THROUGHOUT MOST OF the years since World War II, productivity in the American economy has risen briskly. As it went up, it carried with it people's earnings and standards of living. But the rise began to slacken about a decade ago, and for the past year and a half there has been hardly any rise at all. Of all the changes overtaking the American economy, the behavior of productivity is one of the most peculiar - and one of the most disquieting.

The immediate consequence of no productivity gains is that inflation will become harder than ever to control. But if the present pattern continues, it will also ignite uncomfortable political questions about dividing the pie in a country that has come to expect, and to count on, steady increases in both public and private wealth. Nobody really knows why productivity has stopped rising. All explanations are, to one degree or another, speculative. But the evidence suggests that it is no minor passing blip on the chart. The causes seem to lie deep in the changing structure of the national economy.

Productivity is simply the average output per hour of labor. The Labor Department computes it every three months, and it has just published the figures for the spring quarter of this year. They show that productivity was rising at the minuscule rate of 0.1 percent a year, after having fallen during the winter. Through the 1950s and most of the 1960s, it was going up at an impressive pace of nearly 3 percent a year. In the years after 1968, the trend dropped to half that rate. Since late 1976, it has been almost flat.

That departure is consistent with two other surprises. Inflation has been running considerably higher in recent months than most people expected, and unemployment has been considerably lower. Output over the past year has been raised by putting more people on payrolls, not by improving each person's capacity to produce.

It's a striking departure, and one explanation may well lie in the rather low rates at which business has been investing new capital. That, in turn, may be the result of low profits. Another possibility is the cost of the new environmental and safety rules, requiring industry to invest heavily in equipment to control air and water pollution.

Whatever the influences controlling productivity, they vary enormously from one country to another. In international competition, the United States is currently not doing well. The following brief table compares the increases in productivity in manufacturing, for the decades 1967-77, among some of the major industrial powers:

United States 27 percent

France 72

West Germany 70

Italy 62

Japan 107

Canada 43

Great Britain 27

There is a tendency in this country to regard Britain as the world's great example of industrial decline. But you will note that the rate of productivity gain in British factories over the past 10 years has been the same as in American factories.

Since no one is quite sure why the American rate has fallen, no one is in a position to offer a sure remedy. But these latest productivity figures may well strengthen the impulse in Congress to cut taxes on capital gains, in an effort to increase investment. Beyond that, it's also necessary to consider the possibility that some of this change may lie beyond the reach of government policy. A good many Americans' ideas about work, incomes and economic growth began to change around 1968. These new attitudes may now be showing up in the statistics on the nation's economic performance.