There are some researchers who believe intelligence testing results is due to only genes. One way psychologists came to this conclusion by doing twin and adoption studies. Psychologists believe if identical twins have similar intelligence scores than fraternal twins we can assume that genes are correlated with intelligence. Bouchard and Mcgue (1981) looked at twins and how they were raised, whether it was in the same home or different homes. The study concluded that identical twins who were raised in separate homes had more similar intelligence test scores than fraternal twins raised in separate homes. Polmin, Fulker, Corely, and Defries (1997) found that adoptive children's intelligence scores are highly correlated with their biological parents suggesting that IQ is based on genetics instead of environment.