Since they are defining quality classrooms after the fact, based on how well students did, the third causal explanation might be students who do well in K are the kinds of students who will succeed in life regardless. Though the experience level of teacher and size of class appear to have an impact, many other studies show a poor relation to class size and outcomes.

Yes, the big missing options were no kindergarten, and kindergarten without formal teaching. There was a recent article in the Economist that said that almost all of countries with the highest PISA scores don't start formal teaching until children are aged around 7.

What about the glaring third option, "kids who go to nice kindergartens are probably well off and hence well positioned to succeed in life"?

Yeah, that's definitely a thing as well. I wasn't terribly precise in how I worded things. I was getting at something like "given that going to a nice kindergarten causes better life outcomes than going to a shitty kindergarten, there's two mechanisms that explain that causal link."

That's not a causal explanation, in this context. ThrustVectoring specifically limited the comment to causal explanations: why do better kindergarten classes cause better life outcomes.

Common cause is a causal explanation.

What's the difference between the two? Either way, a better environment leads to a better outcome.

They're the same if you're wondering which school to send your child, but different if you're deciding whether to send your child to school.

Oh, I see what you mean.