Experiencing bullying and forms of aggression in late adolescence and early adulthood is linked to a marked increase in the likelihood of having daydreams or fantasies about hurting or killing people, according to a new study.
While research has shown that significant numbers of people fantasise about inflicting harm*, little is known about the processes behind such "violent ideations".
A team led by a University of Cambridge professor tracked the self-reported thoughts and experiences of 1,465 young people from schools across the Swiss city of Zurich at the ages of 15, 17 and 20.
Researchers gathered data on whether violent thoughts had occurred in the last 30 days, and the types of bullying or aggression experienced over the last 12 months.
They used questionnaires to probe the levels of aggression (humiliation, beatings, murder) and imagined targets (strangers, friends) within young people's darkest fantasies.
The team also asked about experiences of 23 forms of "victimisation", such as taunts, physical attacks and sexual harassment by peers, aggressive parenting - yelling, slapping and so on - and dating violence e.g. being pressured into sex.
While the majority of teenagers had been victimised in at least one way, experiencing a range of mistreatment was "closely associated" with a higher likelihood of thinking about killing, attacking or humiliating others.
Boys were more prone to violent thinking in general, but the effect of multiple victimisations on violent fantasies was very similar in both sexes.
Among 17-year-old boys who had not been victimised in the preceding year, the probability of violent fantasies in the last month was 56%.
With every additional type of mistreatment, the probability of violent fantasies increased by up to 8%. Those who listed five forms of victimisation had an 85% probability of having had violent fantasies; for those who listed ten it was 97%.
Among girls the same age, no victimisation experience had a violent fantasy probability of 23%, which increased to 59% in those who listed five types of mistreatment, and 73% in those who said they had suffered ten.
"One way to think about fantasies is as our brain rehearsing future scenarios," said Prof Manuel Eisner, Director of the Violence Research Centre at the University of Cambridge, and lead author of the study published in the journal Aggressive Behavior.
"The increased violent fantasies among those who experience bullying or mistreatment may be a psychological mechanism to help prepare them for violence to come," he said.
"These fantasies of hitting back at others may have roots deep in human history, from a time when societies were much more violent, and retribution - or the threat of it - was an important form of protection."
According to Eisner, the research hints at the extent of violent ideation in societies as seemingly peaceful as Switzerland - with murderous thoughts surprisingly commonplace.
"About 25% of all 17-year-old boys and 13% of girls reported having at least one fantasy of killing a person they know during the past thirty days. Close to one in five of all the study participants at that age. These thoughts may be deeply troubling to those who experience them," he said.
The team - including researchers from the University of Zurich, University of Edinburgh, University of Utrecht, University of Leiden, and Universidad de la Republica - collected and analysed a wealth of data.
As such, they were able to filter out and 'control' for other possible triggers for violent thinking in the teenagers. For example, they found that socio-economic status played little role in violent fantasy rates.
The study also shows that "adverse life events" such as financial troubles or parental separation had no significant impact. "Thoughts of killing others are triggered by experiences of interpersonal harm-doing, attacks on our personal identity, rather than noxious stimuli more generally," said Eisner.
"It's the difference between conditions that make people angry and upset, and those that make people vengeful."
By following most of the teenagers to the cusp of adulthood, researchers could track patterns over several years. Overall rates of the most extreme thoughts decreased by the age of twenty: only 14% of young men and 5.5% of women had thought about killing someone they know in the past month.
However, the effects of victimisation on violent fantasies did not lessen as they grew up, suggesting the intensity of this psychological mechanism may not fade.
"This study did not examine whether violent ideations caused by victimisation actually lead to violent behaviour. However, a consistent finding across criminology is that victims often become offenders, and vice versa," said Eisner.
"Fantasies are unrestrained, and the vengeance taken in our minds is often wildly disproportionate to the real-world event which triggered it.
"Studying the mechanisms behind violent fantasies, particularly at a young age, may help with targeted interventions that can stop obsessive rumination turning horribly real."
* Examples of previous research on levels of "homicidal ideation" among adults cited by the study include:
- Between 50-80% of university students report at least one fantasy of killing another person in their life;
- 14% of a community sample of adults had experienced daydreams or thoughts about physically hurting other people in the two months prior to the study.
* The research team also found a minor correlation between consumption of violent media and violent thoughts, but the effects were small. A more significant predictor was simply having reported violent fantasies at an earlier age, as well as a belief that violence can be justified, which contribute to what researchers call "trait aggressiveness".
* The study was conducted with the Zurich Project on Social Development from Childhood to Adulthood, which is mainly funded by the Swiss National Science Foundation and the Jacobs Foundation. It is hosted by the Jacobs Center for Productive Youth Development at the University of Zurich.