Thomas M. McDevitt [letters, Nov. 7] raises a fair point in calling my statement on global population trends -- "We've virtually conquered infant and child [mortality]" -- an exaggeration [front page, Oct. 12].

Millions of children continue to die, and their deaths are both tragic and largely unnecessary given current knowledge about natal and child health.

Nonetheless, infant and child death rates are but a fraction of their rates prior to 1900, a more apt comparison than the 1950 figures Mr. McDevitt cites. Worldwide, one in three children died before their first birthday just before 1900, compared with 1 in 17 currently. Indeed, dramatic declines in the death rates of young people have been the major influence on the 20th century's trajectory of world population growth.

Throughout most of humanity's time on earth, population grew slowly not because people had fewer children than they do today -- they had many more on average -- but because much smaller proportions of children survived to have children themselves. Before the 20th century, life expectancy at birth is estimated to have been 20 to 40 years. Today, the vast majority of children do survive through (and beyond) their reproductive years, and that is why the world's population has grown so rapidly.

I agree, however, that "virtually" is too weak a qualifier to place on the idea that infant and child mortality has been "conquered." Perhaps it would be more accurate to say that recent substantial declines in mortality have made birthrates, not death rates, the major determinant of current changes in world population.

ROBERT ENGELMAN

Vice President for Research

Population Action International

Washington