This is the finding in the latest Journal of Economic Behavior & Organization paper by John Ermisch and Diego Gambetta. They provide that that is the case, but add that the mechanisms that underlie this effect revolve around the level of outward exposure of these families:
"[F]actors that limit exposure limit subjects’ experience as well as motivation to deal with strangers."
So, it's not really because family ties is strong, but it's really because of lack of experience in interacting with strangers. And it makes sense. So it's okay to have strong family ties. Just makes sure that you don't isolate your kids.