COMT gene has also been linked to schizophrenia a lower level functioning version has shown poor performance on "stroop" tasks.
DZ twins are no more genetically similar than brothers and sisters born at different times so there should be less of a relationship between IQ scores of non identical twins.
One study examining identical twins as looked at by Bouchard & McGue (1981) who conducted a review of 111 world wide studies on family IQ ignoring studies which they claimed had mythological deficiencies and came up with the following average correlations of IQ
Identical twins reared together had an average correlation of 0.86 and for non identical twins reared together they had an average correlation of 0.6. This shows that there must be a significant genetic influence on intelligence because there is a difference between the MZ and DZ correlations as predicted.
However, studies which simply look at concordance rates for twins reared together can never separate out the influence in the environment so they will always have questionable validity. Twins are renowned for being raised identically which means they must have very similar environmental influences. The solution to this has been separation studies where twins who are separated at birth are compared presuming that they have different environments and so suggesting that this limitation may not be valid for this group of studies.
Sir Cyril Burt (1970s) did separation studies of twins and they found that concordance rates were very consistent with correlations of 0.77 in virtually all 30 trials spanning 30 years. This strongly suggests that IQ test performance has a strong genetic influence Buchard & McCue also conducted a meta analysis of the separation studies and found the results to be accurate at 0.72 correlation.
Meta analysis studies are potentially very valuable sources of evidence for genetics in intelligence because if a finding is consistent across many studies it suggests that it's true. Because it's unlikely to be a fluke.
Attention from the family, if they're second or third born they get less attention, therefore, academic IQ development would be less than the older siblings.
Peer group if their friends are intellectually superior they would be able to
Quality of education can affect how well the receive the information, because if the class sizes are larger the amount they would take in would be less size of class.
Daycare in the early years develops their intellect which could lead to superior intellect.
Nutrition - eating the right things such as protein can help further development and allows the Brian to grow lager. Vitamins and minerals. In the short term, high sugar and not eating correctly, lead to a lack of concentrating which can affect learning over a longer periods of time.
However, there is evidence which challenges the genetic approach and claims that environmental factors can actually affect intelligence test scoring.
One piece of evidence which challenges genetics comes from Skeels (1966) where reports the case of 13 infant orphans who with an average IQ of 64 were transferred to a special institution and were given enriching interaction with older girls. By the age of 7 they had gained an average of 36 points compared to a control group of orphans who remained in the orphanage and whose IQ dropped by an average of 21 from an original average of 86 This shows that an enriching environment can affect IQ and so suggests that environment does effect IQ scores.
Another piece of evidence which suggests that the environment has an effect on IQ is the Flynn effect. Across western nation, IQ's have steadily risen since records began 7 IQ points between 1940 and 1979 twenty countries showed the Flynn affect. People have actually become more intelligent this is as the increases in IQ scores have happened too quickly to be explained through genetic environment.
Education there has been a steady trend for children to attend school and college for longer than previous generations length of education correlates with IQ scores however the Flynn effect is seen more with non verbal IQ where as schooling is likely to affect both verbal and non verbal scores.
However, we need to be cautious about this evidence because these increases may be due to familiarity to tested systems rather than just effects in the environment.
Another piece of evidence which suggests environment effects intelligence test scores is nutrition Oddy (2004) found that babies that were breast fed for more than 6 months had better verbal intelligence at 8 than babies breast fed for less than six months in a landmark study.
Another study which shows that prenatal facts which can affect Prenatal factors was conducted by Mortensen (2005) for instance found that children and mothers who smoked more than 20 cigarets a day during pregnancy did badly on IQ at ages 18 to 19. Can lead to foetals alcohol syndrome in this the baby has low birth rate weight and often suffers mild brain damage and reducing IQ.
One major issue with research relating to a rich learning environment is that people with a genetic intelligence may actually create an ideal environment around themselves which can develop their intelligence and so appear to show that the environment is causing intelligence when in actual fact it's the genes that are shaping the environment. For example, in schools if you appear to be intelligent, teacher will treat you differently in giving you harder assignments to complete this makes it appear that the environment is making the child more intelligent but it's the child's intelligence which is shaping the environment.