Another theory, alluded to by Greenspan, is that high oil prices were symptomatic of global turmoil and uncertainties that caused businesses to pull back on hiring and consumers to cut back on spending.
"I know it has an effect, I know it's there," Greenspan said of the oil price surge. "I'm almost certain that at $30 oil, we would be doing better than we are today, but by how much, I think it's extraordinarily difficult to judge."

Federal Reserve Chairman Alan Greenspan is questioned by a member of the House Budget Committee yesterday.
(Melina Mara -- The Washington Post)
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Video: Federal Reserve Chairman Alan Greenspan said Wednesday the economy has "regained some traction" after a late spring slowdown that was triggered by a sharp spike in oil prices.
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Greenspan's remarks largely echoed the findings in the Fed's latest survey of regional economic conditions, released yesterday.
"Economic activity continued to expand in late July and August, although several districts indicated that the pace had slowed since their last reports," the 12 regional Fed banks reported in the survey known as the "Beige Book."
The results were mixed, according to the report, which compiles anecdotes from businesses across the country to help Fed policymakers in Washington get a feel for the economy as they prepare for their policymaking meeting.
Auto sales, for example, were "sluggish" in the Cleveland and Richmond districts over the past six weeks, but improved in St. Louis and San Francisco.
Kansas City and Dallas said small vehicles were outselling sport-utility vehicles and light trucks; the opposite was true in Atlanta and Chicago.
Women's accessories and jewelry were in high demand. But sales of back-to-school items, particularly children's clothing, were disappointing.
And while consumer prices "were generally flat or up modestly," the Fed districts "continued to cite noticeable price increases for oil."