US Demographics - Glass Half Full Or Half Empty?

Measuring How Births Swell the Population Isn't Child's Play

Pew Research Center says the birth rate is at its lowest level since at least 1920. But the U.S. fertility rate, an estimate of how many children a woman will have in her lifetime, is well above record lows, says the Population Reference Bureau

By Cark Bialik

A recent report said the U.S. birth rate has dipped to a record low level. But another measure of the nation's fertility remains comfortably above its historic low. The mismatch shows that even in a country with comprehensive birth statistics, summarizing population trends is far from straightforward.

Last week, Pew Research Center said the birth rate last year fell to 63.2 per 1,000 women age 15 to 44. That's the lowest level since at least 1920, the earliest year for which reliable data are available. The report made headlines and even spurred calls for Americans to get procreating lest they fall behind economically.

But the U.S. fertility rate, an estimate of how many children a woman will have in her lifetime, is well above record lows. According to the Population Reference Bureau, a Washington, D.C., research center, it fell in 2011 to just below 1.9 per woman, down from 2.12 in 2007—the highest in the last 40 years—but above a record low of 1.74 in 1976.

Demographers disagree on which measure is best for tracking births' contribution to population growth. The distinction matters because birth trends are monitored closely. A record-low level of fertility could augur problems for future economic growth, while a slight drop from a 40-year high may have less serious implications—particularly because fertility often declines during economic slowdowns.

"Births are at a record low, but it's a much more complex story," said Brady Hamilton, a statistician at the Centers for Disease Control and Prevention's National Center for Health Statistics. "The devil is in the details."

Simply computing a per capita birth rate is flawed, researchers say, as it is skewed by the age and gender structure of a population. A country with a disproportionately high share of older men could have a much lower birth rate than one with a large population of women of child-bearing age, even if women in each country have the same average number of children. Calculating the rate of births per women age 15-44, as Pew and the NCHS do, improves on the raw birth rate as it accounts for the share of the overall population most likely to have children. But the measure—called the general fertility rate by demographers—is still subject to quirks in the distribution of women in that age range.

A demographic blip that leaves a country with a population in which there are far more women in their 20s than women between 15 and 19 or between 40 and 44 could result in an artificially high general fertility rate, because in this case most of the women in the 15-to-44 range are in their peak years of child-bearing.

That is what many demographers said happened in the U.S. in the 1980s. Women in their 20s made up a disproportionate share of the 18-44 female population—over 38% in some years, compared with under 31% a decade ago and about 34% now.

Some researchers prefer a measure known as the total fertility rate. To truly calculate how many children that women born in one year will, on average, bear in their lifetimes requires waiting decades, until they've all reached menopause. Instead, researchers assume today's trends will apply. So if a hypothetical group of women has today's birth rate at age 15, and again at 16 and so on, how many children, on average, will each one give birth to? This evens out fluctuations in age distribution, and is preferred by researchers projecting population trends, under the thinking that this fertility rate is likely to remain more stable.

But this measure has drawbacks. Imagine a society where women now in their 30s all had children in their 20s and then stopped, while today's 20-somethings plan to wait until their 30s and then have just as many children as the prior generation. This would mean neither group is having many children today—a shift that would artificially drive down the total fertility rate. Yet both groups will end up with the same number of children.

This is what some researchers say may have made fertility rates in some European countries look lower than they really were in the 1990s. Younger women, who were entering the workforce in growing numbers and had more ability to plan pregnancies due to widespread contraception, were planning to have children later.

Whether the U.S. is experiencing record-low fertility or a gentle dip matters in determining the severity of the decline, and the likelihood that fertility could bounce back.

John Bongaarts, vice president of the New York-based Population Council, a research group, said fertility is hard to predict, as so many factors it depends on are, too—such as the economy. "Around 2005, very few demographers predicted the decline that has happened in the past couple of years" in fertility rate, he said, since most economists didn't expect an economic crisis.

But a full recovery won't necessarily revive fertility. "The decision about fertility is one that will continue to be about individuals," said Jose Miguel Guzman, chief of the United Nations Population Fund's population and development arm, and about how they "see the future—the possibility to have a house, a job, to send kids to a good school."