Economists agree US must boost productivity: the question is, how?

Why can't the United States boost its productivity? US productivity is actually falling while Japan's rises spectacularly at 4 percent a year.

''If American standards of living are to resume their upward course, productivity growth must be restored,'' says economist Lester C. Thurow of the Massachusetts Institute of Technology.

''Unless we improve productivity, our leadership as a world economic power is threatened,'' declares Howard J. Samuels, head of an economic consulting agency in New York.

''Productivity growth, other things being equal, will determine whether workers are going to find themselves winning or losing in the real battle of inflation,'' adds Arnold Packer, consultant to Control Data Corporation and to the US government.

The views of these three economists are brought together by the prestigious Center for Democratic Policy, a public policy research organization headed by former North Carolina Gov. Terry Sanford.

Meanwhile, an anxious search for the answer continues elsewhere.

A US business delegation is going to Japan to explore labor-industry relationships. Another study, ''World Military and Social Expenditures,'' issued here by the research organization World Priorities, argues that US and Soviet civilian productivity is handicapped by heavy arms costs: In the US, military expenditures as a proportion of gross national product amount to 7 to 8 percent. They are even higher in the USSR, which is believed to have a poor productivity record. In contrast, military spending accounts for only about 1 percent of GNP in Japan.

Economists Thurow, Packer, and Samuels don't agree on what depresses American productivity at this juncture. But the three agree productivity is in a slump, and that there is probably no single remedy.

The Thurow study is typical in its reference to Japan. The Japanese, he says, operate through the Ministry of Trade and Industry and the Bank of Japan. There is more investment for plant and equipment; the government helps direct investment capital to new growth areas - ''sunrise'' industries.

''To compete in world markets the US is going to have to develop some equivalent institutions,'' he argues. ''The current Reagan strategy is tight monetary policies (through high interest rates) and easy fiscal policies (big deficits). Given the problems facing the US, the strategy could be exactly the reverse - low interest rates and large surpluses to raise national savings.''

To encourage less consumption and more investment Dr. Thurow advocates a ''progressive value-added tax,'' such as used in Europe. The effect of this tax, he says, is to promote savings and reduce consumption.

''If the productivity problem is left to actions in Washington, D.C., it will not be solved,'' declares Dr. Thurow gloomily. He argues that needed changes must come from private industry itself. Federal tax laws discourage productivity by directing investment to the wrong place. ''President Reagan's tax cuts are highly inefficient when it comes to stimulating saving,'' he asserts.