Natural experiment designs

This paper by Michael Rutter is a couple of years old but has some useful guidelines on various kinds of natural experiment designs. One I liked is the children of twins design where the children of monozygotic and dizygotic twins are compared to assist in separating genetic and environmental factors that contribute to the physical or mental health of the children. In this case it's expected that the children of the MZ twins (comparing the children of MZ twin 1 with the children of MZ twin 2) should have more similar problems than the children of DZ twins if the inheritance is genetic. If parents transfer risk factors to their children socially/through environmental factors then it is expected that the differences between the children of MZ twins and between the children of DZ twins should be comparable. You may have to consult the article for this one!