A couple of commentators have put forward reasons why 2012 might be a better-than-expected year for the economy. Matt Yglesias’ entry into the “happy days are here again” sweepstakes is a bit older, but it’s smarter than most and invokes an obscure, but important, economist of olde to make the point. Thus, it’s a good peg to use to remind people about the case for pessimism.

Yglesias’ post basically sums up multiplier-accelerator models of recovery – the idea that when recoveries begin, they will be self-sustaining and initial improvements in one sector of the economy will generate further increases in activity in other sectors (this reasoning also explains the dynamic of contractions, not just recoveries).

As Yglesias puts it:

“But every downward tick in the unemployment rate is another twentysomething moving out of his parents’ basement, stimulating a return to a more normal level of construction. Multifamily housing starts are already up 80 percent over the past year to accommodate the likely coming flood of renters, and there’ll be more to come once people have more cash in their pockets.

This increase in economic activity will boost state and local tax revenue and end the already slowing cycle of public sector layoffs. Re-employment in the construction, durable goods, and related transportation and warehousing functions will bolster income and push up spending on nondurables, restaurants, leisure and hospitality, and all the rest. Happy days, in other words, will be here again.”

This is indeed what recovery will look like when it comes. But there’s very little evidence that the process has started.

For one, “every downward tick in the unemployment rate” that we’ve seen over the past two years (i.e., since the unemployment rate peaked at 10.1 percent in Oct. 2009) has not represented somebody getting a job (and hence able to move towards independence and spending). Rather, it’s represented somebody dropping out (or choosing not to enter) the labor force. And even over the past year (since Nov. 2010), fully two-thirds of the decline in the unemployment rate was driven by a shrinking labor force and not by employment growth.

The best chart to show that a robust multiplier-accelerator process has yet to begin remains the difference between actual and potential GDP. The size of this gap is the progress that is being made (or not) towards recovery. The free-fall of this ratio that was the Great Recession has stopped, but so has the upward progress of the early part of the recovery (when, by the way, there was an actual boost to the recovery being provided by fiscal support, instead of the drag that will constitute the next year). Until one sees a rapid upward movement in the gap between actual and potential GDP (and, actually, until one sees this movement driven by improvements in actual rather than a deterioration in potential GDP), it seems awfully premature to think that a positive, self-reinforcing cumulative causation has set in or can be banked on for the coming year.