And third: U.S. companies have been generating such huge profits that they have needed to borrow less money than usual. Fewer borrowers means it's cheaper to borrow.

As Merrill pointed out in a strategy session in London earlier Tuesday, any U.S. economic slowdown could drive rates the other way for two reasons: First, companies will generate fewer profits, so some will need to borrow more to finance expansion. And second, if U.S. consumers spend less, then we'll be running a smaller trade deficit with those Asian countries. So they'll have less money to lend us.

The danger, as usual in financial markets, is a vicious circle. If long-term rates rise, they'll take mortgage rates with them. You can imagine the effect on the housing market -- and the effect, in turn, on consumer spending.

None of this is certain. But it's among the reasons I have parked a small amount of money in ProFunds' ( RRPIX) Innovative Rising Rates Opportunity Fund , which rises and falls with long-term interest rates.

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