Donald Trump isn't rich because he's a great investor. He's rich because his dad was rich.

"It takes brains to make millions," according to the slogan of Donald Trump's board game. "It takes Trump to make billions." It appears that's truer than Trump himself might like to admit. An analysis suggests that Trump would've been a billionaire even if he'd never had a career in real estate, and had instead thrown his father's inheritance into a index fund that tracked the market. His wealth, in other words, isn't because of his brains. It's because he's a Trump.

In an outstanding piece for National Journal, reporter S.V. Dáte notes that in 1974, the real estate empire of Trump's father, Fred, was worth about $200 million. Trump is one of five siblings, making his stake at that time worth about $40 million. If someone were to invest $40 million in a S&P 500 index in August 1974, reinvest all dividends, not cash out and have to pay capital gains, and pay nothing in investment fees, he'd wind up with about $3.4 billion come August 2015, according to Don't Quit Your Day Job's handy S&P calculator. If one factors in dividend taxes and a fee of 0.15 percent — which is triple Vanguard's actual fee for an exchange-traded S&P 500 fund — the total only falls to $2.3 billion.

It's hard to nail down Trump's precise net worth, but Bloomberg currently puts it at $2.9 billion, while Forbes puts it at $4 billion. So he's worth about as much as he would've been if he had taken $40 million from his dad and thrown it into an index fund.

But if you compare Trump's performance since 1982, when the stock market started to take off after the early-'80s recession, it looks pretty abysmal. Forbes estimated that Trump was worth $200 million that year. If he'd put that money in an index fund that year at a 0.15 percent fee, he'd have $6.3 billion today after dividend taxes, almost certainly more than he actually does. This jibes with analyzes prior to Dáte's which have found that Trump has underperformed compared with the market since 1988; an AP analysis found that if he'd put his money in an index fund that year, he'd have $13 billion today; the S&P calculator similarly suggested he'd have $11.3 billion, after fees and dividend taxes.

These calculations vary a lot depending on the size of fee you introduce, how much of the investments you take off for living expenses, which exact day of the year you buy the index funds in, etc. If Trump had invested $40 million in an index fund in January 1974, for example, he'd have $500 million less, after fees and dividend taxes, than he would have if he'd invested in August.

But the exact numbers aren't the point. The point is that after decades of touting his business acumen, his ability to negotiate tough deals and spot good investments, and after spending this entire campaign season arguing that he's qualified for the presidency based on his skills in the market, Trump nonetheless has an investment record that at best roughly matches and at worst underperforms the market. He did only as well or possibly worse than a retiree with a Vanguard 401(k) did.

That's not really impressive. Worse, it suggests that his success is almost entirely the result of having inherited money from his father. His own actions might have even cost him money.

Don't be like Trump. Put your money in low-fee index funds from a company like Vanguard or State Street Capital; Vox's Tim Lee has a good explainer of how to do this. Other presidential candidates are smart enough to take this approach. Hillary Clinton's savings are primarily in index funds, including Vanguard's S&P 500 fund.

Disclosure: I have most of my retirement and other savings in Vanguard mutual funds, and because Vanguard is structured as a cooperative, that technically makes me a Vanguard shareholder.

Watch this: How wealth inequality is dangerous for America