[totally]

Yep, the bias is that all of the scouts came from households that decided to enroll their kid/s in the scouts. This is not a joke, and there is a difference in value systems and other factors that cut across cliché dimensions like socio-economic status.

[pablobaz]

As a former scout leader, I think there is also another effect at play. In my experience parents often send children to scouts when they don't fit into more mainstream team sports and when children are struggling to make friends elsewhere.

These children are often particularly socially awkward. I'm not an expert but I think that this group of children may be more predisposed to mental illness.

[takingflac]

Just curious, what was the size of the city/town that this scout troop was in?

[pablobaz]

A town of about 50k people, but the troop had a wider catchment area of maybe up to 200k

[disconcision]

Yes, this seems likely. Even putting aside fine-grained value differences, a friend of mine put it this way: "Kids of parents who don't care enough go to summer camp, kids of parents who care too much go to scouts." (Referring, mostly, to the average expected levels of parental involvement.)

[timthorn]

The study lead was interviewed on R4 this morning, and asserted they they'd controlled for this.

[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.