In 1939 Lionel Penrose published an incidental finding in an investigation whose primary purpose was to ascertain the meanings of the terms ‘insanity’ and ‘mental deficiency’ in different parts of the world (Penrose, 1939). His views were based on two assumptions: the first was the ‘knowledge’ based on Cyril Burt's The Young Delinquent that mental disorder predisposed to crime; the second was the ‘supposition’ that the criminal population contained a group who should properly be labelled as mentally diseased (Burt, 1925).

Penrose said that generally there were two ways of segregating the socially undesirable: wait until a crime is committed and then remove them from society; provide institutional care as material for ‘medical attention’. He emphasized the view that medical attention would be judged by its success in preventing damage to the community from crime. Penrose considered the larger European countries in the year 1934. (For the present discussion three countries have been chosen because of their similar populations in 1934 of about 40 million.) He did not question the reliability and validity of his criterion groups: mental disease was measured by the number of institutional beds in a country; crime by the amount of prison accommodation.

Although the claim was unfounded Penrose concluded:

Penrose then returned to his social hygiene theory. He suggested that attention to mental health may help to prevent the occurrence of serious crimes, particularly deliberate homicide. He drew the inference from the figures set out in Table 6.2.

Whatever caution Penrose showed in interpreting his findings in 1939 had disappeared by 1943 when he wrote in the American Journal of Mental Deficiency on the statistical relationship between mental deficiency and crime:

And:

The prevalence of serious crimes, especially those which imply violence against the person, appears to be much more marked in countries which provide relatively few beds for mental patients.