Some films rated PG - Parental Guidance Suggested - in the US are more violent than films given the more restrictive PG-13 rating, according to a study.
Four Weddings and a Funeral was among the films evaluated
A survey of the 100 top-earning films of 1994 found one in five PG films had more violence than movies not suitable for children aged under 13.
And one in 10 were more violent than some films rated R, which require under 17s to be accompanied by an adult.
Films surveyed include Four Weddings and a Funeral and Pulp Fiction.
The study, published by the School of Film, Television and Digital Media at the University of California, studied how much sex, violence and bad language were in the movies.
Because there have been no structural changes in the ratings system since that year it was considered "a representative sample and indicative of what's going on", said report co-author Theresa Webb.
The Motion Picture Association of America (MPAA) provides information on nudity, violence and language as an "advance cautionary warning".
However, the study concluded that "many films that were rated primarily for language were in fact just as violent as films that were rated for violence".
The study says a "qualitative component" should be added to ratings, with numerical values replacing such "vague qualifying terms" as 'strong', 'mild' and 'moderate'.
The study arrives amid reports that UK media regulator Ofcom is discussing a new industry-wide classification system covering everything from TV shows to online videos.
The scheme would be further reaching than the current film-rating system and would offer "clear, accurate and timely advice about the nature of content".
The BBC is reportedly in favour of the idea, though commercial broadcasters are said to be less enthusiastic.
Ofcom has set up a working group on the labelling of "challenging content".
It is hoped that a single labelling system - similar to that introduced in the Netherlands - will curb the distribution of harmful or inappropriate material.
However, the scheme would require the cooperation of leading media companies, internet service providers and telecommunications groups.
And it would have no influence over content distributed or downloaded from the internet, over which Ofcom has no jurisdiction.