There are some researchers who believe intelligence testing results is due to environmental factors. One way researchers came to this conclusion is by conducting twin and adoption studies. In the same study conducted by Bouchard and Mcgue (1981), found that identical twins who were raised with their biological parents had better intelligence scores than those twins living with adoptive parents. Waldman, Weinberg, and Scarr (1994) studied adoptive children and found that those raised by their poor biological parents had lower intelligence scores than those raised by adoptive middle-class families.