Since the earliest days of intelligence testing, it’s been known that criminals average IQ’s around 90 (about 10 points below the North American average of 100). What’s interesting is that criminals also score 10 points below their non-delinquent siblings who grow up in the same home. Comparing criminals to their siblings is useful because it controls for so many of the socio-economic factors that are so often assumed to confound research on both crime and IQ.
Given that intelligence and morality are two different concepts, some may be surprised by the fact that criminals tend to have lower IQ’s. Indeed criminal geniuses such as the fictional Hannibal Lecter are a pop culture fixture. In the book, The g Factor by Arthur Jensen, three hypotheses are discussed:
1) Criminals have lower IQ’s because they’re not smart enough to achieve success through conventional avenues and thus must stoop to unethical methods.
2) Criminals have lower IQ’s because crimes are often impulsive, and so people who think through the long-term consequences (for themselves and the victims) are more likely to avoid crime.
3) Criminals have lower IQ’s because they’re not smart enough to understand why their crimes are wrong.
Pumpkin Person would like to propose a fourth hypothesis. Criminals have lower IQ’s because some of the same factor(s) that stunted their intellectual development, also stunted their moral development, since both are a function of the brain.
A popular hypothesis among laymen is that criminals actually don’t have lower IQ’s, they just appear to because all the smart ones get away, but this idea was dismissed by Charles Murray and Richard Herrnstein in the book The Bell Curve. They argued that there are not enough unsolved crimes for there to be a secret group of high IQ criminals and that among convicted criminals, the duller one commit more crimes, and used both these facts to dismiss the high IQ criminal theory.