So, during the first two decades of the 20th century, the growth of the US population was due to the high birth rate (in 1900 there were 32 newborns per 1000 inhabitants, or 32 %, in 1910 – 30.1 ‰, in 1920 – 27.7 %) and mass immigration, which from 1901 to 1910 amounted to 8.8 million people, and from 1911 to 1920 – 5.7 million people. As a result, the first decade of the last century was marked by the largest relative increase in the US population in the past 110 years – 21%; a very significant increase was also noted in the 1920s – 14.9%.
In the following decades, the birth rate declined at a fairly rapid pace, and this became the dominant historical trend: by 1930 the birth rate was 21.2 %, and during the years of the “Great Depression” it fell to 18.4-19 %, i.e. almost 1.7 times compared with the birth rate at the beginning of the century. The exception was the period from 1946 to 1964, which (at the end of the 1950s) gave the second highest demographic result since the beginning of the century in both indicators – absolute (28 million people) and relative (18.5%), which is explained by the post-war the birth rate boom – 24-25 %, which was never repeated or surpassed. This is the so-called baby boomer generation.