Two children are playing outside — one child accidentally bumps into the other, and a fight ensues. But what exactly causes children to react to these situations in a violent manner? A study led by a team of University of Iowa researchers aims to answer this question.

The study, spearheaded by Isaac Petersen, an assistant professor in Psychological & Brain Sciences Department, looked at the role of the P3 brain marker, which is associated with aggression. In experiments that measured the brainwaves of 2.5- to 3.5-year-old children, researchers appeared to find that toddlers with smaller spikes in P3 brainwave activity were more likely to respond to stimuli in an aggressive way.

“The P3 wave, given that it may impact the social information processing cascade, is an important aspect of this study,” Petersen said. “If two children are interacting and the child with a shorter P3 wave interprets an ambiguous social cue as hostile, they may react in an aggressive way.”

The P3 wave — a series of brainwaves generated when people evaluate changes to their environment — has been studied in adults, and the scientific community has postulated about its role in the formation of aggression, Petersen said. But in the adolescent population, he said, it was important to track changes in brainwave activity over time.

“It was our intention to conduct this study longitudinally, because we wanted to follow the children’s brain development in order to understand the potential changes in P3 waves over time in relation to the development of aggression,” Petersen said.

Although this study does look at the role of the brain marker in situations of reactive aggression, there are a number of variables that have yet to be explored, he noted.

“We cannot know from this study if the P3 marker contributes to the development of aggression later in life,” Petersen said. “We do know that the P3 marker in adults is one of the processes that reflects the ability to detect changes in the environment and may contribute to mental-health outcomes, including depression and schizophrenia.”

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John Bates, an Indiana University professor of psychological and brain sciences who worked on the study, said he was particularly interested in the toddlers’ reactions to situations that could be interpreted in a negative way.

“More than that, you have to look into the signs of reactive aggression in children to understand where those sources of aggression are,” Bates said. “Even though this study doesn’t look into certain aspects of aggression later in life, this is a great starting point, not an end stage.”

Despite the “what-ifs” not covered by the study, Bates said, there can still be much garnered from its results and hopefully applied to future work in the area.

“An important aspect of this study is the ‘why did it happen’ of situations in which ambiguous stimuli are interpreted as aggressive,” he said. “Gathering information about these social situations can help us paint a clearer picture of what’s happening with things like the P3 wave.”