Probably both nature (instinct) and nurture (depending on circumstances). I think it is too complex for simplistic solutions. For example, the few I've seen wean them at three weeks was in the heat of summer when food was really plentiful. I don't think they would be so quick to wean them in cooler weather.Do you believe the decisions as to when to wean are random? Or do you think the hens are instinctively doing it for the right time for that particular brood?
Interestingly one of the three week ones was back to laying when she weaned them. The ones that take months to wean the chicks are not back to laying for a much longer time.