Shrek 2 broke records for an animation film in the US

PG-rated films grossed $2.3bn (�1.2bn) in 2004, compared to $2.1bn (�1.09bn) for R-rated films, aimed at adults.

Half of 2004's top 10 highest-earning films were rated PG - open to all ages - including The Incredibles and Harry Potter and the Prisoner of Azkaban.

More than 1.5bn people visited the cinema last year, according to the National Association of Theater Owners.

Out of the top 25 highest-earning films, only four carried R-ratings, which prevent children under the age of 17 from attending unless accompanied by an adult.

There will always be a need for R pictures. We just don't need 500 of them John Fithian

However, of 2004's rated films, 540 were rated R, while only about 325 were G, PG, or PG-13.

G is equivalent to a U certificate in the UK, which indicates a film is suitable for all viewers. PG-13 - which warns some scenes may be unsuitable for children - is roughly equivalent to a British 12A rating.

John Fithian, head of the National Association of Theater Owners, urged Hollywood studios to make more family films that carry ratings of PG or PG-13.

Fithian said: "We want to call on our friends in the studios to continue to distribute to us family titles, because that's where we sell more movie tickets.