 74 points.

Rebok (1987) warns that caution is needed when drawing conclusions in regard to later-life IQ from earlier data as intelligence is not as fixed as the original theories presumed. His third proposition indicates that the IQ correlation between biological parents and their offspring should be stronger than any IQ correlation with adoptive parents. Research into this third proposition needed comparisons to be made between the IQ correlation of children and their foster parents and the IQ correlation of children and their biological parents.

Early studies by Burks, 1928 Leahy, 1935 and Skodak and Skeels 1949 did find correlation differences between children and their adopted parents and that of children and their biological parents, 0. 15 between adoptive parents and adopted children and 0. 48 between biological parents and their offspring. These studies were, however, criticised due to the way in which the two different family groups were matched; the adopted children came from families with a poor background and were being placed with families of a higher socio-economic status who had also been carefully screened by adoption agencies.

Later studies (McGurk 1975) showed environment to have a marked effect on the level of IQ of the adopted children, the adopted environment raises the child’s IQ above what it probably would have been had the child remained with their biological parents. This study also came under debate when it was found if the environment remains constant between both adoptive and biological parents and children the IQ’s of the adopted children tended to be much nearer their biological parents (Schiff et al 1978).

The fourth proposition predicts that any attempt to deliberately increase or nurture the levels of intelligence should have little or no effect. In the United States in the early 1960s there was a growing movement to try and close the educational gap between white middle class children and children from socially disadvantaged backgrounds. Two books that contributed to this movement, J McVicker Hunt’s Intelligence and Experience (1961) and Blooms Stability and Change in Human Characteristics (1964), both stated that intelligence was not a fixed attribute and intellectual ability could be increased by outside circumstances.

The first of these programmes introduced by the United States government was known as Operation Headstart. The programme aimed at pre-school children did show early increases in the children’s IQ but these increases were short-lived and the overall educational gains were shown to be minimal. This particular programme came under a lot of criticism, but after further study of the results it was shown that there was in fact a plateauing effect and there was improvements in the beneficiaries cognitive abilities (Brown and Grotberg 1981).

More recent studies on 111 Romanian children adopted into English families within the first two years of their lives (Rutter et al 1998), the children came from impoverished backgrounds suffering both psychologically and physically, compared to a sample of English adoptees showed a considerable catch up, both physically and cognitively, by the age of four years. This catch up was significantly improved if the adoption had taken place before the children were six months old.

Nature, Nurture or Both? From the evidence produced in relation to the four propositions, which supported the genetic theory, it can be seen that the environment does play an important part in the development of intelligence. But how much? The balance between these two factors has produced conflicting results in its own right with Jensen (1969) claiming hereditability to be 80% whereas Bouchard and Segal (1988) said the figure was more likely to be around 50-60%.

The differing research and study results documented do give an insight into perhaps how not only there is a need to have a positive environmental input but also the need to be aware of the actual effects hereditability has on the issue of intelligence. The complexities of the nature and nurture positions could possibly suggest the need to perhaps reframe the debate on the nature of intelligence, perhaps a more useful question could be: how is intelligence influenced by both heredity and the environment?

One possible response to this question was offered by Gardner (1983) who offered the multiple intelligence theory, stating that multiple intelligence is neutral on the issue of heritability, and emphasized the importance of environmental interactions. Although Gardner did not doubt that genetics do play an important part in intelligence, he stressed the interaction between genetic and environmental factors as pre-eminent.


Gross, R (1999) Psychology: the Science of Mind and Behaviour (3rd ed) Hodder & Stoughton Hayes, N (1995) Foundations of Psychology An Introductory Text Routledge Daly, P J (2003)

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