Describe the logic of twin studies.

Twin studies are an attempt to sort out the relative contribution of genetics vs. experience in the trait being studied. It is assumed that monozygotic twins are essentially genetically identical and if they are reared together that they have had significant similarities in their experiences. If they were to be raised apart, then they would still be essentially genetically identical, but the variable of experience may be significantly different. Traits that they are similar on may have a large genetic component in their cause and traits on which they differ may have a significant environmental component. Similarly dizygotic twins are no more genetically similar than other siblings so if they are reared in the same environment and are quite different on a trait, then it may be the case that that trait has a strong genetic component.