An expert believes Donald Trump's "piggy" comment is part of an "emotionally unstable" strategy often deployed by the president.

His comment to a Bloomberg News reporter aboard Air Force One last weekend has been roundly denounced, but it highlights an ongoing tactic utilised by the president, psychologist Alex Iszatt said. Trump, when asked about his ties to Jeffrey Epstein, replied, "Quiet! Quiet, piggy."

Iszatt, speaking to The Mirror, said, "I don't think he has narcissistic personality disorder, though he definitely shows grandiose and narcissistic traits, and his attitude towards women comes straight from that old-school hierarchy. He not only grew up in an elite bubble that enabled this mindset, he also thrived in environments that rewarded aggression and spectacle, where bulldozing and humiliating opponents was seen as an asset rather than a liability."

Iszatt went on to suggest the president is "emotionally unstable" and that he has had "decades" of bad behaviour reinforced.

She said, "Nature and nurture are both at play, and his traits fit a personality that is egocentric, low in empathy, and emotionally unstable, with mean and childish reactions that have been reinforced for decades, because he has never needed to see the world from anyone else’s perspective, and humiliation is a tool he uses like a playground bully to get his own way.

"I don’t think he is mentally ill, he is cunning, and breaking norms isn’t a loss of control, it is part of his strategy."

Fellow body language expert Judi James believes the president's "piggy" comment is part of his ongoing visualization of both insults and praise.

James said, "His insults tend to be visually-prompted. His favorite, constantly-employed compliment word is 'beautiful' and he uses it to praise things like a deal or a bill as much as the people he is meeting."

"It suggests a trait of predominantly visual stimulation, which might also account for his constant boasting about his lavish redecoration of the White House. If he sees 'beautiful' as the highest praise it's probably no surprise he will go for a visual term to insult someone, too."