WHEN A POLITICIAN asks voters whether they are personally better off than they were four years ago, the honest answer for a clear majority is almost sure to be yes. That's because -- barring retirement, disability, loss of a family breadwinner or total economic collapse -- most people move up in the income scale as they get older. The more relevant question, as both contenders in Sunday's debate suggested, is whether the population as a whole is better off. That is not so easily answered.

The president pointed to a gain of almost $3,000 in per-capita after-tax income -- he called it earnings, but all kinds of income are included -- during his time in office. That's a healthy gain, especially given the very deep recession. But income is not evenly distributed on a per-capita basis. If all the gains are ncentrated among a relatively few families, the population as a whole is not going to feel better off.

Looking at the distribution of gains over the period, Mr. Mondale summed up the situation accurately. "Well, if you're wealthy, you're better off. If you're middle income, you are about where you were. And if you are of modest income, you are worse off." As a result, as studies confirm, the U.S. income distribution has become markedly more unequal since 1980.

Many factors affect the income distribution. Over the past decade, for example, sharp increases in the numbers of young people and women in the job market dampened average wage gains; increases in families headed by women and elderly people held down gains in average family income. But over the past four years, factors more directly related to government policies have dominated: big tax cuts, which -- the president's protestations notwithstanding -- primarily benefited the wealthy; government benefit cuts, which, together with rising tax burdens on lower incomes, primarily hurt the poor; and, despite a strong comeback, a generally weak job market that has depressed wage gains.

As a result, the purchasing power of a middle-income family is still well below its 1979 peak. The earnings of average full-time workers, moreover, now buy no more than what their counterparts earned in the late '60s. And some groups, notably black families and farmers, have lost substantial ground.

Still, there are solid reasons why many people may feel better off, even if, in fact, they are not. Just as rising unemployment strikes terror in the hearts of people who still have jobs -- a falling, or at least stable unemployment rate makes people feel more secure. So does lower inflation. When prices are soaring, people can't help wondering whether their pay will keep up. For the moment, both these trends are favorable. The real question, as Mr. Mondale suggested, is whether people believe that the country's current course will sustain those trends into the future.