There is a long-standing debate over the role that genetic factors have to play in the development of intelligence, as opposed to environmental factors. This is known as the ‘nature nurture’ debate. There are supporting arguments and criticisms levelled at both these theories, and most people agree that the real issue is how much of a role each of these things have to play in a childs developing intellect. One of the first studies into genetic factors was conducted by Shields in the 1960’s. Using advertising, he was able to gather together, and study the IQ of 44 pairs of monozygotic twins. He claimed the twins, although identical genetically, were all reared apart.
Shields found a high positive correlation in IQ test results between the twins, much the same as in twins who had been reared together. This would suggest there was strong evidence for the ‘nature’ argument. However, the study comes under fire for several reasons. It later emerged that the MZ twins had grown up in very similar environments, and actually spent a lot of time with each other. Many of them lived within close proximity to each other. Also, Shields only gathered correlational data, which doesn’t prove any causative relationship. That is, it doesn’t show that genes affect IQ in any way.
In support of Shields, other later twin studies give much the same results as his did; high positive correlations between the IQ results of twins. Bouchard and McGue analysed data from many kinship studies, which included twin studies. They wrote an article, which concluded that genetic factors do have a role to play in the development of intelligence, but that environmental factors also have a considerable effect. The current opinion is that around half of the factors which contribute to our intelligence are hereditary.
Genetic research – Adoption studies
Effective adoption studies give researchers the chance to compare IQ scores between adopted children, adoptive parents, and biological parents. If genetic factors were of the greatest influence, then you would expect children to have IQ scores which closely correlate to the scores of their biological parents. If environmental factors are of the greatest influence, the scores ought to match those of the adoptive parents more closely.
A study which investigated these factors was the Texas Adoption Project, which was conducted in the early 80’s. The researcher used records of IQ test results from a large private adoption agency to gather correlational data. The IQ scores were taken from unmarried mothers of 469 children (who were later adopted), the children themselves, and their adoptive mothers. The results show a slightly higher correlation between the children and their biological mother than between their adoptive mothers, but this still leaves about 80% of the scores to be accounted for in terms of environmental influence.
A transracial adoption study conducted in the USA by Scarr investigated black children from lower class backgrounds, adopted by white middle/upper class parents. Scarr found that the average IQ of the black children brought up by the adoptive parents was 106, but genetically similar children who were not brought up in a middle class environment only had an IQ score of 90. This is convincing evidence towards the importance of environmental factors.
Environmental and cultural factors Members of the culture in which a child is brought up, such as parents, teachers, and friends, all directly influence development of intelligence in the child. Home background, parental involvement, and enrichment programmes are all things that have been studied in relation to intelligence. With regard to home background, the Rochester Longitudinal study observed children from birth to adolescence and found there were 10 factors which could be detrimental to the childs IQ.
That is, the more factors present in the childs life, the lower the childs IQ would be. Some of these factors include the mother having a history of mental illness, the mother not going to high school, there being four or more children in the family, and the main earner in the family having a semi-skilled job. The data gathered here is only correlational, and a criticism of the study is that low parental IQ could account for some of the 10 factors. For example, “the mother didn’t go to high school” – this may be due to the fact that the mother herself has a low IQ, and therefore the child has inherited his IQ from her. This works against the researchers original conclusion that environmental factors are of greater influence to intelligence!