What are the influences of environment and heredity on measured intelligence?.

            

             Psychologists are greatly divided over whether heredity or environment has a more dominant influence on individual intelligence.

             Although some animal studies appear to suggest heredity have the greatest influence, a seemingly more accurate conclusion may be drawn form human studies on intelligence. .

             In studies on the similarities between IQ of siblings reared in the same and separate environments, the IQ of fraternal twins reared together showed greater similarity than the scores of ordinary siblings.

             One may wonder why this is so! Parents tend to treat twins more alike than they do ordinary siblings. The influencing factor behind the increased resemblance in the twins IQ proves to be environmental.

             Identical twins who are raised together show even greater similarities in their IQ, which is to be expected, considering their identical genes and the similar environmental influences they receive. When identical twins are raised apart the similarities in their IQ can be seen to decrease, however not by much. As the similarities are still impressive one may be lead to believe that heredity plays a major role in intelligence, but one must not overlook the fact that almost all separated twins are placed in homes socially and educationally similar to those of their biological parents. This tends to falsely increase the apparent effect of genes on intelligence, by making the separated twins IQ more alike.

             Psychologists who place emphasis on environmental influence in regards to intelligence point out that in the cases where separated twins were placed in homes with significant educational and environmental differences to those of their birth parents, some twins IQ scores vary by as much as 20 points.

             Other evidence, which seems to refute the influential importance of heredity on intelligence, comes from parents who raise both biological and adopted children.