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THE NEW YORK TIMES

TUESDAY, JANUARY 7,1997

Model Shows How Medical Changes Let Population Surge

Simple advances in hygiene had the greatest impact.

By GINA KOLATA

Demographers often like to toy with statistical trends. If the world's population keeps growing as it is now, how long will it be before there is just one square foot of space for each person? If the death rate keeps failing, how long before there are more old people than young people in the population?

But Dr. Samuel H. Preston and one of his graduate students, Kevin M. White, demographers at the University of Pennsylvania, decided to do a sort of reverse experiment. Suppose, they asked, the mortality rate at the turn of. the century had not changed? How many Americans would be alive today?

The answer surprised them. They concluded that there would be about half as many Americans in today's population, 139 million instead of 276 million. Half of the missing population would be absent because one of their parents would not have su'rvived to reproductive age. And the other half would have been born but would have died young.

Of course, said Dr. Paul Demeny, a demographer at the Population Council, a nonprofit research group in New York, and the editor of Population and Development Review, it is well known that death rates have plummeted in the 20th century. But, he added, Dr. Preston's paper, published in Population and Development Review in September, "translates this in an uncommonly interesting and gripping way."

Dr. Richard Suzman, who directs the office of demography at the National Institute on Aging, said: "It's a rather simple but profound simulation. I was stunned by the magnitude of the effect."

Never before in history, Dr. Preston said, have life spans increased so much. "The expansion in longevity ranks among the great social achievements of our time," he wrote.

At the turn of the century, life expectancy at birth was 47.3 years. In 1994, it was 75.7 years. Moreover, Dr. Preston noted, people are not only living longer but they are more likely to survive long enough to have families of their own. In 1900, fewer than 60 percent of women lived to the age of 50, while 95 percent of women can now expect to live to be at least 50.

The effects on population size have been striking. "Most people believe that the population has grown because of immigration," Dr. Preston said. But his study, he said, "is a concrete way of expressing how important the health advances are." He added that when he calculated the effects of immigration on the population's size, he found they were only half that of the changing death rates.

Most of the health advances and resulting declines in mortality rates occurred in the first few decades of the 20th century. This means, Dr. Preston said, that they were due to simple changes in hygiene and public health, not to sophisticated medical treatments.

To illustrate this point, Dr. Preston asked how large the population would be if mortality rates had rema ned static since 1950, The result, he said, would be that 94 percent of Americans who are alive today would be still be alive.

The declining mortality rates in the first half of the century benefited children, for the most part. As children who would have died survived and had children of their own, the population jumped. In the second half of the century, in contrast, the improvements in death rates affected mainly older people, who had already had their families, and so had a much'smaller effect on the population's size but a large effect on the number of very old people:

Asked what changed the death rates in the first half of the century, Dr. Preston said he thought it was the ascendance of the germ theory of disease. This resulted, he said, in profound changes in personal and public health practices, like cleaner water, the sterilization of food, keeping flies away from food, washing hands and isolating sick patients.

Dr. Preston said medical advances early in the century could not have so markedly changed the mortality rates. After the advent of smallpox vaccinations, in the 18th century, and vaccinations against diphtheria in the 1890's, there was no major medical advance until the late 1930's, he said, when sulfa drugs were introduced to fight bacteria, and in the late 1940's, when penicillin was introduced.

Most people, Dr. Preston said, are entirely unaware that they probably owe their very existence to some thing so simple as an ancestor's hand washing or to the isolation of a sick child nearly a century ago.

Dr. Suzman, for one, said that the new study opened his eyes. It shows, he said, "how one sometimes has to go back several generations to understand what's happening today."

 

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