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Violent Crime & Intelligence
Association between intelligence quotient and violence perpetration in the English general population.
Written by Matthew Archer.
A friend sent me this horrifying video of a woman being stabbed in South London by a teenager whilst a horde of other children gather to laugh, film, and mock her. So I thought this would be a good moment to look at some fascinating data on IQ and violent crime. You can read a write up of this story here:
This is the study I’ll be walking you through:
Okay, so a few things: the study is addressing the limitations of the usual data on IQ and violence, namely the samples. A lot of studies are either too small or not representative (they focus on inmates, for example). So that’s a tick. They also adjust for some obvious confounds like sociodemographic factors, childhood abuse, bullying, substance abuse, and common mental disorders. Another obvious point: it’s a UK sample, so highly useful considering the rise in violent crime in many major cities over the last decade. The sample is limited to over 16s, although that’s not necessarily an issue when it comes to applying their findings to under 16s given the stability of IQ.
They used a reading test as a proxy for verbal IQ:
Verbal IQ was estimated using the National Adult Reading Test (NART). The NART, a brief measure administered only to native English speakers and widely used in the world, consists of a list of 50 words and is scored by counting the number of errors made in reading out the words (Ali et al., 2013). The reliability of the NART has been assessed by a split-half technique (Cronbach α) which gave a reliability coefficient of 0.93 compared with the Wechsler Adult Intelligence Scale (WAIS) (Nelson and Willison, 1991).
The problem here is that this is a test used for assessing premorbid intelligence. That’s when you have sustained something like a brain injury or dementia and the clinicians want to know what your score would have been like prior to the injury/condition. From the Wiki for NART:
The test comprises 50 written words in British English which all have irregular spellings (e.g. "aisle"), so as to test the participant's vocabulary rather than their ability to apply regular pronunciation rules.
The correlation between NART scores and age 11 IQ was moderately high at 0.60. Not exactly brilliant, but hey-ho. Also, note that they excluded non-native speakers, a potential problem when it comes to assessing the link to violent crime depending on the nationality and ethnicity.
They also rely on the honesty of self-report, but at least it’s only one question (so the low IQ folks can’t fuck up too badly):
Participants were asked ‘Have you been in a physical fight or deliberately hit anyone in the past 5 years?’ with ‘yes’ and ‘no’ answer options, and the number of violence episodes per individ- ual was measured. For those who replied affirmatively, additional questions were asked regarding potential intoxication during the violent episode, persons who were involved in the violent episode
Still, some nice results:
There were 6872 participants aged ⩾16 years included in the cur- rent analysis. The mean (S.D.) age was 46.3 (18.6) years and 48.2% were men (Table 1). The prevalence [95% confidence interval (CI)] of violence was 8.8% (8.0–9.6%), and 8.7, 12.3, 21.1, 19.4, 25.4, and 13.1% of the individuals had an IQ of 70–79, 80–89, 90–99, 100–109, 110–119, and 120–129, respectively. The mean (S.D.) of the IQ score was 102 (15). Trends in the sample characteristics by IQ scores were significant for all variables except life- time bullying and smoking status.
even after adjusting for demographic and behavioral factors, childhood adversity, and psychiatric morbidity, low IQ was asso- ciated with significantly higher odds for violence perpetration with an IQ <90 being associated with more than a two times higher odds for violence perpetration compared with those with IQ 120–129. The strength of the study includes the large sample size, the use of nationally representative data, and the inclusion of a variety of potential confounders in the analysis. To the best of our knowledge, this is the first study to investigate the relationship between cognitive ability and violent behavior with nationally rep- resentative data in the UK population.
For you visual learners:
They don’t show the data in the paper, but there were no significant interactions by sex, age, and ethnicity in the association between IQ and violence perpetration.
The authors come to the obvious conclusion that low IQ is very likely to be causal when it comes to violent behaviour. I find it rather incredible to think anybody could argue otherwise. Being stupid means being impulsive and not being able to map the consequences of your actions. I think you’d see even sharper data if we could assess the schoolchildren involved in that video, who no doubt suffer from a plague of other relevant confounds like fatherlessness, testosterone, etc. Anyway, it’s worth noting the authors final words:
Clinical implications and directions for future research
Before going further, one has to bear in mind that intelligence is not frequently assessed in the general population, while IQ is gen- erally considered to be a non-modifiable risk factor. Thus, pre- ventive programs in this setting are very difficult to implement. That being said, in some settings, IQ is often measured in children with abnormal development or difficulties at school, or indivi- duals with psychiatric disorders. Based on the findings of the pre- sent study, it is important to note that people with low IQ may be at high risk of violence perpetration in the future or that low IQ may be an underlying factor in individuals who frequently engage in violence. Therefore, individuals with low cognitive ability should be regularly followed and several actions should be pro- posed throughout their life. For example, an education develop- ment program might be interesting at young age in order to improve the performance of children with low IQ at school. Academic alternatives might further be proposed to adolescents who failed at school or repeated school years multiple times, and professionalizing training could help them find their path. Follow-up of these individuals may not only help reduce violence but also other adverse outcomes which are known to be associated with low IQ such as obesity (Chandola et al., 2006), alcohol con- sumption (Sjölund et al., 2015), and common mental disorders (Koenen et al., 2009). In addition, we believe that the develop- ment of recreational activities is important for the societal integra- tion of individuals with low IQ. Programs favoring the development of empathy and sensitivity might also help reduce the risk of recidivism in individuals who have already committed violent crimes. Further research is necessary to evaluate the impact of these different actions on the risk of violence perpetra- tion in people with low cognitive abilities. Our study also high- lights the importance of interventions across the lifespan given that the mean age of our sample was around 46 years, and that there was no significant interaction by age, while violence is also frequent in older individuals (Warmling et al., 2017).
Matthew Archer is the Editor-in-Chief of Aporia.