Tis most accurate to state that family, twin, and adoption studies are designed to determine the effect of on human behavior.

These studies are designed to determine the effect of both environment and genetics on human behaviour. The study of human behaviour has always focused on the question of whether nature or nurture play the most important role in the development of human personality and behaviour. Studying biological or adopted siblings and twins can help scientists assess the impact of genes and experience on behavioural phenotypes.