It seems to defy investment logic, but defined contribution plans, haphazardly run by amateurs, actually lost less money in the recession than professionally managed defined benefit plans, according to a new paper from the Urban Institute, a Washington, D.C.–based social and economic policy think tank.

Not only that, but defined contribution plans are also doing better during the recovery,  according to the paper.

Senior researcher Barbara Butrica found that traditional private sector pension plans lost 37 percent of their assets, or $1 trillion, from mid-2007 to 2009. Even after regaining some of those losses in the following three years, they remain about $4 billion below their $2.7 trillion peak.

By contrast, 401(k)s, individual retirement accounts, and other defined-contribution-style vehicles lost only 31 percent and have since bounced back so nicely that they are now about $8 billion above their 2007 peak — to a total $9.5 trillion.

So what gives?

Part of the problem may be precisely the pension plans’ sophisticated, professional investment strategies.

During the five-year period covered by the paper, says Timothy Barron, chief investment officer for the consulting firm Segal Rogerscasey, “there was [investment] area after area that would hurt defined benefit plans and not hurt defined contributions as much.”

For instance, he says that a typical defined benefit plan might have 18 percent in alternative investments like private equity, 14 percent in international equity, and 4 percent in real estate investment trusts. Those were among the asset classes that did worst during the downturn and still lag in recovery.

Meanwhile, in Aon Hewitt’s most recent survey of 120 large defined contribution plans, the typical portfolio was just 5.6 percent in international and 0.4 percent in “specialty” assets. Real estate was too small to merit a separate breakout.

However, asset allocation is only part of the story. More important may be the amount of money coming in and out.

Thanks to provisions like automatic enrollment — now offered by more than half of all defined contribution plans, according to Aon Hewitt — coupled with plain old inertia, 401(k)-type plans keep pulling in new assets through good markets and bad.

The defined benefit pool, on the other hand, has been shrinking, as companies freeze, close and terminate their programs. Where 38 percent of the private workforce had a DB plan in 1979, just 14 percent were covered as of 2011, according the Employee Benefit Research Institute.