Hair, it is said, is a lady's crowning glory, an idea that has inspired a trillion dollar industry. Chris Rock's thoughtful comic documentary sets off with a simple enquiry from his 5-year-old daughter - "Daddy, how come I don't have good hair?" - and fans out into an epic adventure in hairdressing and African-American identity politics.

The economics are startling; between visits to the beauty salon, chemical relaxers used to straighten nappy locks and human hair weaves (most of which come from Indian women), black America spends $9 billion annually on coiffure.

Aided and abetted by a fantastic gaggle of pundits and wags - Maya Angelou recalls getting her first hair relaxer at the age of 70, Ice-T remembers when a gangster was measured by the size of his smoothing rollers – this good-natured picture prefers to laugh rather than criticise follicle related foibles. But Mr. Rock, though mellow and warm as our guide, leaves us with an elephant in the room that nobody addresses directly: if so many African-Americans are slaving away for a straight-haired white ideal, whither emancipation?