Running on Empty

Even as earth’s population tops 8 billion, much public discussion lately has centered on declining birthrates around the world. Elon Musk famously called it mankind’s greatest existential threat. One informative treatment of the subject is Empty Planet: The Shock of Global Population Decline by Darrell Bricker and John Ibbitson.

Half the countries in the world have fertility rates lower than 2.1, the rate needed to sustain a constant population over the long run. (The US is currently 1.78, which is a bit higher than most Western countries.) Most of the other half are very close to this replacement rate. Global population is still rising, but only 8 countries in Africa and Asia will account for the majority of the increase between now and 2050: Democratic Republic of the Congo, Egypt, Ethiopia, the Philippines, Nigeria, Pakistan, Tanzania, and India. (India actually has slipped slightly below 2.1 already, but longer lifespans and the burgeoning elderly population will keep India’s population rising for a few decades.) Bricker and Ibbitson contend that the UN projection for global population to rise to 11 billion in this century before leveling off is in error, for it is based on current birthrates and doesn’t account for the ongoing birthrate decline in presently high-fertility regions. The authors instead project a peak population of 9 billion followed by steady decline. Immigration keeps populations rising in most Western countries despite low birthrates. Low-fertility-rate countries that are culturally resistant to immigration however (e.g. China, Japan, Hungary, Russia, Korea, et al.) are already experiencing actual declines. The authors cite the usual list of economic problems that accompany declining numbers: notably, fewer working-age people struggling to support social welfare programs and a much larger generation of old people who live longer than ever.
 
Lower fertility rates are associated around the world with rising education of women: the more years in school, the fewer kids. It is also independently correlated with increasing urbanization. This was as true in the 19^th century as it is today, and already was well noted by demographers at the beginning of the 20^th century when all but a few national populations were still mostly rural. The one historical exception was the immediate postwar period when the birthrate rose despite ongoing urbanization. The Baby Boom has to be regarded as a freak anomaly in an otherwise century-plus long downward trend – an odd ephemeral reaction to the outsize traumas of Depression and World War. By the mid-‘60s, however, the longer-term fertility decline had resumed.
 
While most commentators propose social and economic causes, some analysts wonder if something more fundamentally mammalian is at work. They refer to the famous rodent studies conducted by the wonderfully named Dr. John Bumpass Calhoun of the NIH from the 1940s to the 1970s. Rats and mice notoriously breed profusely in adverse conditions. All major cities battle rat populations to little avail. Calhoun, working first with rats and later with mice because of the latter’s shorter life cycle, decided to see what would happen if he created rodent habitats with abundant food and ideal environmental conditions. How crowded would they get? The results were counterintuitive. Yes, as expected, the population of each habitat soared at first, but then strangely fertility would fall, eventually below replacement rate. Population would peak at well below the enclosure’s carrying capacity at which point it would start to drop: slowly at first but then headlong. A crash never reversed itself once it started. Mortality in the habitats was 100% every time.
 
A typical example was Universe 25: a mouse utopia abounding with tunnels, nests, nesting materials, plentiful food, pleasant temperatures, and no predators. Universe 25 was able to accommodate 3000 mice easily, but it never got there. Calhoun placed a handful of breeding pairs in the enclosure in 1968. The mouse population doubled every 55 days in the “exploit period” reaching 620 on day 315. Fertility then began a long decline though at this point it still exceeded the replacement rate. The mice acted ever more oddly as crowding grew. Mice huddled together in some nests while leaving other nests nearly empty. The females grew more aggressive while the males became either passive or violent. There were bursts of hypersexuality. By day 560 a generation of mice that had grown up amid this weird adult murine behavior showed diminished interest themselves in mating, competing, or raising young. A few took possession of upper nests (mouse penthouses) exclusively for themselves and a handful of their favorites – Calhoun dubbed them the “beautiful ones.” The beautiful ones didn’t reproduce much either. The Universe 25 population peaked at 2,200 on day 920. The fertility rate then slipped below replacement level and the population began to decline. The rate of decline accelerated even when population dropped back below 620. The last baby mouse was born in 1973. The remaining mice grew old and died to the last mouse.
 
In his paper “Death Squared: The Explosive Growth and Demise of a Mouse Population” (1973) published in The Proceedings of the Royal Society of Medicine, Calhoun comments that part of the problem for the rodents (in addition to simple crowding) was precisely the lack of struggle for resources that keeps urban street rats in their brutal environments socially healthy and relentlessly fecund. Calhoun wasn’t shy about suggesting parallels to human societies: “I shall largely speak of mice, but my thoughts are on man.” To the rejoinder that humans, with few exceptions, are neither mice nor rats, he would answer that in many ways we kind-of are. However much we rationalize our behavior as ideology, philosophy, and lifestyle choices, he suggests our actions may be at least as much rooted in biology.
 
If Calhoun was right, growing global affluence is a big factor in declining human fertility. It is hard to see that as a bad thing. In any event, I think Bricker, Ibbitson, and Calhoun are all too pessimistic. To start, we are not yet at the equivalent of day 920 of Universe 25: human population is still rising. Further, I think the socioeconomic challenges of declining numbers are more manageable than the worrywarts imagine. The population of Japan, for example, is dropping by over a half million per year. Nonetheless, Japan remains a pretty nice and well-run place. There are advantages to smaller numbers including a lower strain on resources. When I was born the global population at 2.5 billion was less than a third of what it is today. The US population was 152,000,000: well under half of what it is today. Yet, no one was complaining back then that there weren’t enough people. No one will make that complaint if we return to those lower numbers either – which even according to Bricker and Ibbitson will not happen in this century anyway or probably the next. That is plenty of time to come up with a “solution” if indeed a solution is necessary.
 
An obvious answer is to pay people to have kids. Many countries already do this both directly in cash payments and indirectly via subsidized child care – in some cases the assistance is extraordinarily generous and includes mandated lengthy paid parental leave. These efforts haven’t made a notable difference in fertility anywhere, true enough, but that just means the payments still aren’t high enough. I’m not suggesting they should be made higher at the current time. I’d rather see some negative population growth first. Taxpayers after the year 2100 then can step in if they wish. Meantime, I think “Empty Planet” has rather a nice ring to it – emptier anyway.
 

Metric – Empty