When Rhett Butler said he frankly did not give a damn in Gone With the Wind, he was almost certainly telling the truth.

That is because people who swear are more honest, a psychological study has found.

It may appear that those who turn the air blue with four-letter words are less concerned about social rules like telling the truth and not hurting people.

It may appear that those who turn the air blue with four-letter words are less concerned about social rules like telling the truth and not hurting people

But swearing is the 'unfiltered, genuine expression of emotions' and those who do it frequently were found to be more sincere.

This perception may explain, the study suggests, why Donald Trump, who liberally uses the word 'hell' and promised to 'knock the s*** out of Isis', was hailed for his authenticity by some of those who voted him in as President.

Meanwhile suspects innocent of crimes have previously been found to swear more than those who are guilty.

The latest study, accepted for publication in the journal Social Psychological and Personality Science, found people who liked more swearwords and used them most often were least likely to lie. People who swore more on Facebook also lied less often online to make themselves look better.

Co-author David Stillwell, from the University of Cambridge, said: 'There are two ways of looking at it. You might think if someone is swearing a lot, this is a negative social behaviour seen as a bad thing to do, so if someone swears they are probably a bad person as well.

'On the other hand, they are not filtering their language so they are probably also not putting their stories about what is going on through similar filters which might turn them into untruths.

'That is what we seemed to land on in this study, that people who use the language that comes to mind first are less likely to be playing games with the truth.'

The study cites 'Frankly my dear, I don't give a damn', said by Rhett Butler to Scarlett O' Hara in the closing scenes of Gone With the Wind, as revealing the conflicting attitudes of society towards swearing.

A classic line from movie history, it also saw the film's production team fined $5,000 for violating the Motion Picture Production Code.

But the researchers note that it 'profoundly conveys Butler's honest thoughts and feelings'.

When surveying 276 people on why they swear, most said it was to express their true self and be honest, or to express negative emotions rather than to insult or intimidate others.

Donald Trump, who liberally uses the word 'hell' and promised to 'knock the s*** out of Isis', was hailed for his authenticity by some of those who voted him in as President

The participants' honesty was measured using a test with questions including whether they had ever blamed someone for their mistake, cheated at a game or taken advantage of someone else.

How much they swore was measured by asking them to list their most commonly used and favourite swear words, and admit how much they used bad language in person, on their own and in writing.

Those who wrote down more swear words they frequently used, liked them more and used them most often were found to lie least.

A second experiment with almost 74,000 people on Facebook found those who swore more online were less dishonest. This was judged using telltale signs from previous studies, such as liars using fewer first person pronouns like 'I' or 'me' and more anxiety-driven words like 'worried' and 'nervous'.

The kind of dishonesty looked at was 'self-promoting' deception aimed at making someone look better on Facebook, rather than serious unethical behaviour.

The research, also involving the universities of Maastricht, Stanford and the Hong Kong University of Science and Technology, says white lies or 'social lies' are the most common type.