Crime and intelligence

Many theorists have tried to find a clear connection between crime and intelligence. Unfortunately, connection between crime and intelligence isn't clear when the experiment is well controlled, and most of the factors are identifiable. It is necessary to point out that crime is a social phenomenon and in most cases is defined by laws, while intelligence is a human ability determined by psychological instruments, i.e. tests (psychometric theory).
Problems when researching crime and intelligence
1. The definition of crime changes geographically.
2. There are at least 20 definitions of intelligence and every definition of intelligence is somewhat different. Some theorists perceive intelligence as an ability that is measured by tests, other theorists as a form of holistic and complex ability or set of identifiable and unidentifiable abilities.

Research on intelligence and delinquency identified two areas of study. The first area was related to the assessment of how intelligence affects the development of criminal career. Second area of research related to the assessment on how low IQ influences on the committing of crimes. 

Research of crime and intelligence  
Many researchers believe that the cause of crime is sociological factors such as poverty or ethnicity, while other researchers stress the importance of intellectual functioning. Some studies explain that the low intelligence is directly associated with poor performance in school, business achievements, social adaptation and integration, and thus indirectly with delinquency.
Richard Dugdale first showed interest to research a connection between crime and intelligence. In 1877,  he published the study of Jukes family tree i.e. "pedigree family tree study" where he analyzed available data on the Jukes family members in past 200 years. The Jukes were very known New York family at that time. Dugdale's goal was to establish a connection between low intelligence and criminal genesis. Conclusion of his study was that members of Jukes family were mostly criminals, degenerates, prostitutes or asocial persons. This study had an impact on forming of public opinion, hostile stance towards such a persons and call for action to eliminate persons like Jukes from the society. This was the turning point for development of Eugenics Movement.
Howard H. Goddard analyzed the Kalikaks family tree in 1912 in the institution for feeble-minded persons in New Jersey. Goddard analyzed the data on six generations of Kalikak's family and came to the conclusion that in case of Kalikaks family members, mental retardation is inherited. To support his claims Goddard used G. Mendel's genetic laws to explain the role of recessive genes and their effects on inheritance of mental retardation. According to Goddard chances for Kalikaks family members to suffer from mental retardation were 3:1. Goddard's goal in the Kalikaks family tree study was to prove that mental retardation is inherited. Goddard personally thought that mental retardation was one of the possible causes of crime and ascertained that 97 % of prostitutes, 80 % of tramps, 70% of prison inmates and 50 % of poor was feeble minded at that time.
In 1919 Goring estimated that there are 10-20% of people among grown-up delinquents who have bellowed average intelligence. In his assessment Goring didn't use any tests, but briefly estimated the percentage based on his experience. Goring used the publication of Royal Commission for England and Wales from 1908th. that determined 0,5 % of feeble-minded persons in England and Wales as control results. According to the results of Royal Commission for England and Wales, 10% of prison inmates was feeble-minded. Goring was radical in his ideas and thought that mental retardation is adequate enough to replace Lombroso's biological substrates and a way to identify future criminals.
In 1950's Mannheim found that previous studies were wrong in an attempt to present the mental retardation as  a cause of crime.

In 1970 Gibson and West made an analysis of intelligence tests. Research results showed that the delinquents achieved lower results on verbal and non-verbal intelligence sub tests.
In 1972 Wolfgang, Figlio and Sellin analyzed the results of intelligence tests of 9945 participants. Delinquent subgroup had 5 -10 points less than the non-delinquent group. On the same sample, Wolfgang and his associates compared the crime perpetrators and crime recidivists. A result of research showed the difference in IQ by an average of 8.1 points for the whites and 10.6 points for the blacks. The crime perpetrators achieved better results on intelligence tests than crime recidivists.
In 1990, Moffitt made a research on the sample that consisted of 536 girls and boys and found an average difference of 8 points in IQ scores between delinquents and non-delinquents. The difference in IQ scores between crime perpetrators and crime recidivists was 17 points. In Moffitt's research, delinquents and crime recidivists achieved lower IQ scores than non-delinquents.

Theories of crime