Maybe he should have put his money in stocks instead.

By the measure of success he holds most dear — wealth — Donald Trump has done well. Since 1987, when the real estate mogul published his best-seller "Art of the Deal," his net worth has jumped 300 percent to $4 billion, according to figures from wealth-tracker Forbes magazine.

Turns out, though, there was an easier way for Trump to add to his wealth than all the deal-making and TV shows — and far more effective. It's the same strategy many wealth advisers are telling middle-class families to follow: Stick your money in an index fund tracking the stock market and forget about it.

If Trump had done that in 1988, he would be worth $13 billion today, more than triple the Forbes estimate.

How Trump's wealth has changed over the years compared with the performance of other billionaires, according to Forbes, and the gains in Standard and Poor's 500, an index of stock market value:

Bill Gates (Microsoft founder)

2015: $80 billion

1988: $1.1 billion

Gain: 7,173 percent

Warren Buffett (investor)

2015: $67.8 billion

1988: $2.5 billion

Gain: 2,612 percent

Donald Bren (real estate)

2015: $15.2 billion

1988: $1.9 billion

Gain: 722 percent

Donald Trump (real estate, TV)

2015: $4 billion

1988: $1 billion

Gain: 300 percent

S&P 500

2015: 2,084

1988: 263

Gain: 1,336 percent (including gain from dividends reinvested in the index)

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Associated Press researcher Jennifer Farrar contributed to this report.