My observation of my friends (mostly SF bay area) is it's about 50/50. I'd like to think that it's my brilliant parenting that we have a one-meal family, but our 4yo has always been very food-motivated. Without getting too much into the nature/nurture debate, I'm willing to accept there's plenty of both, and I try not to judge my friends who make separate meals for their kids.

That said, kiddo doesn't like shrimp (yet!), and I love shrimp, so on shrimp nights he gets something else.