Critically consider the role of genetic factors in the development of measured intelligence

                IQ=        Mental Age           x 100

                        Chronological Age

This produces a mean IQ of 100 and a normal distribution of IQ’s around the mean, where IQ’s of 90-110 are regarded as “normal”.

Although all these measures of Intelligence are far from perfect, they do give us a basis for studying those factors, including genetic factors, which contribute to the growth of Intelligence.

The historical question was the relative contributions of Genetics and Environment (Nature v Nurture). Genetics being expressed in terms of inherited factors and Environment in terms of upbringing and social factors. However, the traditional nature-nurture debate may no longer be valid and I will return to this point later.

Most research into the impact of genetic factors has been with kinship studies of families, twins and adoptions. However, it may be helpful to start with recent work arising from the mapping of the Human Genome- if only to show that it does not provide all the answers.

The work has led researchers to believe that they have come close to pinpointing a group of genes responsible for intelligence. 

Researchers set out to compare the 37 genes on Chromosome 6 for people with high IQ’s with those of average IQ’s. They found that one gene, IGF2R, showed a difference (high IQ groups having 46% IGFR2 Allele 5 compared with average IQ having 23%).

In finding that a clear difference exists, they believe that further research will identify the gene or group of genes responsible for intelligence.

However, the problems with the work to date, is that differing allele could only account for a 2% variation in IQ or 4 points. Clearly, we need to look wider to explain all the contributory factors and the kinship studies, which I mentioned earlier, may help us.

 Galton carried out the first kinship study in 1869. He was convinced that eminent men are sons of eminent fathers (the old idea that “leaders are born, not made”).

Whilst his study contributes little in terms of the source of intelligence, it did highlight the fact that family members share both environment and genes, making it difficult to separate cause from effect. Subsequent studies have aimed to eliminate this factor.

Many studies with identical twins (MZ monozygotic) and non-identical twins (DZ dizygotic) have been carried out. Identical twins have virtually 100% shared genetic material compared with non-identical twins who share around 50% genetic material.

Unfortunately, research in the UK by Cyril Burt in the 1950’s and 1960’s was discredited but it showed a correlation of 0.994 for identical twins raised together and 0.771 for identical twins raised apart. He concluded, that the inheritability of intelligence is 80%, was adopted by both Jensen (1969) and Eysenck (1973).

In1962, Shield carried out further research, which showed the correlation for identical twins raised apart to be 0.77 although it was found that he omitted twins if IQ’s were not similar. In addition, some of his twins brought up “separately” were brought up by members of the same family and in some cases went to the same school.

In 1982, Studds found that a correlation of 0.77 for 44 pairs of identical twins brought up separately, although it transpires that their environments were not very different.

This contrasted with twin studies by Cecci, where one twin was brought up in a rural environment and the other in an urban environment. He found a weaker correlation of 0.27.

Bouchard and McGue reviewed 111 studies of intelligence in twins and calculated averages (excluding Cyril Burt’s studies) of:

Identical twins raised together                0.86

Identical twins raised apart                0.72

Non-identical twins raised together        0.60

Siblings raised together                        0.47

Siblings raised apart                        0.24

Parent and child living together                0.42

Parent and child living apart                0.22

Cousins                                        0.15

Bouchard and McGue concluded that a significant part of intelligence is inherited.

However, there are unresolved issues, since the studies assume that the differences between identical and non-identical twins raised together are due to genetic factors and that they may not have had different environmental factors. Even twins brought up together do not have identical environments.

This issue does not arise with twins brought up apart, but there are relatively low genuine numbers in these studies with Bouchard and McGue finding only 65 pairs of identical twins raised apart in three studies.

Bouchard has carried out further work with the Minnesota Twin Study (1995) where close similarity has been found. However, these twins are volunteers and in some cases have been reunited, perhaps, causing them to seek publicity because of the similarities they found in each other.

More recent work by Plomin and DeFries (1998) on verbal and spatial ability shows that the similarity for identical twins is significantly greater than between fraternal twins across the period from childhood to old age. This provides increasing evidence for the inheritability of intelligence

Let me now turn from Twin Studies to the evidence of Adoption Studies and consider their implications.

At the outset it is worth noting that Adoption is not a neutral process. Adoption agencies select adoptive parents having regard to the child and there will be, therefore, a degree of matching rather than a random placement.

Horn (1983) studied 469 children through the Texas Adoption Project. He found a correlation of intelligence of 0.28 with biological mothers and 0.15 with adoptive mothers, suggesting some level of inherited intelligence. However, it is not a major influence and shows that other factors are at work.

A longitudinal study of 245 children, adopted before the age of one year, showed a correlation with both the natural and adoptive parents. By adolescence, however, no correlation was found with the adoptive parents suggesting that genes have a strong influence.

This contrasts with other studies such as those of Scarr and Kidd (1983), which showed a strong environmental impact. They examined black children adopted into white middle class American families and found IQ scores of 10-15 points above that of the biological mother. Perhaps this just indicates that they learnt the skills to complete IQ tests better?

In 1998, Plomin and Defries published work from the Colorado Adoption Project, which measured special and verbal ability of children up to the age of 16 years.

1. Children with birth parents
2. Adopted children with birth parents
3. Adopted children with adoptive parents

This shows that adopted children’s intelligence resembles that of their birth parents and that there is little resemblance with the adoptive parents.

The evidence from adoption studies suggests that the IQ’s of adopted children are more like their birth parents than their adoptive parents.

However, if genetic inheritance is not the whole story, it appears that environmental factors have a role to play.

A wide range of studies has been carried out into environmental factors. All of these studies indicate that there are environmental factors which contribute to the growth of measured intelligence thus demonstrating that genetic factors are not the only forces at work.’’

This returns us to the nature-nurture debate.

Sternberg and Grigorenko (1997) suggest that this debate is no longer valid-at least in its traditional form. The current challenge facing psychology is to establish the way in which nature and nurture interact in the development of intelligence

In conclusion, the extensive work into the development of measured intelligent indicates clearly that there is a genetic inheritability factor, thought by many psychologists to be around 50%.

More recent research indicates that it could be as high as 75% bringing us full circle to Galton in 1869, who thought eminent sons came from eminent fathers or those who used to talk of “good blood” and “bad blood.

Other References

Psychology-The Science of Behaviour       Carlson, Buskit and Martin

Psychology in Focus-A Level                    Haralambos and Rice

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