There are many studies to substantiate the belief that intelligence is an inheritable trait. Most are linked to IQ scores that are linked to the genetic factors. It is commonly noted that highly intelligent parents tend to have highly intelligent children. .
One of the most specific studies to prove the theory that intelligence is hereditary is that of identical and fraternal twins. It was found that the IQ of identical twins that are raised together is very similar to each other. This finding suggests a genetic influence in intelligence. On the other hand, fraternal twins IQ are more similar to each other than those of ordinary siblings. This finding suggests that the environment affects intelligence. Another important study is that of adopted children's intelligence. The IQ of these children has a small relationship with the IQ of their adoptive parents. In general, the IQ of adopted children is more similar to those of their biological parents vice their adoptive parents. These findings suggest the importance of hereditary factors of the intelligence. It is now generally accepted that individual differences in IQ and information-processing capacity are strongly influenced by hereditary factors, with genetic variance constituting about 70% of the total population variance in IQ (Jensen, 1981). There is also evidence that the genes for superior intelligence tend to dominant, which is what would be theoretically expected if intelligence is a fitness character in the Darwinian sense, and if it had been subject to natural selection through the course of human evolution (Jensen, 1983). Both adoption and twin studies have been used to examine genetic influences on intelligence. Both types of studies have clearly indicated that intelligence is, at least in part, genetically transmitted from parent to child. (Henry, 2002).
It is obvious that some traits, such as human intelligence will depend upon a complex combination of both hereditary and environmental influences.