[redcalx]

Thanks. Seems they were acutely aware of this; it's the first thing discussed in the paper:

http://jech.bmj.com/content/early/2016/10/05/jech-2016-20789...

They're comparing with people who went to other groups, and concluding that there is a quality of scouts and guides in particular that is protective.

I do wonder if there is still selection bias, since you may choose to avoid certain types of group, and attend others.