We know that a complex web of biology, psychology, and environment drives aggressive behavior in humans. But a recent, massive review of existing research sheds light on a highly specific piece of the puzzle: intelligence.
Anecdotally, criminologists have noted a link between lower cognitive abilities and general criminal behavior. However, the exact relationship between intelligence and direct, physical violence against others remained murky.
Researchers at the University of Valencia in Spain decided to dig deeper. They wanted to know if a lower intelligence quotient (IQ) is just an incidental background trait or a core facilitator of violent acts.
“The main motivation for this study was the absence of a systematic analysis assessing whether violence is truly related to the intelligence quotient (IQ) or whether, on the contrary, it is an independent factor,” explained Ángel Romero-Martínez, a professor of psychobiology at the University of Valencia.
Breaking Down the Data
To find answers, the research team conducted a massive systematic review and meta-analysis. They gathered every piece of relevant, previously published research they could find to identify overall trends.
The team scoured major scientific databases like PubMed and Scopus, initially pulling over 5,000 articles. After stripping away duplicates and irrelevant papers, they focused on 131 empirical studies that met their strict criteria.
They split their statistical analysis into two main chunks. First, they compared the IQ scores of violent individuals against non-violent control groups. This included 1,748 violent individuals and 2,623 controls. Second, they looked at the broader statistical correlation between intelligence and aggressive behavior across 33,118 participants.
What they found was that violent individuals scored significantly lower on full IQ tests compared to non-violent people.
This gap held true across both verbal intelligence — our ability to use and understand language — and nonverbal intelligence, which involves abstract reasoning and visual problem-solving.
“What was truly surprising was just how clear and robust the relationship turned out to be,” Romero-Martínez told PsyPost. “Beyond finding a general link, the most striking aspect was the consistent relationship across all different types of intelligence (verbal and non-verbal IQ).”
Reactive Versus Proactive Violence
You might wonder if these differences simply stem from wealth or educational gaps. Interestingly, the researchers found that socioeconomic status did not explain the intelligence gap. Many of the included studies accounted for economic backgrounds, and the IQ divide remained consistent.
This gap was particularly pronounced when the violent individuals also suffered from diagnosed mental or personality disorders. But here is where the research gets truly fascinating. The link between lower intelligence and violence does not apply to all types of harm.
The researchers discovered that lower IQ strongly correlates with reactive violence. This is the kind of impulsive, emotional outburst that happens in response to a perceived threat or intense frustration.
On the flip side, the study found no significant link between lower IQ and proactive violence. Proactive violence is planned, calculated, and goal-oriented. Navigating complex social manipulation or planned aggression might require a completely different set of cognitive tools.
Violence Drops as IQ Increases
The findings are very similar to a 2018 study led by Louis Jacob, which asked the same question: Does a person’s IQ affect their likelihood of behaving violently?
To find out, they analyzed data from a large, nationally representative study in the United Kingdom. They looked at nearly 7,000 adults, aged 16 and older. The team measured verbal intelligence using the National Adult Reading Test (NART) and asked participants a direct question about their past. Specifically, they wanted to know if the subjects had been in a physical fight or deliberately hit anyone in the past five years.
In this study, the prevalence of violent behavior dropped steadily as IQ scores increased. For individuals with IQ scores in the 70 to 79 range, 16.3% reported engaging in violent behavior. Compare that to the brightest cohort: just 2.9% of people with IQs between 120 and 129 reported similar violence.
You might think other factors were at play, like a rough childhood or struggles with addiction. But there too, the researchers controlled for demographics, childhood adversity, substance use, and mental health. Even with those variables accounted for, the link between higher intelligence and lower violence held firm.
Cognitive Overload
So, why does a lower IQ make someone more prone to impulsive rage? The researchers propose a cognitive resource-based explanation.
When we face high-stress situations, we rely on our mental resources to navigate the conflict. Strong verbal skills help us talk our way out of a dispute. Good problem-solving skills allow us to see alternative solutions.
Without those cognitive tools, a person might struggle to process their frustration peacefully. In the heat of the moment, physical or verbal aggression becomes a default reaction. However, the scientists are incredibly careful to point out that not being cognitively gifted does not automatically make you a person others should stay clear of.
“The most important takeaway is that while our study found a correlation between lower IQ and reactive violence, having a lower IQ does not mean a person will be violent,” Romero-Martínez explained.
“It is crucial to understand that intelligence is just one factor within a much more complex problem involving biological, social, and psychological variables. Rather than a direct cause, a lower IQ acts as a facilitator.”
Rewiring Rehabilitation
Historically, society has often stigmatized individuals with lower intellectual abilities. This study seeks to do the exact opposite. By pinpointing how cognitive limitations fuel reactive violence, we can fundamentally change how we rehabilitate offenders.
“The practical significance of these effects should not be interpreted to blame or stigmatize individuals with lower IQ scores,” Romero-Martínez continued during an interview with PsyPost. “Instead, the real value of these findings lies in identifying the therapeutic needs of people involved in violent acts.”
Current intervention programs often use a one-size-fits-all approach. But if an individual lashes out because they lack the verbal skills to de-escalate a fight, traditional therapy might not actually help them.
The researchers acknowledge that their meta-analysis has limitations. The included studies used various different intelligence tests, which can introduce inconsistencies into the data. Also, by only reviewing studies published in English and Spanish, the findings might not perfectly reflect global populations.
Future research will need to untangle how specific cognitive functions — like impulse control and mental flexibility — play into these violent outbursts.
The new findings appeared in the journal Intelligence.
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