Mortgages are bumping along at or near multi-decade lows and a flood of Treasury bond buyers has led to a very appealing yield curve for corporate and government borrowers.

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The slope of the yield curve can be measured in varying ways but is conventionally thought of as the long-term government bond rate minus the short-term government bond rate.

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They found that it made sense to buy long bonds in almost all interest-rate scenarios but one: when rates are low (so the risk that they will revert to the mean, thus undercutting the value of the bond, is high) and the yield curve a line drawn through the interest rates on government securities with different maturities is flat (so there is little extra reward for taking that risk).

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