Paris (AFP) - Donald Trump has one big lesson to learn from outgoing US President Barack Obama -- he needs to learn to be cool about crying.

"If he could squeeze one tear out during his inauguration" on Friday, "people will turn around quite quickly their feelings towards him", British behavioural expert Judi James told AFP.

"Clearly he is going to go swanning out there with his alpha male feathers strutting, but if at the right moment he could squeeze out a tear it would have an amazing effect on people," said the author of "The Body Language Bible".

"I think there is quite an emotional guy in there somewhere," said added.

Showing his emotions certainly worked for Obama, James argued.

The president has wept at least 10 times on camera since he took office, most tellingly when tears poured down his cheeks last year when he talked about the toll gun violence was taking on America.

He wiped away another few earlier this month when paying tribute to his wife Michelle and his teenage daughters in a farewell speech in Chicago.

Tears are one of the most powerful weapons politicians have in their arsenals, James insisted.

And for Trump, "crying would be ideal" to allay the worries of a world fretting about what his presidency will hold, she said.

"I think so many people are depending on Trump now that unmasking himself as quite a warm, well-meaning character with values that he has been hiding all this time" would go a long way.

For her, Obama's tears helped show his sincerity, connected him with the public and undercut his reputation as an intellectual.

- 'Tears show empathy' -

"Tears help to bring him down from the pedestal. He is a bit more of a New Man. You can imagine him doing the shopping whereas up to now the US presidency has been an alpha male preserve. And it is again (with Trump)."

Obama's comfort with big emotions was also evident when he surprised his tearful vice president Joe Biden with the highest US civilian honour last week as they prepared to leave office.

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Crying specialist Lauren Bylsma, an assistant professor of psychiatry at the University of Pittsburgh, said it was clear that "Obama is a man that is comfortable expressing his emotions in public.

"In terms of character, we know that crying is associated with personality traits such as higher levels of empathy," she said.

Despite his hardman image, Russian leader Vladimir Putin has also shed tears several times in public, mostly notably after winning back the presidency in 2012 despite street protests he claimed were orchestrated by the West.

Crying at the pain of others makes politicians particularly attractive, James added.

But psychologist Jean-Pierre Friedman insisted politicians well up more because they are afraid of losing power.

"Most of them are not the sensitive type. They cry the day they lose power," said the French psychoanalyst and author of two books on politician's inner lives.

- Danger of faking it -

They would take the loss of power "as a bereavement, or not being able to put their projects into force".

Whatever the case, Friedman said he could not imagine Trump ever blubbing publicly. "He's someone from another generation. Fifty years ago the attitude was 'men do not cry'. He's a hardened old cowboy."

Indeed, the president-elect has ridiculed those who cried at his election victory and was famously irked by a crying baby at one of his rallies

James, however, is convinced that he is capable of shedding a tear in public.

"I think Trump could cry, but it would be more to do with self-pity or even anger", which does not go down so well.

But tears can also be a politician's undoing, James warned, particularly if they are seen as put on.

She pointed to the former British prime minister Tony Blair whose emotional openness was first seen as a breath of fresh air only to be later ridiculed.

"Subliminally people are very good at spotting real tears from fake ones.

"When Tony Blair blinked back the tears after the death of Princess Diana... it created a huge moment of empathy, particularly as the British royal family were not coming up with the tears. That bonded people with Blair.

"But he used the same technique a lot over the years, usually when he was put on the spot.

"We saw him do it too much, particularly over the war in Iraq. It looked like tears to order," she added.