Genetic and environmental influences
Twin and adoption studies suggest that a large component of the variation in IQ is due to genetic factors. Other evidence comes from gene mapping studies (e.g. Chorney et al, 1998) where individual genes associated with high IQ have been identified.
However... There is equally strong evidence for the ffects of nurture, for example the Flynn effect is the observation that IQs all over the world increased by as much as 20 points in 30 years (Flyn, 1987), which must be due to environmental enrichment. Furthermore, a recent study illistrated a gene-environment interaction. Turkheimer et al (2003) showed that the extent to which genetic factors mattered depended on the socioeconomic status of the children. In children from affluent families, about 60% of the variance in IQ scores, could be accounted for by genes, but for children from impoverished families, genes accounted for harly any of the variance.
Twin and adoption studies again show that the closer two individuals are genetically, the more likely that both of them will develop the same mental disorder. For example, the concordance rate for schizophrania is about 40% for MZ twins and 7% for DZ twins (who, on average, share 50% of their genes) (Jospeh, 2004).
However... There is clearly a significant environmental component. This is well described by the diathesis-stress model which represents both nature and nurture: individuals who have a genetic vulnerability (diathesis) for a particular disorder only develop it under certain conditions (stress). For example, Tienari et al (1994) found that adopted children who had schizophrenic biological parents were more likely to become ill themselves only if the adopted family was rated as disturbed. In other words, the illness…