The type of brain damage that will make the participants more likely to commit acts of violence in the future due to impulse control issues (see the high incidence of prior concussion among violent offenders in prison, high incidence of domestic violence among people who've suffered head trauma previously, many known instances of people who became irritable, impulsive, violent after concussions/mtbi).
Even those who don't commit violent acts are likely to suffer depression, anxiety, mood swings, difficulty concentrating, etc.
Society at large bears the burden of the increased healthcare costs for these individuals later on in life. Every slap can be directly linked to future costs to medicare/hospitals, police/first-responders, judicial system, etc.