Intelligence and Crime

V. IQ Differences Between Criminal and Noncriminal Groups

The majority of studies have found IQ differences between offenders and nonoffenders (e.g., Ellis &Walsh, 2003). On average, the IQ for chronic juvenile offenders is 92, about half a standard deviation below the population mean. For chronic adult offenders, however, the average IQ is 85, 1 standard deviation below the population mean. A study of Texas inmates who entered the prison system in 2002 indicated that approximately 23% of the inmates scored below 80, almost 69% scored between 80 and 109, and only 9.6% scored above 110 (Ellis & Walsh, 2003).

To give readers an understanding of the relative proportions of individuals with IQs in those ranges, we offer the following statistics, from Ellis and Walsh (2003): Only 9.18% of individuals in the general population score at or below 80, 63.39% have an IQ between 80 and 109, and 25% have an IQ at or above 110. These data clearly show that low-IQ offenders (below 80) are substantially overrepresented in the Texas prison population (23%–9.18%), that those with scores between 80 and 109 are modestly overrepresented compared with the nonincarcerated population (69%–63%), and that individuals with IQ scores at or above 110 are underrepresented in the Texas prison population (9.6%–25%). Data from every other state reveal the same pattern.

IQ scores derived from prison inmates depict a clear relationship between IQ and offending; however, it is important to note that some scholars question the validity of this association. They question whether criminal justice processes function so that intellectually dull offenders are more likely to be incarcerated. If so, the association between IQ and imprisonment would be substantially inflated. Data from nonincarcerated offenders, usually matched on criminal record, cast doubt on this criticism. Studies have found that low-IQ offenders are more likely to be involved in crime over their life course, that they are more likely to be involved in chronic property crime, and that they are more likely to commit acts of violence (Ellis & Walsh, 2003). Their overinvolvement in crime, especially crimes involving violence, account for the reasons why they are incarcerated, not their low IQ.

Even so, it is important to point out that when data are collected through self-report questionnaires, whereby respondents are asked questions about their involvement in a range of criminal and delinquent acts, the magnitude of the association between IQ and criminal/delinquent involvement diminishes (Ellis & Walsh, 2003). Whereas some scholars point to this empirical regularity as evidence of the limited explanatory power of IQ, others correctly observe that the types of behaviors being measured influence the IQ -> delinquency association. For example, it is relatively common for adolescents to cheat on tests or to stay out later than their parent-imposed curfews. The majority of adolescents self-report involvement in these types of relatively innocuous behaviors. Because these behaviors are very common (some would argue normal), adolescents from all IQ ranges are equally likely to cheat or to violate their curfews.

This should not be taken as evidence that IQ is unimportant in delinquency or criminal behavior. When researchers examine self-report data that are based on measures of relatively serious crime, such as armed robbery, burglary, or assault, they note substantial IQ differences. Individuals with relatively lower IQs are more likely to report engaging in these serious criminal acts. The association between IQ and misbehavior therefore depends on the seriousness of the behavior being analyzed, with the association becoming stronger as the behavior becomes more serious.

The strength of the IQ -> crime association also depends on how frequently the individual engages in criminal and delinquent behavior. Low-IQ individuals are more likely to engage in serious misbehavior more frequently than their higher IQ counterparts, and they are more likely to engage in serious misbehavior over a longer span of their life course. Most life-course-persistent offenders also score relatively low on tests of IQ.

Another important aspect of the IQ -> crime association has to do with the difference between performance IQ and verbal IQ. Verbal IQ reflects an individual’s ability to read and comprehend written material and to use words correctly. Performance IQ is assessed through measures of spatial visualization, pattern recognition, and object assembly. Research has consistently shown that offenders are more likely to score lower on measures of verbal IQ than on measures of performance IQ. Explanations for this pattern are in short supply, but the association likely has to do with deficits in the language centers of the brain, specifically, Wernicke’s and Broca’s areas, that are indirectly assessed by the IQ test.

Language skills and abilities are crucial for healthy human development and appear universal to humans. For this reason, many linguists view language ability as innate, with the neuronal structures necessary for the development, use, and comprehension of language embedded in our DNA. Indeed, so strong is the “language instinct” that, barring any biological or genetic insult, all humans will develop the use of a language.

The use of language allows individuals to discuss problems and negotiate conflict. It allows for the use of instructions in learning, and it allows for feedback, teaching, and training. Reading comprehension, moreover, gives one the ability to learn from outside sources and to understand complexity in day-to-day encounters.

Language abilities emerge early in the life course, with verbal deficits identifiable by age 3. Unfortunately, language abilities become resistant to change by about age 9 or 10, when the language centers of the brain appear to formalize. These abilities are highly heritable, so whereas approximately 80% to 85% of the words an individual has in his or her vocabulary overlap with his or her parents’ vocabulary, the architecture that allows for these abilities appears to be genetic.

Verbal IQ also correlates moderately with the ability to think abstractly. Individuals capable of abstract thinking tend to be able to see the nuances in situations and relationships. They better understand not only the simple but also the complex. They see the interconnections between their attitudes and behaviors and the consequences that flow from their beliefs and behaviors. More important, they can understand how their behaviors and attitudes affect and influence others. Criminals, research tells us, tend to be concrete in their thinking—that is, they view the world in simplistic ways, often much like that of a young child (Ellis & Walsh, 2003). They are strongly influenced by the here and now, they do not tend to make effective generalizations from one situation to the next, and they tend to be very literal in their understanding of life events.

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