But the Social Security Administration’s advisory panel on assumptions and methods disagrees, arguing that because of the long-term consequences of decreased smoking, the decline in mortality rates will speed up. The Congressional Budget Office counters that because of offsetting factors such as obesity and improving medical technology, it is most defensible to assume that the trends of the past six decades will continue.

This dispute may sound abstruse, but the real-world consequences are enormous. Life expectancy today averages 78.5 years. The Social Security trustees project that this figure will rise to 83.6 years over the next two generations. But if their own advisory panel is correct, average life expectancy will be more than two years higher—85.8 years. Simply extrapolating current trends, as the CBO suggests, generates an average of 84.9 years—1.3 years more than the Social Security trustees foresee.

If the CBO is right, the population of Americans 65 or older would rise by 37% over the next decade and 85% by 2038. The number of Social Security beneficiaries would rise correspondingly—from 57 million today to 76 million in 2023 and a staggering 101 million just 15 years later. The system’s actuarial shortfall would amount to 3.4% of taxable payroll, significantly more than the 2.72% the trustees expect. And the surge in Social Security outlays is only one of many challenging consequences of a rapidly aging society.