– Christopher Swann is a Reuters columnist. The views expressed are his own –

People can be divided into three classes, it has been said: the haves, the have-nots and the have-not-paid-for-what-they-haves. The prevalence of the third category may be the biggest single source of vulnerability for the U.S. recovery.

A stress test of the consumer could reveal more distressing results than the one conducted on the banking system.

Debt is at high levels — 130 percent of disposable income, or more than twice its peak in the late 1980s. A slide in net wealth has reduced the collateral Americans can draw upon for emergency loans. Finally, it is now harder to borrow money for new consumption or to roll over existing debt.

Like a compromised immune system, this weakness makes consumers extremely susceptible to further shocks. Traumatic as the recent bout of retail restraint may have felt, worse may be in store. After all, consumption rose by 18.5 percent in the seven years to 2008. So far it has only fallen back by less than 2 percent.

There are several potential mishaps that could swiftly undermine consumer spending and set the recovery back to square one.

Among the most likely problems would be a continued slide in house prices. Even on the conservative measures used by the Federal Reserve, the value of residential real estate has fallen 18 percent since 2006.

With signs of recovery still tentative, a further 10 percent slide is well within the realm of possibility — inflicting a further blow on the balance sheet and sense of well-being of American households. (If nervousness over swelling government debt pushes up bond yields, the outcome could be still worse.)

Homeowner’s equity, already down to 40 percent from close to 60 percent, would plunge to 35 percent. This would send household wealth down to its lowest level since the mid-1980s. While Americans can’t immediately rebuild the $12 billion of net worth lost over the past 2 years — equivalent to more than a year’s worth of consumption — further losses will heighten their sense of caution.

A second threat is also looking increasingly likely — wage cuts. Fifteen percent of employers surveyed by the Society for Human Resource Management reduced pay in the past six months, a threefold increase from earlier this year. It is no longer implausible to imagine nominal wages starting to decline on a nationwide basis.

The Employment Cost Index is already rising at its slowest rate since the early 1980s. Wage deflation would make it even harder for Americans to repay the $10.5 trillion of mortgage debt they hold or the $2.5 trillion of consumer credit.

Martha Olney, a professor at Berkeley, envisages disproportionate cuts in spending if wages dip. “For households that are paying 60 percent of their income in mortgage and credit payments a 10 percent pay cut does not mean a 10 percent fall in disposable income,” she says. “It’s a 25 percent fall.” This is the danger of leverage.

Even under a rosy scenario it could take years for American consumers to repair their finances. If the savings rate rises to 8 percent of disposable income — about $860 billion a year — it will now take four years for debt to return to its 2002 level. This steady drain alone is equivalent to almost 10 percent of consumer spending.

After such a substantial loss of wealth, Americans will be pressed to be even more frugal. Along the way American will be hyper-sensitive to any jolts from weaker than expected employment, house prices or financial markets.

Consumer spending may not merely stagnate as most economists expect. It could yet decline more sharply. And like the banks, consumers will almost certainly need more support from the central bank and government before this economic malaise is over.