Trump’s net worth has grown about 300% to an estimated $4 billion since 1987, according to a report by the Associated Press. But the real estate mogul would have made even more money if he had just invested in index funds. The AP says that, if Trump had invested in an index fund in 1988, his net worth would be as much as $13 billion.

The S&P 500 has grown 1,336% since 1988.

Other billionaires’ net worths have beaten the stock market’s growth in that time. Bill Gates, for example, saw his grow increase 7,173% since 1988 to $80 billion. Warren Buffet’s wealth grew 2,612% in the same time period, to $67.8 billion.

Another recent Associated Press report found that Trump is a much more cautious businessman than he lets on. “He holds few stocks for someone of his wealth and has grown increasingly dependent on making money by lending out his name to others rather than developing real estate himself,” the AP wrote.