R.J.
It doesn't have to be karate, any martial art (taught at a good dojo) teaches discipline and emotional regulation in a way that gels REALLY well with how our minds work. In large part, because we're forming mental and emotional regulation *while our bodies are moving*. Dance works for some, but the fact that it's a preforming art AND a main focus of dance *isn't* honor, respect, evaluate, act, reevaluate, act, correct, recover nature of martial arts, means that a person just sort of has to get lucky with the right instructor.
It's the evaluate act reevaluate act evaluate act repetitive but constantly changing nature of martial arts (so it doesn't get boring)... plus that focus on personal responsibility, honor, choice done in a *positive* way, aspects that just really takes our emotional swings and "centers" us.
Horseback riding is also REALLY useful, but in an entirely different way. My son is in Aikido just having started this year, but because of budget concerns, we're waiting to get him riding regularly.