The second half of the Twentieth Century was a period of unprecedented and remarkable population growth in the United States.  By 1950, the U.S. population had grown from the nearly four million people enumerated in the nation's first census (in 1790) to over 151 million.  In less than fifty years, from 1950 to 1996, the population of the United States exploded to 268 million.  This increase of over 116 million people represented the largest population growth of any fifty year period in U.S. history and more than all population growth in the first one hundred and fifty years of the nation's history.  The 116 million new Americans added between 1950 and 1996 were 41 million more than the 75 million Americans added to the population in the previous fifty years, as well as 41 million more than the entire U. S. population at the beginning of the Twentieth Century.

The second half of the Twentieth Century also saw a remarkable shift in the demographic shares accounting for most of the growth.  During this relatively short time, the driving force behind U.S. population growth dramatically shifted from native-born natural increase to immigration, both directly and indirectly through births to immigrant women living in the U.S.  The period began with a baby boom and bust during which U.S. total fertility fell from an average of 3.5 births per woman to below replacement level (the average 2.1 births per woman necessary to replace the mother and father) in less than one decade.  The boom and bust was followed by a sustained period of sub-replacement fertility by the native-born population, which continues today.

At the very time that the great majority of native-born Americans were voluntarily choosing to limit their family sizes to levels which could have led to the end of U.S. population growth, Congress was making changes in immigration policy which has ensured ever more growth.  The result of these changes was the highest sustained immigration and greatest population growth in U.S. history — population growth which shows no sign of ending as the U.S. prepares to enter the Twenty-first Century.  Ironically, were it not for the dramatic increase in immigration, this extraordinary period of population growth would have been a prelude to the end of U.S. population growth in the first part of the Twenty-First Century.

In many ways, the 1950s and early 1960s were typical of population growth patterns throughout U.S. history.  Natural increase of the native-born population was the primary component of growth.  Net immigration, which during this period averaged about 240,000 annually, also approximated traditional historic averages.  Somewhat atypically, natural increase attributable to immigrants, was actually negative.  That is, deaths outnumbered births to immigrants, a reflection of the fact that many foreign born had entered the U.S. during the great wave of immigration from 1896 to 1915.  As a result, immigration's overall share (net immigration plus natural increase attributable to immigrants) of U.S. growth during the 1950s was small, less than 4 percent.  By the 1960s, however, increased immigration began to push this share gradually upward, more than doubling it to 9 percent during the decade 1961 to 1970.

Nonetheless, the primary demographic force during the 1950s and 60s was the Baby Boom. The Baby Boom was a post World War II period of high domestic fertility, during which American women gave birth to some 76 million babies from 1946 to 1964 — the largest generation in U.S. history.  Almost all population growth during the 1950s was attributable to births to U.S. born women with annual population growth rates peaking in 1957 when the U.S. population grew by 1.8 percent.

By the mid 1960s, however, two events occurred which were to have profound long-term consequences for the United States' demographic future.  First, the Immigration and Nationality Act Amendments of October 3, 1965 marked a major shift in U.S. immigration policy. Although the impact was gradual at first, the change in emphasis (from a policy based on national quotas and labor market needs to one based on family reunification), resulted in dramatically increased immigration. By the 1980s and 1990s, immigration swelled to the highest sustained levels in 
U.S. history.

The second major event was a sudden and remarkable drop in U.S. fertility. At the end of the Baby Boom (1960-1964), the Total Fertility Rate (TFR) stood at 3.5.  By the end of the 1960s that rate had fallen to 2.6 (1965-69) and by the early 1970s (1970 to 1974) was at replacement level, the average of 2.1 births per woman necessary for a biologically stationary population.  From boom to bust, this extraordinary drop in fertility served to set the stage for the unprecedented shift in the shares of U.S. population growth that was to occur in the next three decades.

The 1970s were a demographic watershed for several reasons.  First, in 1972, the U.S. TFR went below replacement level for the first time since the 1930s, an astounding event considering that just over 10 years earlier the United States had experienced record numbers of annual births.  Another milestone, occurred just two years later, when, for the first time in the second half of the Twentieth Century, births to the foreign born began to outnumber foreign-born deaths.  In the next two decades these two divergent trends would continue.  As native-born fertility remained at sub-replacement levels and dropped as a share of total U.S. population growth, direct immigration and births to the foreign born began to rapidly increase their shares of U.S. population growth.

The remarkable drop in native-born fertility continued throughout the 1970s, with the U.S. TFR reaching a historic low of 1.7 in 1976.  This marked the beginning of a sustained period of sub-replacement level fertility, which continued into the 1990s.  Overall, U.S. TFR remained at sub-replacement levels throughout the 1970s and 1980s: 1975-79, 1.8; 1980-84, 1.8; 1985-88, 1.9.  Although the overall TFR has since risen back to replacement level (largely due to the higher fertility and growing number of foreign-born women in the U.S.), the native-born population has continued to maintain below replacement fertility of about 1.9.

Had immigration remained at replacement level (in and out migration equal) after 1970, this sustained period of sub-replacement level fertility would have resulted in the U.S. population peaking at about 250 million around the year 20301.  Assuming that domestic fertility remained at about 1.9 births per woman and immigration remained at replacement levels, the U.S. population would have then begun a very gradual transition to a smaller and ultimately optimum population.  This was the demographic future that the majority of American families have, since the 1970s, chosen.
 

Table 1.  Components of U.S. Population Growth By Decade 1951-1996
Decade	1951-60	1961-70	1971-80	1981-90	1991-96
Total Growth	27,664,479	22,493,591	19,974,785	24,828,693	16,751,176
( % of Total )	(100%)	(100%)	(100%)	(100%)	(100%)
Native-Born Share1	26,610,726	20,466,805	13,001,265	14,206,174	7,040,510
	(96%)	(91%)	(65%)	(57%)	(42%)
Net Immigration	2,224,479	2,597,677	6,297,314	8,349,062	6,638,213
	(8%)	(11%)	(32%)	(33%)	(40%)
Foreign-Born Natural Increase2	-1,170,726	-570,891	376,206	2,573,457	3,072,453
	(-4%)	(-2%)	(2%)	(10%)	(18%)
Immigration's Share3	1,053,753	2,026,786	6,673,520	10,922,519	9,710,666
	(4%)	(9%)	(34%)	(43%)	(58%)
Notes: 1. Natural increase attributable to the U.S. born population minus after 1970 emigration of U.S. born persons estimated at 48,000 annually. 2. Natural increase attributable to immigration, that is births to foreign-born women minus deaths of the foreign born. 3. Net immigration plus natural increase attributable to immigrants.

By the 1970s, however, another demographic future — this one chosen for the American people by the politicians in Washington — was beginning to take shape.  With the chain migration provisions of the 1965 amendments kicking in, legal immigration during the decade rose to nearly 5 million, more than double the annual average of the previous four decades2.  Illegal immigration also significantly increased with an estimated net three million illegals establishing permanent residence in the U.S. from 1971 to 1980.  The rapidly growing number of immigrants was also reflected in the growing stock of foreign born in the U.S. which rose from slightly over 10 million (4.9% of the total population) in 1971 to more than 14 million (6.2% of the total population) in 1980.

As a result, the 1970s saw immigration's total share of U.S. growth dramatically increase.  In 1970, immigration (net immigration plus the net natural increase of the foreign born) accounted for 280,949 (13%) of total U.S. growth that year (2,111,281).  By 1980 immigration's contribution had tripled to 897,965 or 38% of total annual population growth. Most of this growth in immigration's share of U.S. growth was due to rapidly increasing levels of immigration.  Net foreign-born migration for the decade totaled 6,297,314 and accounted for 34% of total growth.  Because of the age structure and relatively small size of the foreign born population in the U.S., natural increase attributable to immigration (births to foreign-born women minus deaths to the foreign born) was still minor, only 376,206, or 2% of total U.S. growth for the decade (19,974,785).  When combined with net migration, however, immigration accounted for fully one-third of all U.S. growth for the decade 1971 to 1980 (6,673,520 or 34%).  Astoundingly, this was over eight times immigration's growth share from the 1950s and more than triple the contribution of immigration to U.S. growth in the previous decade, the 1960s.

The dual trends, of rapidly increasing immigration and a growing number of births to immigrants already in the country, greatly intensified in the 1980s.  The latter 1970s marked the beginning of a prolonged period that saw the highest sustained immigration in United States' history.  According to INS figures, from 1977 until 1996 15,566,019 immigrants were granted legal permanent residency in the U.S. This is more than the 14,818,902 immigrants who entered during what had, until recently, been the historic period of highest immigration, the 20-year period 1896 to 1915.  This figure also does not include the estimated 5.5 million illegal immigrants added to the U.S. 
population from 1977 to 1996 (illegal immigration was not a significant factor around the turn of the century).

After tripling in the 1970s, immigration's share of annual U.S. population growth was on the path to double again.  In 1981 immigration accounted for 881,886, 38% of annual U.S. growth.  By 1990 that share would grow to 2,068,828—58% of all U.S. population growth.  Directly, this radically changing share of growth was due to very high levels of legal immigration.  Indirectly, however, as the total number of foreign-born women permanently residing in the U.S. rapidly grew, so too did the number of births attributable to immigrants.  The growth in total numbers of foreign-born women was compounded by the much higher fertility of large segments of the foreign-born population.  A 1995 Census Bureau report has confirmed that foreign-born women had fertility one-third higher than that of native-born women of childbearing age.3 Women born in Mexico, who comprise one-third of all foreign-born women of childbearing ages, had fertility two-thirds higher.

While direct immigration was the most obvious contributor to U.S. population growth, the full importance of the indirect component, births to immigrants living in the U.S. has largely gone unrecognized.  It was only recently, that the Census Bureau finally acknowledged that, "most of the importance of net migration in understanding population growth is the natural increase of the population it adds.4"  Of all the components of U.S. growth, natural increase attributable to the foreign born was the fastest growing, nearly tripling (a 178% increase) between 1981 and 1990 (from 157,286 to 436,945).  The decade of the 1980s, also saw births to the foreign born jump from 10% to 15%, of total births as did the foreign born share of total natural increase, which rose from 9% to 22%.  Likewise, the share of foreign-born natural increase as a percentage of total growth, increased from 7% to 12%.  These figures are all the more remarkable considering that the foreign born were still a relatively small fraction of the total U.S. population, 6% in 1981 and 7% in 1990.

Overall, the numbers for the decade 1981 to 1990 clearly substantiate these trends, especially when compared to the earlier decades.  For the 1980s, immigration's share increased to 10,922,519 or 43% of total U.S. population growth (24,828,693) for the decade.  The enormous number of new immigrants entering the U.S. were likewise reflected in the rapidly shifting shares which included natural increase attributable to the foreign born of 2,573,457 (10% of total growth) and net immigration of 8,049,062 (32% of total growth).  From 1981 to 1990, immigration's annual share of total U.S. growth increased an astonishing 152%.
 
 

Table 2.  Contribution of Foreign-Born Births to U.S. Population Growth
Year	Total  Foreign- Born Population  (% Total Population)	Births to Foreign-Born Women (% Total Births)1	Percentage of U.S. Natural Increase2	Immigration's  Share of  Total Growth3
1950	10,347,000	103,578	-7%	1%
	(6.9%)	(3.4%)
1960	9,661,000	150,498	-4%	5%
	(5.4%)	(4.2%)
1970	9,740,000	222,642	-1%	13%
	(4.8%)	(5.9%)
1980	14,080,000	335,358	+7%	38%
	(6.2%)	(9.3%)
1990	19,767,000	645,589	+22%	58%
	(7.9%)	(15.5%)
1996	25,208,604	747,167	+34%	61%
	(9.5%)	(19.2%)
Notes: 1. Births to foreign-born (immigrant) women permanently residing in the United States. 2. Foreign-born natural increase (foreign-born births minus foreign-born deaths) as a percentage of U.S. natural increase. 3. Net foreign-born migration and foreign-born natural increase as a percentage of total U.S. population growth.

Two further changes in immigration policy, helped to accelerate this shift from native-born births to immigration as the driving force behind U.S. population growth.  The 1986 Immigration Reform and Control Act (IRCA) not only gave permanent legal status to some three million illegal aliens, but more importantly, made them eligible to bring additional family members into the country under the generous family reunification provisions of current law.  Second, the 1990 Immigration Act directly increased total legal immigration by about 35%.

During the 1990s, these two changes worked together to solidify immigration's role as the primary component of U.S. growth.  Including the IRCA legalizations, the nearly two million — 1,827,167 — legal immigrants granted permanent residency in the U.S. in 1991 marked the highest annual level of immigration in U.S. history.  This was over 42% higher than the previous record for annual immigration, 1907, when 1,285,349 immigrants entered the country. 

Natural increase of native-born Americans has continued to play an important role accounting for 48% of total U.S. population growth in the 1990s.  Much of that growth, however, was population momentum from pre-1970 fertility, that is, births to the Baby Boom generation.  Although women from the Baby Boom generation have averaged less than two births each, population momentum dictates that their total births will outnumber the total of people dying, most of whom are from earlier and smaller generations.  Thus, Baby Boom population momentum was essentially predetermined population growth that could not be changed.  By the 1990s most of the women from that generation had already completed their families or were nearing the end of their childbearing years (15 to 44) and their population momentum was, likewise, nearing its end.

While Baby Boom population momentum has been winding down, births to the foreign born have continued their remarkable and steady rise as a percentage of total U.S. births.  In the relatively short period from 1991 to 1996, foreign-born births (as a percentage of total U.S. births) increased from 16% to 19%, and natural increase attributable to immigration (births to immigrant women minus deaths of immigrants) increased from 25% to 35%.  These figures are even more remarkable given the fact that the foreign born still comprised less than 10 percent of the total population during this time.

The growing number of foreign-born births combined with high immigration were clearly evident in immigration's share of U.S. population growth which totaled almost 10 million (9,710,666), or 58%, for the entire period 1991-96 (16,751,176).  For the last year in which full data is available, 1996, the foreign born population in the U.S. was 25,208,604, or 10% of the total U.S. population.  Yet, births to the foreign born totaled 747,167, 19% of total births. Likewise, 34% of all natural increase (1,576,804) and 21% of total U.S. growth (2,524,704) was due to immigration.  Similarly, net immigration was extraordinarily high, approaching one million  (995,900) and accounting for 39% of total U.S. population growth in 1996. Taken together, direct immigration and births to immigrants accounted for more than 1.5 million, or 61%, of total U.S. population growth.

The rapidly shifting shares of population growth in the second half of the Twentieth Century were unprecedented in U.S. history.  Also without parallel was the extent that a single federal government policy — immigration — has fostered continued population growth.  By the 1970s, most Americans were voluntarily choosing to limit their families to a size which would have stopped population growth.  Contrary to the direction native-born fertility was taking, the U.S., Congress raised immigration to the highest sustained levels in U.S. history.

While the direct impact of immigration was important, the indirect effect of immigration, births to the foreign born, quickly became an increasingly powerful engine of additional population growth.  Immigration policy dramatically increased the total number of U.S. births in two ways.  First, it greatly increased the total number of women giving birth in the U.S.  Second, the total fertility of most immigrant women has been substantially higher than the native-born population.  Currently, immigrants account for about 10% of the total U.S. population, yet contribute well over one-third of U.S. biologic growth.

The last twenty years of high immigration has also created tremendous population momentum, which will drive U.S. population growth far into the next century.  Due in large part to immigration, the Census Bureau now projects that "beginning in 2012, the number of births each year will exceed the highest annual number of births ever achieved in the United States⁵."  The full extent of immigrant driven population momentum and the consequences of continuing current immigration policy are provided in projections done by the Census Bureau and confirmed in a study by the National Research Council⁶.  According to the Census Bureau middle series projections, which assume net immigration of 820,000 a year, immigration will boost U.S. population growth by almost 80 million (61% of all growth) between 1995 and 2050⁷.  That is, the U.S. population will be 393 million rather than the 314 million if there was no immigration.

Because of population momentum, U.S. immigration policy has already determined a demographic future of continued growth through 2050 — even if immigration were stopped today.  Rather than a population that would have peaked at 250 million and begun a transition to an optimum and sustainable level, the U.S. is now committed to a growing population of at least 314 million⁸.  Americans and our political leaders can, however, still choose a demographic future different from the one that current high levels of immigration are taking us toward.  The tale of America's demographic future in the Twenty-first Century is being written today in annual levels of immigration and the family size choices of all Americans.  The choice between our two possible futures is clear: continued growth, environmental degradation, and a declining quality of life or population reduction and long-term sustainability.

This NPG Forum was adapted from a longer study analyzing the changing shares of U.S. population growth from 1950 to 1996.  Unless otherwise noted, the source for all figures mentioned in the report are published birth, death, immigration and population data (or direct derivations from that data) provided by the National Center for Health Statistics, the Census Bureau and the Immigration and Naturalization Service (INS).  Where reliable data is simply not available, such as estimates of emigration and illegal immigration, figures are taken directly from Census Bureau or INS estimates or derived by comparing available data and estimates from both government and private analyses.  A complete listing of sources and references is available in the full study.

 Immigration's share of total population growth was determined by looking at both its direct and indirect contributions.  The direct component is simply the net addition of those who immigrate into the country and establish permanent residence minus the foreign born who permanently leave — net immigration.  Immigration's indirect impact on population growth is determined by subtracting annual deaths of permanent resident immigrants from annual births to immigrant women, that is, the natural increase attributable to immigration.

 It should also be noted that this methodology for calculating immigration's share of growth underestimates the long-term impacts of immigration on population growth.  In limiting immigration's share to direct immigration and births to immigrant women, the broader impacts of generation after generation of immigrant descendants are not included.  A full discussion of the methodology behind the figures, is also available in the full study.

The full demographic analysis from which this Forum was adapted, "The Changing Shares of U.S. Population Growth: 1950 to 1996," is available from Negative Population Growth.

1. If U.S. immigration had remained at replacement level after 1970, U.S. population growth would have peaked around 2030 at about 247 million.  Based on projections done by demographer Leon Bouvier in Roy Beck, "Immigration No. 1 in U.S. Growth."  The Social Contract, Vol. 2, No. 2, Winter 1991-92.

2. The INS lists total immigration between 1931 and 1970 at 7,400,626, an annual average of 185,016 and a decade average of 1,850,157.

3. U.S. Census Bureau, Current Population Reports, Series P20—499, "Fertility of American Women: June 1995." Forthcoming.

4. U.S. Census Bureau, Current Population Reports, Series P23—194, "Population Profile of the United States: 1997."  U.S. Government Printing Office, Washington, D.C., 1998, p.9.

5. U.S. Census Bureau, Current Population Reports, Series P23—194, "Population Profile of the United States: 1997."  U.S. Government Printing Office, Washington, D.C., 1998, p.9.

6. Smith, James P., and Barry Edmonston, Editors. 1997. The New Americans: Economic, Demographic, and Fiscal Effects of Immigration.  National Academy Press, Washington, D.C.

7. U.S. Census Bureau, Current Population Reports, Series P25—1130, "Population Projections of the United States by Age, Sex, Race, and Hispanic Origin: 1995 to 2050." U.S. Government Printing Office, Washington, D.C., 1996.

8. Bouvier, Leon. 1998. "The Impact of Immigration on United States' Population Size: 1950 to 2050."  NPG Forum, Negative Population Growth, Washington, D.C.

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