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JK Rowling has taken another swipe at Donald Trump by focusing on his huge handwriting and what it says about his personality.

Publishing a link to a graphology guide which studies meanings behind handwriting styles, the Harry Potter author suggested that the US president’s oversized signature could be a sign that he "over-values" himself.

A man called Russ on Twitter initially drew attention to Mr Trump’s signature.

Posting a photo of the President holding up a proclamation declaring his intention to withdraw from the JCPOA Iran nuclear agreement after signing it, Russ asked: “Why? Why is his signature so big? Why, god, why?”

Ms Rowling replied with a link to a study on handwriting.

“I didn't believe in graphology until about three minutes ago,” she wrote.

The guide details what handwriting can reveal about a person.

According to the website Handwriting Graphology, large handwriting could indicate someone who “over-values himself, may compensate inferiority complexes showing the opposite.”

It said a person with a large scrawl could also have “tendencies of vanity, pride and excessive self-esteem that may become megalomania.”

Large handwriting also indicates a person with “a great global vision, expansive, who has difficulties interiorising experiences and concentrating on specific details.”

Ms Rowling’s tweet quickly went viral with many people weighing in on the subject.

Erica McDermott wrote: “Thank You for presenting a hearty laugh to me ‘free of charge.’”

Another person said: “I love graphology! Such an interesting science!”

But some people pointed out that Ms Rowling has a large signature herself, posting photos of signed Harry Potter books.

One person wrote: “Yeah? You are definitely onto something...here's another rather huge signature from someone who overvalues themself with an inferiority complex ...”

Other’s pointed out that Mr Trump is not the first president to have a large signature, sharing photos of former President Barack Obama’s writing.

Leaders around the world reacted with shock and disappointment as Mr Trump announced the US's withdrawal from the Iran nuclear deal earlier this week.

Former president Barack Obama said the decision was a "serious mistake" that could leave the US facing "a losing choice between a nuclear-armed Iran or another war in the Middle East".

In a joint statement, Theresa May May, German chancellor Angela Merkel and French president Emmanuel Macron said the decision was a matter of "regret and concern".

Mr Trump said the move was a sign that the US "no longer makes empty threats".

"When I make promises, I keep them," he said.

In a joint statement, Mrs May, Mrs Merkel and Mr Macron said they remained committed to the deal.

The Joint Comprehensive Plan of Action was signed by the US, China, Russia, Germany, France and Britain with Iran in 2015.

Under its terms, Iran is committed to a peaceful nuclear energy programme.