Once a few aggressive or deviant individuals start engaging in a practice like sack tapping, it can quickly come to seem normal, drawing in other boys who want to feel like part of the majority, said Catherine Bradshaw, a developmental psychologist at Johns Hopkins Bloomberg School of Public Health.

Bradshaw said she has seen a similar type of aggression in schools in Maryland. Called “kickbacks,” students standing in line would kick each other between the legs from behind. The problem was bad enough that kids mentioned it to researchers who had come to investigate ways of reducing violence in schools.

Bradshaw said it’s hard to know whether sack tapping is a form of bullying, which has three defining characteristics, she said: a power difference between the bully and the victim; the intention to hurt; and repetition over time.