When a Rockefeller turns 21 he or she publicly introduces him or herself to the wider Rockefeller clan at one of the bi-annual family gatherings - in June at the Playhouse on the grounds of the historic family estate in New York's Hudson Valley or in New York City over Christmas. "They tell a little about themselves and people -welcome them," Peter Johnson, a longtime family consigliere says. "The idea is to say you are part of the family."

This coming-of-age ritual - you might call it a Wasp bar mitzvah, although there are now, six generations on, Jewish Rockefellers, too - is part of a broader tribal culture that has allowed the Rockefellers, more than 70 years after their founding patriarch's death, to remain a powerful force in US civil society.