The results of the small number of existing investigations of criminality among twins have mostly been considered as contributions to the discussion of the relative importance of heredity and environment. I attempt to look at the problem in a slightly different context, but find it useful to give a brief summary of the older investigations. The results of previous studies utilizing the twin method to throw light on the problem of crime appear from Tab. I.

I am sure you will remember Lange's conclusion from 1929: “ As far as crime is concerned, MZ twins on the whole react in a definitely similar manner, DZ twins behave quite differently. In accordance with the significance of the twin method we must conclude that innate tendencies play a preponderant part among the causes of crime”.

There is one fundamental assumption behind the conclusions about the heredity of criminal behaviour, which is not fulfilled. The environment of each of the two twins in a pair must be equally similar or equally different, irrespective of zygosity. However, the milieus of the cotwins in a DZ pair are in fact more different than in a MZ one.

The Danish study is carried out in cooperation with the Danish Twin Register (Harvald and Hauge, 1965). The material consists of data about nearly 6000 pairs of twins, i. e., all twins born on the Danish islands in 1881-1910, where both twins have survived to the age of at least 15 years. The source material may be considered complete. About 900 pairs have been entered into the Central Police Register and/or into the local penal registers.