NEW YORK ( Real Money -- When policymakers consider their options in trying to ramp up economic growth and, in turn, bringing down unemployment, the forces of nature -- or, at least, economics -- seem conspired against them. They might as well try to outlaw bad weather. The fundamental problem with the Great Recession and the following anemic recovery is that this period is different from what we've experienced since World War II. The primary factor is the role of credit. Leverage, as it is known, can work both ways: It can either magnify economic growth on the way up or magnify contraction on the way down.You've probably already heard that recessions associated with excess leverage and ensuing credit busts are often worse than are more "normal" recessions, and that the former enjoy less robust recoveries than do the latter. However, there is another factor to consider. Regarding the strong economic expansions to which we've become accustomed, the reason for such recoveries has been the rapid growth of credit that began several decades ago. In other words, we're using the wrong benchmark in comparing ourselves with where we should be. So not only is the U.S. recovery weaker than average, but we'd become accustomed to economic growth that had been above average. That has made this recovery seem especially painful. The San Francisco Fed did some research into the role of credit in economic cycles ( PDF), examining 14 countries over 140 years. The researchers determined that, when leverage is above average, economic expansions last about one and a half years longer and that the cumulative increase in output is 4% higher. More recently, though -- after World War II -- the differences became even more pronounced. On average, high-leverage expansions have resulted in 38% accumulated gains in output, compared with 28% for low-leverage expansions. They've lasted 9.7 years and produced average annual rates of growth of 3.4%, compared with 8.9 years' duration and 2.4% growth rates for low-leverage expansions. Starting in the early 1980s, the growth of credit products took off -- and so did economic growth. After the two recessions of the early 1980s, the rest of the decade boomed and so did the next one, punctuated only by a relatively mild downturn in the early 1990s. In real terms, economic growth averaged 3.7% from 1983 to 2000. During that period, total domestic non-financial-sector debt increased from 152% of U.S. gross domestic product to 183%. In other words, debt grew by an annualized 1.1 percentage point more than did GDP -- a 1.3 to 1 ratio.