The Effect of Enviroment on Intelligence

In the ongoing debate about which factor has more of an influence on human behavior and intelligence, certain methodologies have been used to determine this in several different studies. The genetic component is made up of additive and non-additive components. The environmental component is separated into the effects of a shared environment where both subjects used experience the same environment and a non-shared environment where the two subjects experience different environments.

The studies that are used to determine the respective effects of genetics and environment are studies of how twins resemble each other, studies of adoption effects people compared to their relatives, and identification of genes. Twin studies use monozygotic twins who share 100% of their genes and dizygotic twins who are estimated to share 50% of their genes. Genetic identification tries to identify genes responsible for personality traits but is made difficult by the fact that many behaviors are caused by multiple genes.

Intelligence or IQ has been studied extensively. It has been found that in childhood the correlation of IQ between twins is about .5. This correlation has also been found to increase with age. Similar results have been found in adoption studies indicating that environmental differences are not as much of a factor on IQ scores.

In addition to IQ several other things have been studied to find out if their cause is in the environment or genetics. Personality has been studied using the Big five traits that are commonly used to test personality. Personality disorders have also been studied as well as occupational interests and beliefs and attitudes. For all of the heritability has been rated to be the majority of the cause. There has been a lot of effort invested in finding specific genes that cause people to be a certain way however they have not been very successful.

Studies have been conducted to determine the effects of shared environment on twins. Since they are known to have the same genes differences can only be attributed to environment and it has been found that twins raised in different environments do not necessarily have more in common with regard to personality. However it has been found that shared environment does have an effect on cognitive ability and juvenile delinquency.

While shared environment has not been shown to have a large effect non shared environmental factors have been shown to be a major source of variation.