On the positive side, the United States clearly has more social safety nets in place and a broader range of policy responses today than it did during the Great Depression. While the higher standard of living we enjoy may be exposed to a greater drop in absolute terms, few expect to see the dislocations and pain that characterized the 1930s.

However, while the U.S. economy today is more diversified, it is also more service-oriented and highly reliant upon debt, with consumer credit leading the way. And this change may have trumped the gains of economic diversity.

The economic impact of U.S. consumer spending is difficult to overstate. It equals 70% of U.S. gross domestic product and is nearly triple the entire GDP of China. The productive capacity supporting it is truly global, with leveraged business models, inflated tax revenues and excess square footage all facing retrenchment on a global scale.

This structural decrease in credit will create a more powerful headwind than what Japan faced 20 years ago, which was a fairly localized capital-expenditure bubble. Impressive as that episode was, with Japanese capex bumping 20% of GDP (twice the rate of the U.S. then and now), the global impact of Japan's subsequent economic unwind was muted by growth elsewhere.

But who will step in for the overextended U.S. consumer? Many thought that global economic activity had become less dependent on the U.S. than it had been in the past, but that view has not held up.

The impact of federal programs may take time to emerge, especially in consumer spending, and may be muted as programs work their way through thestates. Employment gains may fail to meet projections given mismatched skills among job seekers. All the while, we can expect to see consumption being taken out of the economy in favor of saving and debt reduction.

The U.S. savings rate, chronically low and occasionally even negative, had been supported by two forces that have withered significantly: asset inflation and credit availability. As both housing and retirement assets have lost value, the need to reign in spending and add to savings has taken hold. The savings rate has entered positive territory with a vengeance and will continue to grow at the expense of discretionary consumption.

Credit, whether collateralized by inflated asset values or supported by easy terms, had been the high-octane fuel for spending. Consumers are now running on fumes and it is reasonable to expect a reversion of consumer spending to historic patterns that, given current levels, would remove about $700 billion from GDP.

The remainder of the decade will be unlike any period in recent memory. Investment decisions should be made with ample consideration of the fundamental changes that will be with us for some time. Spending and growth expectations must include the potential for stagnation. Still, there will be winners alongside the losers, both with investments and business models, and continued opportunities will emerge for active strategies grounded in realism.

Tim Knepp, CFA, serves as chief investment officer of Genworth Financial Asset Management (an Encino, Calif.-based unit of Genworth Financial), as well as chairman of the firm's investment management executive committee. He can be reached at tim.knepp@genworth.com.