The nature versus nurture debate, is a classic debate in psychology. Modern psychology has since stopped trying to attribute behaviour to one cause, but instead has tried to figure out to what extent it is influenced by them. There is empirical evidence from twin and adoption studies, that suggests it is a combination of both. This essay will focus on homosexuality, and intelligence, and schizophrenia. The most common research methods used when studying genetics, are twin and adoption studies. Twin studies using monozygotic (MZ) and dizygotic (DZ) twins, can investigate the differences in genetic factors. Adoption studies are used to investigate environmental differences as monozygotic twins raised together (MZT), and apart (MZA. While these studies are useful to compare environmental and genetic influences, there are too many variables for us to determine cause and effects. Therefore, we can only compare concordance rates, and draw conclusions based on these rates. One study that used MZ and DZ twins, was Bailley and Pillards’ study of homosexuality. They discovered that the concordance rates for DZ twins was 22%, whereas it was 52% for MZ twins. From this straightforward statistic, it is clear that genetics must be a factor as there is a far higher concordance rate for those with 100% DNA in common. However, if homosexuality was purely genetic, the concordance rate for MZ twins would be 100%. As it is only 52%, there must be some environmental factors as well. Similarly, Heston investigated the levels of schizophrenia in children who belonged to foster families, to study genetic inheritance. This study found that the levels of schizophrenia in children with no family history of the disorder, was the same rate (1%) as the general population. However, in children who did have a family history of the disorder, that level rose to 10%. This is evidence to suggest that this particular disorder has a genetic component. One behavio