WJEC A2 Psychology PY4 - Role of Genetic Factors in the Development of Intelligence

3 Genetic Factors in the Development of Intelligence

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Kinship Research: Twin Studies

1. Kinship Research: Twin Studies

  • Bouchard + McGue (1981) conducted review of 111 worldwide studies on family IQ (ignoring those that had methodological problems including controversial British psych Cyril Burt who allegedly made up his results). Reported average correlations:
  • MZ twins raised together (0.86), apart (0.72)
  • DZ twins raised together (0.6), apart (no data)
  • siblings raised together (0.47), apart (0.24)
  • cousins (0.15)


(+) concordance rates of IQs of MZ twins raised apart (0.72) is much higher than concordance rates of IQs of DZ twins reared together (0.6), which suggests the more closely related a person is to another person, the more similar their IQs even in completely separate environments, suggests genes are important

(-) however concordance rates for MZ twins is not 1.0 which should be if intelligence is 100% genetic

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Kinship Research: Twin Studies

(+) but even concordance rate for same person measured twice on IQ tests is never 100% (0.87), may explain the concordance rate of MZ twins reared together (0.86) as chances of 100% concordance is low; MZ twin concordance is interestingly similar to person doing IQ test twice, strengthening the important role of genetics in intelligence and increasing the validity of Bouchard + McGue's research

(-) critics of genetic influence of IQ argue difference between MZ and DZ twins may be explained by environmental treatment of MZ twins because it may be more similar if twins are identical/same gender, which isn't always the case with DZ twins - difference in concordance between MZ + DZ may not be solely genetic

(-) Voracek and Haubner (2008) found twins are on average 4.2 IQ points lower than singletons (possibly due to premature births, foetal development or attention growing up), therefore the studies lack population validity as twin pps not true reflection of average intelligence

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Genome Research

2. Genome Research

  • as intelligence is such a complex set of behaviours, unlikely to be a 'gene for intelligence', as so difficult to define 'intelligence' (multiple or single), also difficult to predict which genes affect it. Genes likely to be related to production of proteins that ultimately affect speed and integration of cognitive processing
  • Robert Plomin claims intelligence is determined by number of genes (polygenic). Claims high intelligence determined by having variety of high quality 'plus' genes, those with average IQ will have mixture of plus and minus genes
  • Chorney et al (1998) compared genes of 51 'super-bright' children aged 6-15 (median IQ of 136) to those of 51 'average' intelligence (median IQ of 103) to identify candidate genes. Examined chromosome 6 and found allele 5 (of 7 alternatives) of the gene IGF2R (insulin-like growth factor 2) was present in 32% of super-bright and 16% of average group.


(+) suggests there are potential genes more frequently found in people with high IQs

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Genome Research

(-) but gene cannot be the only explanation of intelligence - of high IQ group, more of children didn't have gene compared to those who did, and of average IQ group, some of them still had gene

(+) advances in technology are improving the complexity and speed of genome research which may make identification of genes related to intelligence easier in future

(-) Chorney et al's research has low reliability as others have failed to replicate their findings

(-) despite large amount of research, specific 'genes for intelligence' have failed to be identified, as main function of genes is to code for proteins - therefore likely to be a great number of genes that affect intelligence (eg any genes that affect speed of info processing, production of neurotransmitters, language etc)

(?) each gene, therefore, will have small role to play and role of each gene interacts with many othersdifficult to identify intelligence genes eg gene which codes for size of female birth canal could influence amount of O2 child receives at birth, affecting brain development and therefore one of the possible genes for intelligence

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Interactionist Theory

3. Interactionist Theory: Gottesman's (1963) Reaction Range Theory

  • suggested every individual is born with genetically predetermined limits of IQ (genotype IQ). The quality of the environment then determines where the individual falls within this range (phenotype IQ)
  • therefore, although many different environmental factors may play a role in development of intelligence, there may be individual differences in the effects of these env factors that are determined by specific genes (eg breastfeeding) and the reaction range set by the genetic material of the individual


(+) Caspi et al (2007)

  • found that children with one version of FADS2 gene scored on average 7 points higher on IQ tests if they were breastfed, whereas children without this gene showed no benefits of breastfeeding, supporting and exemplifying the potential principle of reaction range
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Interactionist Theory

(-) Reaction Range only theoretical and impossible to test as unfalsifiable - it cannot be disproven because no actual measurable minimum or maximum IQs are ever suggested for an individual, therefore a researcher could never seek an individual who could exceed their maximum IQ and disprove the theory


In conclusion, it would seem genes (genotype) could influence expressed intelligence and variations in intelligence potential (phenotype), particularly in groups who have experienced the same high level of cognitive nurturing. However this in itself means nurturing is important and can develop or hinder the development of raw material. Therefore intelligence is likely to be a interaction of genes and environment.

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