(Bloomberg) -- Population aging is expectedto drag on U.S. growth, and the hit could besubstantial.

The retirement of baby boomers in thedecade between 2010 and 2020 will lower GDP growth per capita by1.2 percentage point a year from what would have been thecase if the nation's demographics had held steady, accordingto a National Bureau of Economic Research study out thisweek.

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The bright side is that the dent is only half as deep between2020 and 2030 as the pace of aging slows.

The study is based on a simple idea: population aging is alreadylong underway and has been playing out with varying degrees ofintensity across different regions of the country.

By looking at variations in state population aging, authorsNicole Maestas at Harvard Medical School, and Kathleen Mullen andDavid Powell at policy researchgroup RAND Corporation, are able to estimatehow a graying workforce affects output, participation rates andproductivity.

What's surprising is the composition of the slowdown. Justone-third is driven by slowing workforce expansion and the rest bya drop in productivity gains.

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The productivity slump isn't reserved to olderworkers: it takes place across age groups, the researchersfind.

The authors suggest a few theories about why that's the case. Itcould simply be that younger and older workers complement oneanother.

Or the most productive older workers might be leavingthe workforce, while less-productive old timers stay on thejob. "How much of it is that relatively productive workers arethe ones who are choosing to retire? It's very hard to say,"Maestas said in an interview.

Regardless of what's behind it, the discovery that the agingworkforce could be weighing on productivity comes incontrast to other guesses and is important.

As Federal Reserve officials meet in Washington this week, tepidgrowth in ouput-per-hour is likely to be one of the economicquestions they're pondering.

It's not clear why productivity growth has dropped off, andthe change has real-world implications: it's one factorthat caused Fed officials to lower their projections forwhere interest rates will settle in the longer-run, based onmeeting minutes from their June meeting.

Another notable feature of the new findings: they'rereally pessimistic. If growth over the next 20years otherwise held near its average for the 1960-2010period — about 1.9 percent — adjusting forthe demographic shift would lower per-capita GDPgains to 0.7 percent this decade and 1.3 percent next,based on the estimates.

Other research on the topic has suggested that population agingwill have a smaller drag on output growth — for instance,a 2012 National Research Council report foundthat aging could knock 0.3 to 0.6 percentage point off ofgrowth over the next two decades as the workforce structurechanges, which it characterized as a "modest" macroeconomicconsequence. The difference arises because the council didn'tfactor in productivity as a major way aging could drag on growth,Maestas said.

"The real challenge next is to take a closer look atproductivity and whether there are ways to redesign the way wework, and take a look at policy that can inadvertently encourageretirement by productive workers,'' she said.

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