Dear Ben,

       Good news is not newsworthy, so journalists, politicians, and populists need to put a negative spin on events. It is very likely that the population explosion is not going to be quite as explosive as was expected a few years back. That’s the good news. However, it was a big bang and isn’t over yet. Let’s look at some absolute numbers, rather than rates.
       World population today is 5.9 billion, up from 1.6 billion at the beginning of the century and 2.5 billion in 1950. By the year 2050, the U.N. Medium Variant projects a population of 9.4 billion. Thus the next 50 years are likely to see an increase of world population greater than one-half its current population and greater than the increase since 1950.
       In the last five years of the present millennium, births outnumber deaths by about 81 million per year (133 million births, 52 million deaths). World population grows by this 81 million per year, a net gain equal to one-third of the total population of the United States every year. By the year 2050, even under the new, lower U.N. projections, births will still outnumber deaths by 44 million a year (131 million births, 87 million deaths).
       Now let’s talk about rates, and particularly current low rates in the established market economies. First, the Total Fertility Rate is a fertility measure, not a replacement measure. It has no mortality component. In a very low mortality setting such as the United States, a TFR of 2.1 children per woman (or, more accurately, about 2.08) is needed for replacement, to allow for a slight mortality loss prior to childbearing and to allow for the higher proportion of male to female births. However, in a high mortality setting, much higher TFRs are needed for replacement. Roughly speaking, if the risk of dying by age 30 is 50 percent, the “replacement” TFR is over 4. This is important when comparing fertility today with fertility in, say, 1950, when mortality was much higher: The world TFR has declined by about 47 percent, while the Net Reproduction Rate, which is the key indicator from a population-growth perspective, has declined by less than 30 percent.
       In your article in the New York Times Magazine, you compare the most recent U.N. projections with earlier ones and conclude that “demographers were caught with their projections up,” and that “650 million people were ‘missing’.” Amartya Sen has effectively used the word “missing” to highlight excess female mortality in certain regions of the world: The women were born, but died prematurely. To describe those never born as missing seems incorrect; we should call them “delayed.”
       As demographers, we should admit that our projections are always wrong: sometimes up, sometimes down. We tend to assume that changes in trends are short-term aberrations that will quickly reverse. Both the baby boom and the baby bust were unforeseen. In 1950, making projections to 1960, the U.S. Census Bureau assumed that TFR would fall by 25 percent by 1959-60; in fact, it rose by nearly 20 percent. In 1958, making projections to 1975, the bureau made some fertility assumptions: that TFR would rise by 10 percent, stay constant, fall by 14 percent by 1967, and then stay constant; or fall by 28 percent by 1967, and then stay constant. In fact, it fell by over 50 percent. We do not know what makes fertility go up or down, so we can’t forecast it at all accurately even for 10 years, let alone 50.
       One thing we do know about fertility is that period rates–rates for particular calendar years–tend to exaggerate trends. At the peak of the baby boom, in 1958, TFR reached 3.7; no cohort of women (women born in a particular time period) averaged more than 3.1 children at the end of their reproductive lives. At the trough of the baby bust, in 1976, TFR reached 1.74; no cohort of women has averaged less than 1.97 children at the end of their reproductive lives. Timing of childbearing effects are superimposed on underlying trends to exaggerate the rapidity of change. Current very low fertility in Italy probably reflects both an underlying trend and strong period effects.
       I expect the current very low fertility levels in Europe and Japan to move higher over the next 10 years. However, even if they don’t, the consequences are not as dramatic as you suggest. Take the example of Japan. You state that if the current Japanese TFR of 1.4 is maintained, the Japanese population will be halved by the middle of the next century. The U.N. Low Variant projection for Japan uses a constant TFR of 1.43 to 2050, and the population declines from 125 million in 1995 to 96 million in 2050. Not quite halved by my arithmetic.
       What are the social and economic consequences? You talk of “missing children, and missing grandchildren” and a lonelier human species. If couples are having as many children as they want (the objective of family-planning programs), it is hard to see that they have missing children. Grandparents have less control (though are not without influence), but even here perspective matters: fewer grandchildren per grandparent means a greater share of a grandparent per grandchild. Personally, I don’t think I would find a globe with 9.6 billion people on it very lonely: even with 5.9 billion, it has more traffic jams than I care for. The economy will see winners and losers, as with all change. Housing prices are unlikely to escalate, and diaper manufacturers face a dampened market. On the other hand, retirement communities and manufacturers of prosthetic devices are likely to do well.
       And then there is the migration issue. Any “problem” raised by below-replacement fertility in market economies is a distributional problem: too many graybeards. Worldwide, births still outnumber deaths by a substantial margin. Any shortage of labor that may arise (and it certainly hasn’t arisen yet in Europe, with unemployment rates in excess of 10 percent) can be met through immigration. The United States, whether by luck or by judgment, has followed such a policy for decades and is now probably reaping the benefits of it.
       Bottom line: The population explosion is still with us. Population issues in the market economies are different from those in Africa. Policy responses should be different too.
       I’m enjoying our chat already! I don’t get many opportunities to speculate on a grand scale!

Best wishes,