Adoption studies are further methods of studying genetic factors in the development of characteristics such as IQ. If two children raised apart have similar such characteristics, it is likely to be genetic as they do not share the same environment, so genetic factors can be isolated from environmental factors when looking at their similarities and differences. The role of genetic factors is further supported by Plomin et al., whose ongoing Colorado Adoption Project has found that children have similar IQs to their adoptive parents in their youth, but by adulthood are more similar to their biological parents in terms of IQ. This indicates a latent genetic influence, therefore supporting the genetic explanation, and furthermore has received support from a similar project in Texas, which has found a 28% correlation between adopted children and their adoptive parents at age 8, but 0% correction at age 18.
On the other hand, cultural and environmental factors must play a role, since none of the studies mentioned above have found a 100% correlation between the IQ scores of MZ twins, as could be expected. It may be, instead, that twins have different experiences and it is these experiences that influence their IQ, and that it is because these influences are similar that there is a strong correlation.
Compelling evidence for environmental factors as an influence in the development of MI comes from Flynn, who found a global trend for IQ scores to be rising at a rate of around 3 points per decade. This is unlikely to be genetic as the rise is so rapid, and may therefore be a result of factors such as increased quality of education and healthcare which facilitate the development of MI. This therefore supports the role of environmental factors in the development of MI.
Further support for environmental factors comes from the 'HOME Inventory' study, whose researchers identified six key factors in the development of MI, such as the availability of stimulus materials such as books. They found adefinite corerlation between IQ scores and the amount of key factors in a household. This therefore supports the role of environmental factors in MI development, but others argue that this in fact supports the role of genes too. Burns & Farina suggest that genes affect how a person interacts with their environment, and are thus affected in a different way to the same environment than others would. They argue that we create our own 'microenvironment', in which we may elicit more positive or negative responses from others accordingly, and it is through this process tha MI develops. Therefore, the factors in the HOME inventiry may exist because of indirect genetic influences.
The main problem with all of the above research is that IQ tests may not be a reliable or valid measure of intelligence, as they can be biased, both culturally and according to gender. Jensen, for example, found that black Americans had, on average, an IQ 15 points lower than the rest of the population; but critics argued that this was due to cultural differences and bias in the test. A group of American researchers created an IQ test aimed at black cultures, and found that white children underperformed on it. Therefore, it may be impossible to assess whether MI is affected by genetic or cultural factors, or indeed whether MI even exists, by means of an IQ test.
