Over a quarter of cyberbullies suffer from post-traumatic stress disorder symptoms, the study, published Tuesday in the Archives of Disease in Childhood Journal, found.

Clinical psychologist Ana Pascual-Sánchez, one of the authors of the study at Imperial College London, said her team was surprised by those results.

“Perpetrating aggression exposes bullies to potential violent situations in which they can lose control and even feel vulnerable at some point or regret from it, having intrusive memories,” Pascual-Sánchez said.