The Great De­cel­er­a­tion

by Roberto

As prob­a­bly all STC read­ers know al­ready, the end of ex­po­nen­tial growth of the hu­man pop­u­la­tion is near. The dou­bling time for hu­mans to go from 2 to 4 bil­lion was about fifty years. The pop­u­la­tion main­tained pretty much the same dou­bling time, go­ing from 4 to 8 bil­lion. The fu­ture is al­ways un­cer­tain, but one thing is clear to me: the hu­man pop­u­la­tion will not dou­ble again. In that re­gard, pretty much like an E. coli cul­ture in LB when it reaches 8 bil­lion cells per mL, hu­mans are en­ter­ing sta­tion­ary phase. Things are go­ing to be uniquely in­ter­est­ing for hu­mans (and con­se­quently for planet Earth) over the next few cen­turies.

Fig­ure 1. To­tal Fer­til­ity Rate (World­wide). Source. Fron­tispiece adapted from source. 

Most pro­jec­tions of the hu­man pop­u­la­tion fo­cus on the peak num­ber and the year of when that peak will oc­cur. In­ter­est­ingly, these pro­jec­tions keep get­ting re­vised. Just a few years ago (2014), most pro­jec­tions placed the peak at around 12 bil­lion in 2100. Then we heard (2023) that we would peak at 10 bil­lion in 2085. The lat­est I read placed the peak sooner and lower: 9.7 bil­lion in 2064. I rec­om­mend this in­ter­ac­tive tool if you want to vi­su­al­ize var­i­ous pro­jec­tions up to 2100. I be­came very cu­ri­ous as to why the peak num­bers keep get­ting smaller.

To come up with es­ti­mates of fu­ture pop­u­la­tion num­bers, all pro­jec­tions fac­tor in the world­wide to­tal fer­til­ity rate (TFR, the num­ber of chil­dren born per woman). It's in the TFR where we en­counter per­haps the most im­pact­ful hu­man change of the last fifty years. As re­cently as 1970, the world's TFR stood at around 5. The drop since then has been dra­matic; world­wide, the TFR to­day is about 2.2! (Re­call that a TFR of 2.1 is the es­ti­mate of what yields a con­stant-size pop­u­la­tion. It's not 2.0, to ac­count for the fact that not all chil­dren reach re­pro­duc­tive age.)

Fig­ure 1, ob­tained from Sta­tista and then mod­i­fied to go deeper into the num­bers, presents data col­lected from 1950 to 2023. From 2024 to 2100, it's all pro­jec­tions. The solid blue line reaches a TFR of 2.1 in 2050 and con­tin­ues to de­crease, end­ing at 2100 with a TFR of 1.84. Is the TFR likely to come back up any time soon? I will ar­gue, not very likely. Sev­eral coun­tries have had TFRs be­low re­place­ment for decades and have tried in­cen­tives to re­verse the trend. They have not suc­ceeded. If any­thing, TFRs are likely to de­crease faster than the pro­jec­tion. That's why I have marked the graph with a dashed line and a ques­tion mark. Why? To il­lus­trate this, let's look at pro­jected and ac­tual TFRs for a re­gion near and dear to my heart, Latin Amer­ica.

Fig­ure 2. To­tal Fer­til­ity Rate (Latin Amer­ica). Source

Here are the TFR curves from 2010 to 2050 for eight Latin Amer­i­can coun­tries. The thin lines of dif­fer­ent col­ors are the pro­jec­tions made at dif­fer­ent times by dif­fer­ent en­ti­ties in the past, go­ing up to 2050. In bold red are the ac­tual TFR num­bers for each coun­try. Con­trary to ex­pec­ta­tions, these rates are de­creas­ing much faster than es­ti­mates. Much faster! Here's my lo­cal ex­am­ple: the birth rate in Colom­bia is de­creas­ing so fast that, over the last three years, there were 800,000 fewer births than ex­pected from es­ti­mates. That's a huge re­duc­tion given that there were about 400,000 births in 2024!

Why are TFRs de­clin­ing so fast all over the world? There are, of course, many rea­sons; for a good global overview, I rec­om­mend this ar­ti­cle. I quote from it: "There are three ma­jor rea­sons for the rapid de­cline in the global fer­til­ity rate: (1) the em­pow­er­ment of women – in­creased ac­cess to ed­u­ca­tion and in­creased la­bor mar­ket par­tic­i­pa­tion, (2) de­clin­ing rates of child mor­tal­ity, and (3) ris­ing costs of bring­ing up chil­dren, with the de­cline of child la­bor." One key fac­tor that made this drop in TFR pos­si­ble is the var­i­ous tech­no­log­i­cal in­no­va­tions that led to ef­fec­tive and easy-to-use con­tra­cep­tive meth­ods. But em­pow­er­ing women to use them made the drop in TFR pos­si­ble. Many ar­gue that there is a cor­re­la­tion be­tween in­creased gross do­mes­tic prod­uct (GDP) per capita and de­creased TFR. But that is only true at the very low­est GDP lev­els; be­low US$10,000 (see here). No! In­stead, the great­est cor­re­la­tion of de­crease in TFR is in­crease in the num­ber of years of school­ing that women have. Let's hear it for ed­u­ca­tion!

Fig­ure 3. Hu­man Pop­u­la­tion Num­bers. Source

As I noted above, the hu­man pop­u­la­tion is en­ter­ing sta­tion­ary phase, a topic that is cur­rently very much in vogue. How­ever, not too many pre­dic­tions of the pop­u­la­tion dy­nam­ics go be­yond 2100. That might be ex­pected; we are not very good at look­ing much be­yond one or two gen­er­a­tions ahead. That is why I found the mod­el­ling far into the fu­ture pre­sented in this pa­per so in­ter­est­ing (Fig­ure 3). While most peo­ple now re­al­ize that hu­man pop­u­la­tion will peak soon, I think few re­al­ize that, if the TFR is main­tained at just slightly be­low 2.1, say 1.8, the de­crease will be ex­po­nen­tial. Just as the pop­u­la­tion rose dra­mat­i­cally over the last 100 years, it would drop dra­mat­i­cally fast. From the per­spec­tive of world his­tory, the time of high hu­man pop­u­la­tion num­bers would be but a short blip. That is, of course, if the TFR were to re­main be­low 2.1. While it is im­pos­si­ble to pre­dict when the TFR will re­bound, there are al­ready in­di­ca­tions that such a re­bound is not likely to hap­pen quickly. We will soon en­ter "The Great De­cel­er­a­tion."

Let me stop here and state out­right that I view this de­crease in world­wide TFR as the key rea­son to be op­ti­mistic about the fu­ture. An Earth with far fewer hu­mans, achieved by ra­tio­nal TFR re­duc­tion (ver­sus by mu­tual de­struc­tion through the in­san­ity of war), is a bet­ter Earth. Those of us who feel this way are cur­rently in the mi­nor­ity.

Most gov­ern­ments and cor­po­ra­tions see this as a trend that must be re­versed. In­tent on stay­ing the course based on un­sus­tain­able growth economies, they seem un­able to un­der­stand the im­pos­si­bil­ity of hu­man­ity dou­bling every fifty years, the im­pos­si­bil­ity of two tril­lion hu­mans in a mere four hun­dred years (eight dou­blings). They in­sist on the need to in­crease birth rates. In­stead of de­vel­op­ing poli­cies hop­ing to in­crease the TFR, which will likely fail any­way, we should fully em­brace pop­u­la­tion de­clines and new ways of think­ing, such as cir­cu­lar economies. We should not fear the "el­der swell" but rather wel­come it; with in­creased "healthspan" comes the op­por­tu­nity of ag­ing in­di­vid­u­als to re­main as fully ac­tive par­tic­i­pants in so­ci­ety.

The Great De­cel­er­a­tion rep­re­sents humanity's great­est chal­lenge but also its great­est op­por­tu­nity to live dif­fer­ently and build dif­fer­ently. The chal­lenge will soon no longer be ex­plod­ing ur­ban­iza­tion but im­plod­ing ur­ban­iza­tion. We need to take ad­van­tage of the unique op­por­tu­nity of emp­ty­ing cities. To­day, we have the op­por­tu­nity to com­pletely rein­vent how we live.

Which brings me back to the par­al­lels of a bac­te­r­ial cul­ture. Once in sta­tion­ary phase, fit­ter vari­ants, ini­tially as a mi­nor­ity, ex­pand in the pop­u­la­tion and even­tu­ally take over. Their long-term sur­vival de­pends on such evo­lu­tion­ary in­no­va­tions. The par­al­lel for hu­mans need not de­pend on bi­o­log­i­cal evo­lu­tion. Hu­man in­tel­li­gence led to the agri­cul­tural and in­dus­trial rev­o­lu­tions. To avert a chaotic and un­sta­ble fu­ture for hu­man­ity, what is needed is more than just tech­no­log­i­cal in­no­va­tions. These need to be ac­com­pa­nied by a com­plete change in world­view, a change as dra­matic as the agri­cul­tural and in­dus­trial rev­o­lu­tions, but even more so and more ur­gent. The most ur­gent in­no­va­tion to­day is so­cio-cul­tural evo­lu­tion, a fun­da­men­tal mind­set change in how we re­late to each other and to the Earth in the ab­sence of ma­te­r­ial growth. Such a mind­set change may start in a small mi­nor­ity. But it can also spread and grow through ed­u­ca­tion. Let that mind­set change spread world­wide now, while there is still time.

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