The Birth Dearth: WHAT HAPPENS WHEN PEOPLE IN FREE COUNTRIES DON’T HAVE ENOUGH BABIES? by Ben J. Wattenberg (Pharos: $16.95; 182 pp.)

During the first two decades of the 20th Century, there was a great deal of talk in the United States of such subjects as “race suicide,” the decline of Western civilization and the Yellow Peril. As a number of influential authors observed, the birth rate was declining in all Western nations. If this trend was not reversed, they warned, the inhabitants of Western Europe and the United States were likely to find themselves overwhelmed by peoples who were breeding more vigorously.

The theme was one that President Theodore Roosevelt returned to again and again. He repeatedly warned the American people of the “race decadence” that would be an inevitable consequence of a low birth rate. If marriages did not produce an average of four children, he said, then the Germans or the Slavs would eventually achieve world dominance.

Others thought that the rapidly increasing Asians posed the greatest threat. For example, the novelist Jack London believed that it was the Chinese who should be feared. According to London, they would be led by the Japanese, who had ruling and organizing capacities that the Chinese lacked.

These ideas are apparently not so outmoded as one might think. In his new book, “The Birth Dearth,” syndicated columnist Ben J. Wattenberg issues warnings that are barely distinguishable from those of Roosevelt and London. For the last several decades, Wattenberg says, fertility rates have been falling in all Western countries. If we do not do something to reverse this trend, he warns, we are likely to lose our dominant position in the world to the Soviets and the peoples of the Third World.

During the last 15 years, the author says, fertility rates in Western countries have been so low that the number of children being born is not sufficient to maintain present population levels. In order to maintain a population, each woman must bear an average of 2.1 children. But the average American woman now has only 1.8 offspring. In some Western countries, the fertility rate is even lower. For example, it is only 1.3 in West Germany.

Even though there is a widespread “birth dearth,” the populations of Western nations are still increasing. But this is simply a consequence of the fact that the members of the “baby boom” generation are currently of childbearing age. If present trends continue, the author says, the total population of the “western democratic industrial nations” will have begun to decline in 30 years.

On the other hand, Wattenberg warns, the populations of the Soviet Union and other Communist nations will continue to grow rapidly. So will the population of the Third World. The fertility rate in the Communist bloc is 28% higher than that in the United States. Third-World birth rates are higher yet.

Wattenberg estimates that, if present trends continue, the Soviet Union will have a population that exceeds that of the United States by 30% in 2065. Comparisons between the West and the Third World are even more dramatic. In 1950, he says, Westerners made up 22% of the world population. But today they constitute only 15%. By 2035, the figure will be no more than 8%.

The consequences of this may be disastrous, the author says. As populations decline, Western cultural influence in the Third World will inevitably lessen. Our ideals will no longer be admired. Our political system will no longer be imitated. We will no longer be in a position to export our art and literature. Television audiences in Algeria may even cease to watch “Dallas.” In fact, the Third World could eventually become dominant and erode our culture. In other words, there may really be a Yellow Peril after all, even though it might not take the form that London and other early 20th-Century authors envisioned.

According to Wattenberg, the most serious consequence of falling population may be a decline in military power. As he points out, military technology has become expensive indeed. It now costs almost $3 billion to build a heavy aircraft carrier, and the “Star Wars” project is likely to cost a trillion. Obviously, large populations of taxpayers are needed to support present-day military establishments.

It never seems to occur to the author that there might be anything unreasonable about spending so much money on a missile defense system that probably wouldn’t even work, or on huge ships that would only be vulnerable targets in wartime. But I suppose we can’t really blame him for that. After all, the theme of the book isn’t military expenditure.

“The Birth Dearth” is a fascinating and frightening book. When I read an author who maintains that Western cultural dominance of the Third World is wholly beneficial, who urges that we breed more prolifically so that we can continue to increase our already huge military expenditures, I can’t help experiencing a sudden chill.

Fortunately there are humorous passages in the book. If there were not, my mounting feeling of horror might have prevented me from finishing it. The only trouble is that this humor is sometimes unintentional. For example, at one point, the author informs us that “there will be much personal misery . . . if fertility is so low that four grandparents must share one grandchild, and if lots of grandparents don’t have grandchildren.”

I must say that I, too, would hate to live in a world in which grandparents had no grandchildren. It would be nearly as bad as one in which there were parents who had no children.