Why then do almost all public pronouncements limit the impact of immigration to one-third or even less?

It happens because it is one accurate way to state proportions and is the easiest, Bouvier says. If you consider the start of each year a blank slate, you simply add up the number of known net immigration during that year—say, 500,000—and relate that to total population growth—say, 1.5 million. Thus, you have a one-third proportion.

But that method never considers the fact that in ensuing years immigrants have babies and their babies grow up and have babies, all at significantly higher rates than for other Americans—hence the caveat: "and their descendants." An example of how misleading the simple method can be is found in "The World Almanac and Book of Facts 1991." It shows that "natural increase" of Americans was responsible for 71 percent of growth during the '80s while new immigrants made up 29 percent of it. But by the end of the decade, a larger and larger portion of that "natural increase" was the descendants of early-'80s immigrants.

Bouvier's method provides a much cleaner picture and also accounts for illegal immigration without having to rely on estimates. He did this by asking the question: What would have happened to the US population had there been no immigration? He built on reliable US Census data. He took the multi-ethnic 1970 US population of 203 million and the way it was known to have broken down by age and gender. He applied fertility and death rates of the period and ran the age groups through the computer program.

Once he found how many people today were here in 1970 or are descendants of that population, he knew the rest of the population had to have arrived as immigrants since then, or be their descendants.

The year 1970 was chosen as the benchmark because it began the decade in which the modem environmental ethic was formalized in many government policies. It also was the time of the first Earth Day and part of a period of much public clamor for halting population growth for the sake of the environment. In 1972, a presidential commission concluded there were no substantial benefits from adding any new population and that resolution of many environmental, social and economic problems would be enhanced by population stability. Bouvier's figures point dramatically to the ignored warning.

Even without immigration, of course, the '70-vintage Americans were bound to grow on their own because the gigantic generation of baby boomers was moving into child-bearing years. Growth was not as high as it might have been because boomer women averaged less than two children each. Nonetheless, they were responsible for an extra 11 million Americans by 1980, Bouvier found.

But their environmentally and socially responsible behavior on family size was substantially negated by the nation's immigration policy which more than doubled the population growth to 23 million by 1980. Without net immigration, Americans would not have added 23 million to their number until nearly 10 years later. Imagine life today with only 1980's population—only the same number of automobile drivers as then and without the urban sprawl that has been added during the last decade!

In fact, the nation added 48 million people from 1970 to 1990. Of these, almost 25 million were the result of US immigration policies.

The United States population would have increased by less than 24 million persons between 1970 and 1990, but immigrants and their descendants radically increased the growth by more than 24 million additional people.