A bad day for billionaires

March 8, 2012, 12:29 AM UTC

Lakshmi Mittal: No longer in the top 20

Tuesday was a day to forget for the world’s richest people. We know that now. Should we care?

FORTUNE — You may have thought that yesterday’s 200 drop in the Dow Jones industrial average, the worst day for 2012 so far, was bad day for your portfolio. Consider Bernard Arnault, who owns a controlling stake of luxury brand conglomerate LVMH Moet Hennessy Louis Vuitton. Yesterday, he lost $1.2 billion. Casino magnate Sheldon Adelson saw his net worth fall $766 million, or about 36 times what he has dumped so far on Gingrich’s presidency. Warren Buffett’s wealth dropped by $407 million.

And how is this for a blow to the ego: Lakshmi Mittal, owner of the world’s largest steel company, is no longer one of the 20 richest people in the world. With an estimated net worth of $22.3 billion, as of Tuesday evening, he now ranks as 21, at least for today. Ouch.

How do I know this? This week Bloomberg began tracking the net worth of the world’s 20 richest people. And it’s updating its list daily. The question is how much we should care.

Clearly, when markets around the world fall, as they did yesterday, so will the net worth of the richest people in the world. What’s more, it’s certainly wrong to say, as Bloomberg does in a headline, that the world’s richest people “lost” a collective $11.3 billion on Tuesday. They are indeed worth less than they were the day before. But did they really lose that money? Not quite. Buffett isn’t a day trader and neither are the rest of these guys. Their “losses” will never be realized. Many of the other people on the list are rich because of holdings in family companies, which often never get sold, at least not for generations.

But there is something interesting about the list, and looking at net worth of the top 20 on a daily basis. Wealth volatility generally rises the richer you get. That’s because as you become more and more wealthy you generally become more exposed to markets. Less money is in your illiquid house, and more in your stock heavy 401(k). Most studies show that the top 1%, in general, saw their net worth fall more in 2008 and snap back faster since then than the rest of us. And since the U.S. stock market is by far the largest in the world, you would expect the wealth of the richest of the rich to be highly tied to the Dow Jones industrial average. And yet, it appears the 20 wealthiest people in the world are somewhat less exposed to U.S. markets than you would expect. While the S&P 500 dropped 1.5% yesterday, the net worth of Bloomberg’s billionaire dropped only two-thirds of that, or 1%.

This says one of two things or both. One, that the really stinking rich have come up with a better way to shield their assets from volatility than the regular rich. Or two, that the wealth of the really rich is tied less to the U.S. than it used to be. When Forbes launched its list of the 400 richest Americans back in 1982, the person at the top of the list was shipping tycoon Daniel Ludwig, and he was worth $2 billion. And I think it was reasonable to assume he was the richest person, privately wealthy that is, in the world. These days the richest person in the world is Mexican Carlos Slim, who on Tuesday lost $889 million, but at $67.5 billion is still plenty rich.