The kids-stress continuum

Ok, here's another question I've been asked a lot, though not as much as the Katie Couric one.
Does having more kids make you more and more brainy?
Mostly this question has been asked in a jocular way -- except, I suspect, when it was asked, fairly recently, by a TV news interviewer from Utah. Even so, it worries me. I spend a lot of time writing about the environment, and the last thing I'd want to do is encourage people having more babies for the sake of their brainpower. Nor do I think that's a logical argument. (When I raised it with one scientist, he reminded me of the famous Groucho Marx line, "Lady, I like my cigar, but I take it out of my mouth every once in awhile.") Playing bridge is reportedly good for your brain, but certainly not if you do it all day. And one of the strongest arguments in The Mommy Brain is that the smarts you get from having kids really depends on your level of engagement and focus. It seems obvious that staying engaged and focused -- even if you're avoiding going overboard with the child-centric stuff, becomes harder the more you have to split your time and attention. But maybe this is controversial. I'm sure you'll let me know.