Reese Witherspoon starred in the film 'Legally Blonde'. Photo / Getty

From Marilyn Monroe to Paris Hilton, they have been mocked because of their hair colour.

But now it turns out that 'dumb blondes' may have been unfairly maligned - new research says they are actually smarter than other women.

A study has found that on average, women with blonde hair had a higher IQ than brunettes or redheads.

Overall their IQ was up to three points higher than those with different hair colours.

There was good news for men too, with blondes having IQs about equal to men with other hair colours.

The researchers said that they wanted to break down the stereotype about blondes and end the discrimination against them. Dumb blondes have been the subject of numerous films such as Legally Blonde, starring Reese Witherspoon as a sorority queen who goes to Harvard Law School to impress a boy who jilted her.

Goldie Hawn played a dumb blonde in films such as Private Benjamin - even though she is a member of Mensa and has an IQ of 140, said to put her in the top 1 per cent of people in the world for intelligence.

And American hotel heiress Paris Hilton was written off as a dumb blonde over her reality TV shows.

The study of 10,878 Americans was taken from the US National Longitudinal Survey of Youth. The research is a survey of people who were between 14 and 21 when they were first interviewed in 1979.

In the subsequent years they were given an intelligence test and asked about their hair colour. The results showed that blonde white women had an average IQ of 103.2. This compared to 102.7 for those with brown hair, 101.2 for those with red hair and 100.5 for those with black hair.

Jay Zagorsky, author of the study and a research scientist at The Ohio State University, said: 'Research shows that stereotypes often have an impact on hiring, promotions and other social experiences.

"This study provides compelling evidence that there shouldn't be any discrimination against blondes based on their intelligence".

Mr Zagorsky said that blondes may grow up in homes where they are encouraged to read more than other women. He said: "If blondes have any slight advantage, it may simply be that they were more likely to grow up in homes with more intellectual stimulation."

- Daily Mail