In his June 25 op-ed column on Social Security, Thomas J. Healey questioned the reliability of trends in fertility and immigration but did not point out other unmistakable and potentially relevant developments.

Family structures and intrafamily economic relationships were considerably different in America during the 1940s and '50s, when the baby boom was in full swing. Less than 30 percent of all women, and only 16 percent of married women, participated in the workforce in the 1940s.

Today, the participation rates of both groups are 60 percent or higher. Their economic independence, probably permanent because it is driven by technological advances, is one potential explanation for today's higher divorce rate and greater incidence of single-parent households. These new gender-economic relations likely have reduced fertility rates permanently.

Human longevity continues to increase. The rate of increase is uncertain, but we have no doubt about its direction. Economic conditions are improving in the world's most populous regions -- India and China -- because of slow but probably irreversible trends in economic liberalization. This probably will reduce the pressure to emigrate.

And in the new era of worldwide terrorism, how likely is it that U.S. immigration rates will be allowed to soar significantly? All of these factors suggest that we may be unduly optimistic about the factors driving Social Security's finances.

JAGADEESH GOKHALE

Washington

The writer is a senior fellow at the Cato Institute.