RETIREMENT INCOME, in a time of rapid inflation, has become a matter of sharp anxiety for middle-class Americans. Private pensions and annuities generally don't begin to keep up with inflation. But Social Security benefits hold their value reliably. That's why you aren't likely to hear a great deal of complaint about Social Security taxes and the New Year's increase.

People's ideas about Social Security have changed remarkably over the last 15 years, as the inflation rates were steadily rising. Originally, it was designed to provide only a minimal income -- just enough to rescue elderly people from destitution and despair. But now it's becoming common among people in solidly comfortable circumstances to think of Social Security as a mainstay for retirement. Unlike their other resources, it is guaranteed by law to keep step with the Consumer Price Index, point by point.

The 1981 payroll tax increase comes in two dimensions. The tax rate on earnings rose half a percentage point. For the average weekly paycheck, now about $240 a week, that increased the Social Security deduction $1.25. But Congress has also increased the amount of salary on which the tax is collected. The maximum in 1980 was $25,900, but in 1981 it will be $29,700 -- which means that the heaviest shares of the coming increase, some $387 a year, will be paid by those who earn the new maximum or more. But resentment of this disproportionate bite will be mitigated by the thought that Social Security is one program in which higher taxes mean higher personal benefits to the taxpayer.

The most immediate, and most commonly offered, reason for the tax increase is to keep the Social Security fund balanced over the next few years. But it's also clear that Congress has responded to pressures to balance it in ways that provide much larger inflation-proof benefits in the future to people in the upper-middle-income ranges.

As recently as 1970, the top payment to a single man was $189.80 a month -- a decidedly modest amount even in 1970 dollars. By the end of 1980, the top payment was $653.80. Congress not only has adjusted benefits for inflation, but has raised them well beyond the inflation rate. In real terms, after inflation, the 1980 ceiling was 60 percent higher than 10 years earlier.

That ceiling is now rising substantially once again. As people who make more than $25,900 begin paying the higher 1981 taxes, they will be earning eligibility to larger retirement checks. How much larger?Because of the excessively complex formula that Congress has invented, that depends on economic data still to be collected.

For the philosophers of politics and the budget, a couple of points here catch the eye. Social Security demonstrates the manner in which inflation increases the government's role in people's private affairs. Because the government alone can provide firm protection from inflation, inflation makes government pensions more valuable and private benefits less so. Expansion of Social Security makes the federal budget bigger, bringing louder cries about federal spending.But those Social Security checks don't constitute spending, by any careful definition. They are insurance payments, and a reminder that nearly half of the budget is now an insurance fund -- protecting Americans against, among other things, inflation.