“The biggest hit will be in 2009,” Nariman Behravesh, the chief economist of Global Insight, a research and forecasting firm, told me, “and it probably won’t be until 2011 until we see any kind of pay gains.”

What will make this recession different, no matter how deep or shallow it is, is that it’s following an expansion in which most families received little or no raise. The median household made $50,200 last year, slightly less than the $50,600 that the equivalent household earned in 2000, according to the Census Bureau. That’s the first time on record that income failed to set a new record in an economic expansion.

Why has it happened? There is no single cause.

Medical costs have risen rapidly, which means that health insurance premiums take up a bigger chunk of workers’ paychecks than they used to. Some of this money goes to good use; it pays for treatments that weren’t available even a few years ago. But some of it, the part that disappears into the inefficient American health care system, is clearly wasted.

And in the last couple of years, the value of the typical worker’s benefits package has stopped growing. Since 2005, benefits packages have become slightly smaller, notes Jared Bernstein of the Economic Policy Institute. So health benefits can’t come close to explaining the recent pay stagnation.

The bigger factors are probably some combination of the following: new technologies, global trade, slowing gains in educational attainment, the rise of single-parent families, the continued decline in unionization and the sharp increase in inequality, which has concentrated income gains at the top of the ladder. Your political views will probably determine the relative weights that you assign to those causes. Economic research hasn’t yet definitively answered the question.

Whatever the cause, though, the effects of the pay slump are going to be significant. Households have already begun to cut back their spending, and they will do so even more next year. Mr. Behravesh predicts that inflation-adjusted consumer spending in 2009 will be somewhere between flat and down 1 percent. If he’s right, it would be the first year that consumer spending didn’t grow since 1980, which just happens to be the last time that the country suffered through a deep recession.

The pay slump will also make it harder for people to pay off their loans. Last week, Bank of America reported that its losses on consumer credit had tripled over the last year.