There is a general consensus that intelligence is a product of the complex interaction of heredity (nature) and environment (nurture).

Evidence for hereditary influences comes mainly from studies on twins and adopted children. The intelligence of identical twins reared together correlates almost 0.90, while those separated early in childhood also show considerable similarity. Identical twins reared in different environments correlate 0.72. In contrast, fraternal twins reared together correlate almost 0.60, and siblings reared together correlate about 0.50, while siblings reared apart correlate about 0.25. Furthermore, studies of adopted children show that their intelligence is more similar to their biological parents rather than adoptive parents.

Regarding the role of environment, studies report that as children grow in age, their intelligence level tends to move closer to that of their adoptive parents. Children from disadvantaged homes adopted into families with higher socio-economic status exhibit a large increase in their intelligence scores. Environmental deprivation lowers intelligence, while rich nutrition, good family background, and quality schooling increase it.

Therefore, heredity can best be viewed as something that sets a range or limits within which an individual’s development is actually shaped by the support and opportunities provided by the environment.