He spent something like $100 million on his Japanese-style mansion in Woodside, Calif., making it among the more expensive private residences ever built. But that is only about three weeks worth of the interest he earns on his wealth. And a house doesn’t actually spend down his net worth because it is an asset that can be resold. At least part of the $100 million is just a different way of saving.

Mr. Ellison would have to spend that $30 million a week — $183,000 an hour — on things that can’t be resold, like parties or meals, just to avoid increasing his wealth. While somebody might be able to spend like that — Paris Hilton, maybe — it certainly wouldn’t be easy, and it can’t explain why the super-rich accumulate.

The last of the seemingly rational explanations is that the billionaires want to pass it on to their children. But, again, their fortunes are growing far faster than their number of heirs, so each of the children will have the same problems spending the money that their parents had.

Sam Walton’s fortune is now divided among his family, and the Forbes list will probably show that his children account for 4 of the 10 richest Americans in the world (with his wife being No. 11). The children are in their 50s and 60s, and if they live to be 80, and their wealth grows at 10 percent a year, their fortunes will rise by four to eight times and they will each have more than they can ever spend or their children can spend, and so on.

Further, the data, according to Professor Carroll, just doesn’t indicate that children make much of a difference. He found in the government’s 1992 Survey of Consumer Finances, for example, that only 4 percent of the richest Americans said that providing an inheritance ranked in their top five reasons for saving. On top of that, he says, the data shows that elderly super-rich people who do not have children save just as much as the ones who do.

If it isn’t to spend, to give to their children, or to give to charity, then why do the rich save so much? Professor Carroll says maybe they love money, not for what it can buy but just for its own sake. Perhaps they get something different from having money — clout, power, the ability to dominate an industry. Or perhaps these are just competitive people who care about their position compared with other people on the list.

They accumulate more so they can lord it over the other families who have less — a bit like having enough nuclear weapons to blow up the world several times but making more to stay ahead of the other guy.

However you look at it, though, it isn’t for the reasons that everyone else saves money.

In a few weeks, you will see the list of the world’s wealthiest people and how vastly their wealth has increased. Warren Buffett will probably be the only one pledging to give his fortune away. The other billionaires will probably think he’s crazy, but it may make him the most rational person on the list.