Last week, when I wrote my post on how to boost employment, the list started off unambiguously:

The first—and this can’t be stressed enough—is simply extending the federal unemployment extensions. As Menzie Chinn notes, the CEA has scored this, and the numbers are enormous: already, the program has increased the level of employment by 793,000 jobs. If the extensions are kept dead, there will be 593,000 fewer jobs in a year’s time than there would be if they were resuscitated, including more than 46,000 jobs in Florida and more than 26,000 jobs in Michigan.

This is not intuitive, especially to economist types who think that incentives matter and that at the margin, paying people to remain unemployed is not going to increase their chances of getting a job. But the fact is that those unemployment benefits are spent, and the extra economic activity naturally creates employment.

There is of course some effect by which paying people to stay unemployed will increase their chances of doing so. Rob Valletta and Katherine Kuang, of the San Francisco Fed, did the math back in April, concluding that the effect is “relatively modest”:

The question arises whether this extended availability of UI benefits has contributed to a lengthening of unemployment spells because jobless workers are staying in the labor force longer in order to continue collecting benefits. Such a dynamic could raise the unemployment rate. However, analysis of data on unemployed individuals decomposed by their reason for unemployment, which affects their eligibility for UI, suggests that extended UI benefits have had a relatively modest effect. We calculate that, in the absence of extended benefits, the unemployment rate would have been about 0.4 percentage point lower at the end of 2009, or about 9.6% rather than 10.0%.

Peter Coy has taken a detailed look at the interplay between the two effects:

Do the extra checks make unemployment higher than it would otherwise be by paying people to sit at home? Or do the checks sustain growth by supporting the spending power of households with out-of-work breadwinners?

In truth, unemployment benefit extensions do both—they raise the jobless rate a bit, and they make the economy grow faster. What’s clear is that extending jobless benefits makes more sense when the unemployment rate is exceptionally high, as it is now, at 9.8 percent in November… Because aid to the jobless is almost immediately spent (as opposed to tax refunds for the wealthy), it is the most effective means of stimulating demand.

Coy’s “bottom line” is clear: “Although the Obama-GOP tax deal extends unemployment benefits, it probably will not dissuade many jobless from seeking work.”

This all adds up to something reasonably clear. Unemployment insurance isn’t only just from a fairness perspective, it’s also extremely effective as stimulus. Any effect whereby it encourages people to stay unemployed is, in comparison, modest.

Which is why it’s very odd to find Kelly Evans, in the WSJ, writing the exact opposite.

More jobless benefits, more unemployment.

A likely rise in the U.S. jobless rate is the unfortunate reality of the government’s move to fund extended unemployment benefits for another 13 months.

The effect probably won’t be huge, but it will be significant. And it may well hamper any recovery in investor and business confidence.

Evans isn’t very good at explaining the math of why more unemployed people add to the unemployment rate:

Individuals not actively searching for work or willing to take available jobs may claim they are unemployed in order to receive benefits. That could artificially boost the size of the labor force, which is used to determine the unemployment rate.

Well yes, the labor force is indeed used to determine the unemployment rate, but it’s the denominator in that calculation. If the denominator goes up, the rate goes down. The problem is rather that in any ratio less than 100%, if you increase the numerator and the denominator by the same amount, then the ratio goes up.

Evans concludes:

Policy makers are hoping that extending benefits—along with other tax breaks—will generate enough short-term strength in spending and growth to overshadow any rise in the unemployment rate.

Felix Salmon is an Audit contributor. He's also the finance blogger for Reuters; this post can also be found at Reuters.com.