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The term "baby boomer" is used to describe anyone born in the United States between 1946 and 1964. As of 2011, baby boomers have been driving the growth of the senior citizen population, and by 2029, when all of the baby boomers are 65 years or older, it is projected that more than 20 percent of the U.S. population will be over the age of 65. By 2056, the population 65 years and over is projected to become larger than the population under 18 years.

Figure 1 shows the age and sex structure for the population of the United States between 1945 and 2012. The bulge in the pyramids are shaded in gray and represent the baby boomer cohort.

Figure 1   Age and Sex Structure of the Population for the United States: 1945 to 2012 (Numbers in millions)

The dependency ratio of a population can be defined as the ratio of the number of "dependent" members of a population to the number of "productive" members of the same population. In Western societies such as the United States, demographers measure the dependency ratio according to the following formula:

(Equation 1)

In the United States, the total dependency ratio increased steadily from 1945 to 1964, peaking at about 0.8 in 1964. After the baby boomers entered the workforce, the ratio decreased slowly until 2010, when it reached its nadir at 0.59. The projected dependency ratio for 2060 is 0.75, and this is mostly due to the large effect that the baby boomer generation will have.

Source: Adapted from Colby, Ortman, The Baby Boom Cohort in the United States: 2012 to 2060; United States Census. Issued May 2014 P25-1141

Although the fertility rates observed during the baby boom were not the highest ever seen in the United States, the number of births during those years was the highest ever. This is most likely because: