Immigration into the United States fluctuated throughout the 20th century because of varying economic conditions. But the changes made by the Immigration Reform and Control Act of 1986 drastically increased the number of immigrants allowed into the United States. The chart below depicts the increase in population per decade during the 20th century with a brief description below of how immigration numbers affected the growth.
The Great Wave of Immigration began in 1880 but exploded into peak numbers during the first decade of the century. The massive numbers of immigrants reached a cumulative total that began to substantially change the character of the entire country from one primarily of towns and farms into one of densely packed urban centers. This decade saw more growth than any previous decade in U.S. history. The rapid population growth destroyed huge sections of the country's once bountiful natural resources, leading to the establishment of the federal parks system and other preservation programs.
World War I slowed immigration considerably during the middle of the decade. But high immigration at the beginning and end, and high immigrant and native fertility, kept total population growth high.
Americans of nearly every station in life rose up in revulsion at the extreme congestion, widespread joblessness and underemployment, increasing gap between rich and poor, and other serious social problems caused by the previous two decades of immigration-driven population growth. By 1925, Congress had reduced immigration numbers toward more traditional levels. The annual population growth rate at the end of the decade had been cut almost in half from the beginning. But the very high immigration of the first half of the decade, and the momentum caused by the high fertility of the greatly enlarged population, helped the 1920s to set yet another record for highest population growth.
The Great Depression Decade
The 1924 immigration law and the Great Depression kept immigration below traditional levels. And Americans greatly reduced their fertility in response to the dire economic times, cutting total population growth for the decade nearly in half from each of the previous three decades.
The Baby Boom Decades
After the end of World War II in 1945, immigration grew back toward traditional levels and Americans began to create very large families. The giant spike in fertility came to be known as the Baby Boom, a demographic phenomenon that changed every aspect of American society and that continues to drive a lot of the social and political agenda to this day.
This was the peak of the Baby Boom, adding nearly the equivalent of the entire U.S. population at the time of the Civil War. Combined with other factors, this led to an enormous conversion of farmland and natural habitats into sprawling suburbs. This new record for the biggest population boom ever was widely thought to be a special phenomenon reflecting pent-up pressures from the Depression and World War II and one that would never be repeated or exceeded.
Exhausted from years of frantic efforts to expand the nation's infrastructure to handle its large families and burgeoning population, Americans rapidly reduced their fertility through the last decade of the Baby Boom. The growth rate at the end of the decade was a third lower than at the beginning. A vigorous social and political movement emerged calling for Americans to keep their fertility to a replacement level rate to enable the country to eventually stabilize its population.
Low-Fertility/Immigration Tidal Wave Decades
The American fertility rate fell to replacement level in 1972, making it possible for the nation to eventually reach a widely held dream of a stable population. A national government commission recommended that the nation would be best served in reaching its environmental, economic and social goals by a stabilizing population. Numerous experts and commentators predicted that each decade would see lower and lower population growth until early in the 21st century there would be no growth at all.
Despite continuing below-replacement-level fertility, population growth continued at the level of the previous decade. The reason was that Congress had created a system of chain migration that snowballed and doubled annual legal immigration over traditional levels. Further adding to the population, Congress for the first time rewarded illegal aliens -- about 3 million of them -- with a path to citizenship. Federal immigration policy was negating the results of Americans’ choosing to have smaller families.
Prospects for the Future
The immigration tidal wave of the last three decades has made it impossible for Baby Boomers to ever enjoy the 1970s dream of a stabilized country — even if all immigration were stopped tomorrow. The Census Bureau states that if immigration were reduced to replacement level, the United States population would still be growing at the end of the century because of the momentum created by the last three decades of immigration.