A friend of mine put this as her facebook status last week:

"I have a theory about why it appears that self-injury, including tattoos and piercings, have increased so much. It involves the decrease in physical pounding that comes from daily active play as children and adolescents; I think there is a neurobiological need for a certain level of touch/pressure feedback that our most recent generations have not been getting in their "diet," leading to cravings for it later on."

I commented that if that's the truth, then my kids will never get tattoos. :)