But also even if we allow that there is benefit in a stable, two-parent household in which the parents don't change around at all, if we can even isolate that variable away from other factors, it seems tautological.

To make an analogy that's close to my heart right now -

In a drinking water capture system, you can probably collect the most water in a pristine, well-designed system where all the pipe gauges are the same and the fixtures are all well fitting and designed for the duty they perform. But if you haven't got one of those, if the one you have is full of holes and leaking all over the place every time it rains, and the pipes are jammed onto each other with no consideration of size or gaps, then ramming bits of cork into the leaks and praying isn't going to make it perform as if it was. Sometimes you're going to get better results if you rip out the downpipes, dig up the junctions and fit major parts of the system again.

Sure, it still might not be quite as good as that well-designed, perfect system, and you're probably going to get PVA glue all over your hands and clothes while you do the work, but it'll be a darn sight better than watching your drinking water piss out all over the veranda every time it rains...

I'm not sure if this comment is about divorce any more. Right, I'm off to buy more pipe fittings, grrr mumble mumble.