Twin Studies
The first twin study was carried out by Sir Francis Galton. Even though genes had not yet been discovered, he reasoned that there are two types of twin: identical twins from ‘two germinal spots in the same ovum’ and nonidentical twins ‘each from a separate ovum’. Galton reasoned that if identical twins were more alike in behaviour than non-identical twins, then this would support his view that personality and intelligence were inherited.
He wrote to 35 pairs of identical twins and 23 pairs of fraternal twins to collect information about their similarities and differences.
His results were astonishing: twins who looked alike at birth were similar throughout their lives, not only in appearance, but also ailments, personality and interests (Ridley, 2003).
One pair had toothache in the same tooth at the same age and another pair bought each other identical champagne glasses as presents at the same time, despite being at different ends of the country.
Twins that did not resemble each other became more different with age.
Although there were severe flaws in Galton’s twin studies the reasoning behind twin and family studies remains: If IQ is highly genetic, closely related family members should have more similar IQs than less closely related family members.
If IQ was entirely genetic the correlation between the intelligence of MZ twins would be 1.00 (Galton also invented statistical correlation).
More modern twin studies are more objective and use more precise measures of intelligence using intelligence tests. Bouchard & McGue (1981) conducted a worldwide review of 111 such studies and found the following results:
Monozygotic Twins (MZs) reared together
- 4672 pairs in 34 studies
- Expected correlation = 1.00 (if IQ genetic)
- Actual correlations = between 0.58 and 0.96
- Median correlation = 0.85
Monozygotic Twins Reared Apart
- 65 pairs in 3 studies
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Expected correlation = 1.00 (if IQ genetic)
- Actual correlations = between 0.62 and 0.77
- Median Correlation = 0.67
DZs reared together
- 992 pairs in 8 studies
- Expected correlation = 0.50 (If IQ genetic)
- Actual correlations = between 0.21 and 0.87
- Median correlation = 0.58
Ordinary siblings reared together
- 26,473 pairs in 69 studies
- Expected correlation = 0.50
- Actual correlations = between 0.12 and 0.90
- Median correlation = 0.45
Ordinary siblings reared apart
- 203 pairs in 2 studies
- Expected correlation = 0.50
- Actual correlations = between 0.23 and 0.25
- Median Correlation = 0.24
These results show that there are strong genetic factors involved in the development of measured intelligence. This is because there are higher concordance rates for monozygotic twins than for dizygotic twins and ordinary siblings.
Other factors must also be involved, however, as the concordance rates are not 100% and the concordance rates for MZ twins reared together are markedly higher than when they are reared apart.
What is striking, nonetheless, is that MZs reared apart have more similar IQs than DZs reared together. This suggests that genetic factors are much more important than environmental factors.
Adoption Studies
Another method of investigating the nature/nurture debate is the use of adoption studies. Adopted children share half of their genes and none of their environment with their biological parents, while they share no genes and some environment with adoptive parents. So, if IQs of adopted children are more similar to biological parents than adopted parents it would support the idea that genes are a more important contributor to intelligence than the environment.
Munsinger (1975) found that the IQ of adoptive children had a correlation of 0.48 with that of their biological parents and 0.19 with that of their adoptive parents. This seems to indicate that the genetic contribution of the biological parents to IQ is stronger than the contribution of the adoptive environment.
Nevertheless, some adoption studies have shown that the contribution of the environment is very important: when the environments of the biological and adoptive parents are very different large differences in IQ scores have been found.
Scar & Weinberg (1976) studied 101 above average intelligence and social class families who adopted black children (Blacks in the US typically score 15 points below whites on IQ tests. They compared IQ before adoption with IQ some time after:
Average IQ before adoption = 90
Average IQ after adoption = 106
This suggests that the impoverished environments of the black children who were not adopted prevented them from developing their intelligence to its full potential, whereas the black adoptees benefitted from the improved socioeconomic circumstances and were able to reach their potential.
This study also highlights the fact that within an environmentally homogenous population (one where members of the population have relatively similar environments) IQ differences are mostly attributable to genetic factors, whereas IQ differences between groups whose environments are very different are mostly due to environmental factors.
Other studies have also supported the idea that a poor environment can limit the development of measured intelligence. For example, Schiff et al (1978) carried out an investigation into economically deprived French mothers who had given up one child for adoption and kept another:
Average IQ of child adopted into
middle class family = 110
Average IQ of non adopted child = 95
Genetic Studies
A more recent contribution to the nature v nurture debate comes from genetic studies. Genetic studies involve examination of the DNA of participants with particular traits (e.g., high intelligence) and comparing it with the DNA of participants without the traits. If people with the traits have particular gene variants in common, it is regarded as evidence that the gene is somehow involved in producing the trait.
Once candidate genes are identified, their functions can also be examined, which may or may not support the idea that the gene has an influence on the trait being examined; for example, if a gene suspected of being involved with intelligence is found to produce a protein used by neurons it would support the view that that particular gene contributes to the development of intelligence.
Burdick et al (2006) examined variations in a gene called dysbindin-1 in participants with schizophrenia or schizoaffective disorder and healthy volunteers. A common symptom of schizophrenia is cognitive impairment.
They found that a particular variation of the gene was common to participants with low cognitive ability in both schizophrenics and nonschizophrenics.
The dysbindin gene has been found to be highly involved in producing proteins in brain areas linked to problem solving, learning, memory and comprehension.
Burdick et al.'s research suggests that the dysbindinis a factor in intelligence; nevertheless it only accounts for about 3% of the variation in human intelligence. It is very likely that intelligence involves a large number of different genes.
Robert Plomin (1998) examined a region of chromosome 6 in children with ultra-high IQs and children with average IQ. He found that those with ultra-high IQs were twice as likely to have a particular variant of the Insulin like growth factor 2 gene (IGF2). Nevertheless, only half of the ultrahigh IQ participants had this variation and it only accounts for about 2% of the variation in human intelligence.
These genetic studies may appear conclusive; however, it is important to be aware that this type of research produces correlational evidence and, thus, care should be taken in assuming that a causal relationship exists between these genes and intelligence.
Click here for more genetic evidence in the nature nurture debate
Enrichment Studies
Enrichment studies involve providing an enhanced, more stimulating environment for disadvantaged children and comparing their IQs before and after the programme.
Operation Headstart was designed to give culturally disadvantaged preschool children enriched opportunities in early life. The children spent a year attending the project and their IQs were compared before and after. They found that the children’s IQs improved significantly in the short term; however, within two years the IQ gains had been lost. Headstart was criticised for being inappropriate to the child’s needs, it did not provide them with the skills they failed to develop at home, which are developed by most middle class children (Hunt, 1969).
However, it seems that the early criticisms of Headstart were premature. There seemed to be a sleeper effect, whereby the academic achievement of participants was greater than for non participants. This “gap” widened during ages 6 – 14 (Collins, 1983). According to Collins (1983), participants:
- Scored higher in reading, language and maths
- Were more likely to meet basic requirements of schools
- Were less likely to drop out.
- Were more motivated to succeed.
Conclusion
In general, the above research does not come down entirely in favour of nature or nurture, but appears to suggest that intelligence is due to interactions between genetic factors and the environment. Genes may give a person an advantage when it comes to the development of intelligence; however, a nurturant environment is necessary for that person to utilise that advantage. Moreover, genetic disadvantage may be overcome by suitable interventions.
