How does Donald Trump determine his net worth? David Cay Johnston, a Pulitzer Prize journalist, says he just makes it up. And he should know: He wrote The Making of Donald Trump, A New York Times best-seller, and has spent 30 years investigating him.

The alleged billionaire has repeatedly bragged about his wealth, claiming he’s worth $8, $10, even $11 billion dollars. But on Wednesday, Forbes knocked him down a few pegs and reported Donald Trump’s net worth had fallen by $800 million to a scant $3.7 billion. And yes, even that $4.5 billion Donald Trump had last year still isn’t even half of the $10 billion he so often claims. No wonder the GOP candidate is so reluctant to release his tax returns.

So what has caused this precipitous drop in the Donald’s net worth? It has a lot less to do with the money he’s not spending on charity or his campaign, and more to do with a drop in New York City’s real estate values. Forbes explains:

‘Forbes’ new investigation into Trump’s wealth pegs his fortune at $3.7 billion, down $800 million from a year ago. A softening of New York City’s real estate market, particularly in retail and office, where valuations are trending down, has diminished his estimated net worth. New information was also a factor.’

Except for the value of Donald Trump as a cringe-worthy brand, most of Donald Trump’s assets are tied to his properties.

‘Of the 28 assets or asset classes scrutinized by Forbes, 18 declined in value, including his trademark Trump Tower on Manhattan’s Fifth Avenue, his downtown jewel 40 Wall Street and Mar-a-Lago, his private beachfront club in Palm Beach.’

Oh, and during Monday night’s presidential debate, Donald Trump flat out lied about his net worth again.

‘The bombastic Republican presidential candidate said on Monday night that his FEC filing showed income of $694 million for the past year. It doesn’t, because in the document he freely mixed revenue with income, and it covers a period of 17 months.’

Watch: David Cay Johnston explains how Donald Trump determines his net worth.

And back to David Cay Johnston, on Tuesday he told Business Insider what he knows about Donald Trump’s not-terribly-impressive wealth:

Trump is, without a doubt, a wealthy man. But the lifestyle he has doesn’t require a billion dollars of assets, and there’s no evidence that he is a “billionaire.” What there is evidence of is how Donald Trump calculates his wealth.

He goes on to explain how most of us figure out how much we’re worth by running through our various assets, adding them up and then subtracting our debt. But not Donald Trump.

‘His net worth is determined by his emotional state.”

Seriously, the man testified in court a couple years ago, “Well, it depends on how I’m feeling.” When the opposing lawyer pressed him for an explanation, the not-as-rich-as-he-feels billionaire confirmed, “yes, it’s a function of how I feel.”

Whoa. As David Cay Johnston points out, that’s pretty danged scary coming from someone who’s running for president of the United States on the GOP ticket. After all , if some guy sitting next to you in a bar said he felt like he was worth “at least a billion dollars today,” you’d say, “he’s nuts!”

How Donald Trump determines his net worth https://t.co/yoou12OxkY

David Cay Johnston: 'There's no evidence Donald Trump is a billionaire' — smebulok (@smebulok) September 27, 2016

So how’s Donald Trump reacting to people like David Cay Johnston questioning his net worth? He’s threatening to sue.

Trump called me at home 4/27 saying he'll sue me if he doesn't like what I report. pic.twitter.com/0ev3qdaXPO — David Cay Johnston (@DavidCayJ) September 19, 2016

Featured image: Tom Pennington via Getty Images (Public domain crown image added via Pixabay).