Tony Schwartz, the journalist who co-wrote Trump's 1987 book The Art of the Deal, has called the president a "psychopath" who is "responsible for the deaths of tens of thousands of people".
Schwartz discussed Trump's psychology with CNN's Brian Stelter in the lead up to the publication of a book by Trump's niece, Mary Trump, which will scrutinise how he was shaped in the early years of his life.
Stelter explained to Schwartz that Mary Trump is unable to give press interviews because of an ongoing court case alleging that the publication of her book would violate a confidentiality settlement.
I can give interviews, and my view of this is remarkably similar to Mary Trump's. I think we both had long exposure to this man. I have studied psychology myself for 30 years.
Trump is a man without conscience and without empathy, and that is the most dangerous thing you can have in a human being. That's what allows people to do what he's doing right now, which is to be responsible for the deaths of tens of thousands of people and not give a damn. That's how he can go forward with his policies, Brian. Because he doesn't actually care.
The end always justifies the means and the only end is being re-elected.
Schwartz's comments likely refer to Trump's handling of the coronavirus pandemic, which has included such colourful moments as the president appearing to recommend drinking bleach as a treatment during a press conference.
More than 137,000 people are confirmed to have died from coronavirus in the United States, the highest figure of anywhere in the world.
Schwartz spent 18 months with Trump, beginning in late 1985, writing the majority of their book The Art of the Deal.
He is credited as a co-writer on the book's cover and collected half of its royalties.
He has since expressed his "deep remorse" for doing this, admitting that he made Trump "more appealing" and introduced him to a "wider audience".
He told The New Yorker in 2016 that he had "put lipstick on a pig" and would name the book 'The Sociopath' if he wrote it now.
Schwartz went on to refer to Trump as the "psychopath-in-chief", a term he first used in a Medium blog post.
Mary Trump, who wrote Too Much and Never Enough: How My Family Created the World’s Most Dangerous Man and holds a doctorate in psychology, believes that Trump is a narcissist.
In the prologue of her book, she wrote:
I have no problem calling Donald a narcissist – he meets all nine criteria as outlined in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5).
These criteria include "a grandiose sense of self-importance" and "a need for excessive admiration".
But some psychologists have warned against attempting to publicly diagnose Trump with mental health conditions.
The American Psychological Association maintains that psychologists should not publicly diagnose politicians.
It is unethical for a psychiatrist to offer a professional opinion unless he or she has conducted an examination and has been granted proper authorization for such a statement.
Of course, Schwartz is a journalist, not a psychologist, and is not beholden to this rule. Mary Trump is a psychologist and is familiar with Trump, but has not officially assessed him.
It is important to remember that the definitions of terms like 'narcissist', 'sociopath' and 'psychopath' are contested even amongst psychiatrists and have derogatory connotations from their popular use.
Trump may well qualify for one or more of the disorders laid out in the DSM – but we'll leave it up to the experts to decide.