THERE IS NO denying the importance - psychological and otherwise - of the Census Bureau's report that the rate of growth of the world's population is apparently declining for the first time in three centuries. Population itself, it needs to be emphasized, is still growing at an epidemic rate of perhaps 1.9 percent, adding 80 million people (two Egypts, one Bangladesh) a year. And the rate of growth is down by a mere tenth of a percentage point, which is only about 4 million people (another Chad or El Salvador). But the decline conveys an unmistakable sense of a shift of immense forces. At least, it suggests the world is not fated to be overwhelmed by a rising tide of people. It implies that a measure of control over the global condition is perhaps restored.

The Census Bureau is the most conservative estimator around, and it is only confirming estimates previously made by others. So there seems little reason to challenge the basic slowdown it has now detected. The estimated relies heavily on China, which contains almost a quarter of mankind, but other expert estimates of China's trends have indicated even greater declines in its birth rate.

Less certain is why the rate has declined. Is it a result of family-planning programs, or overall development, or some mix of both? A "great debate" on this question continues. It's not academic. On the answer hinge fateful decisions on the direction and pace of economic and social planning all over the world.

The Census Bureau, inspecting its own new figures, declares that "the world population crisis appears resolvable." One must take this as an expression of hope. The new slower rate of world growth is still substantially more than double the American rate of population increase, and the rate in selected regions (Africa), countries and sections of countries has not turned around. Even if - a huge if - the overall rate continues to fall steadily, a zero rate would not be reached for almost 50 years. Even then the ratio of young people would ensure that population would continue to rise. One only has to think of the planet's current fix and then note that optimists see the world's population doubling, to 8 billion, in 30 years.

Population growth, whether at the rate estimated in the last generation or the rate expected in the next, is a relentless force, imposing brutal demands on the world's resources and feeding social and political turbulence everywhere. Relief at a welcome but small change in the numbers cannot be allowed to disguise this pervasive fact.

President Eisenhower, regaridng family planning as a "personal" matter, kept the United States officially aloof. A connection between runaway growth rates and the sort of global instability harmful to American national security began to be perceived in the early 1960s. But only under President Johnson did the United States start developing effective family-planning programs. More recently, the Third World's concern for development, too often gratuitously counterposed to family planning, has diminished official American ardor for these programs. Ironically, this is happening just as their fruits, or so the family planners believe, are showing up in the global population statistics.