Parents are better at teaching the social niceties of more formal settings -- how to act in public, how not to embarrass oneself at the dinner table, for example. But siblings are better role models of the more informal behaviors -- how to act at school or on the street, or, most important, how to act cool around friends -- that constitute the bulk of a child's everyday experiences.This all makes sense to me and fits well into what we already know. Kids are more influenced by peers than parents, which means it really is important to have your kids hang around with decent kids. Yes, I know, your idea of what constitutes a decent kid may differ from mine. I merely mean you should have your kids hang around kids you want them to be like, whatever that is. Lord Chesterfield was right when he wrote to his son that you become whom you hang around.
That's why I have mixed feelings about things like academic tracking in elementary and middle schools and allowing a percentage of rich kids in State Preschool. It is better for a kid to be in a class with kids who are smarter than she is, unless the difference is too great. When kids are more than a couple of standard deviations apart, they have little to say to each other. But when your kid is in a class with kids, say half an SD smarter, or one SD, being with the smarter kids helps.
Of course, this also applies to the smart kids. When you mix really smart kids with normals, it helps the normals but hinders the smarts. To let the really smart kids thrive, you would put them all in one class and let them push each other. How many smart kids to cluster together is a compromise between these two goods.
And for State Preschool kids, we would expect them to be more influenced by being around rich kids (because poor kids come in knowing fewer words and concepts) than middle-class normal kids would. So it would help State Preschool kids if a certain percentage of rich kids were in their class. This happens in college campus CCTR (now CSPP) programs, where full-pay kids of staff and faculty are mixed in with low-income kids. We should encourage this kind of mixing.