Population Overshoot?

OK, it's heresy time again. I'm reading the Malmberg Four Phases piece another time, while I'm preparing something to send him. And I stopped in my tracks on this paragraph:

In recent years, students of demography have focused mainly on gross population growth, while the problem of long-term changes in the age structure hasattracted less attention. In consequence, the demographic transition model has been formulated in terms of (gross) death and birth rates. The analysis has focussed on the impact of the transition on the rate of population growth. According to this classic analysis, the rate of population growth is low as long as both birth and death rates are on a high level. When the death rate begins to fall, without any corresponding decline in the birth rate, the population starts to grow. Eventually, as the birth rate falls to the low levels typical of modern, industrialised societies, the rate of population growth is once again reduced.
LINK

OK, so what's the fuss. This is standard and obvious, now isn't it. Of course, probably that's just the point. But let's stop and think about it in terms of what we know about economic processes and financial markets: let's think leads, lags, and overshoot. Then it all becomes incredibly simple. We have a trip switch: the industrial revolution. This provides a shock to what was otherwise a more-or-less homeostatic situation. Population prior to the IR was more or less self regulating, and Malthus was only partially right. Malthus, it will be remembered, made a distinction between positive and preventive checks. The positive checks are the inevitable mortality rates produced by an excess of poulation above what the land could produce at a given level of technology.

Here Malthus was undoubtedy influenced by Godwin and Condorcet (and by, of course, Cantillon - god bless him - and the left to themselves 'they breed like mice' thesis). But of course the theory of positive checks (which of course do exist), like so many theories of its type, was based on a gross oversimplification of human affairs which was never exposed to the critical scrutiny of a 'reality check'.

Well today we are not constrained by this lack of research and evidence. In fact we now know that the 'preventive check' has been much more representative of human conduct than Malthus would ever have dreamed. (A good place to start on this would be the excellent piece On Fertile Ground
from Harvard anthropologist Peter Ellison, or another point of departure could be the work of economic historian Greg Clark). The fact is, whichever way you look at it, most societies incorporate cuturally grounded 'preventative checks'. So along comes the IR and gives things a big whallop. Mortality rates - for the reasons that you choose to offer, living standards, what Mokyr calls recipes of child care, hygiene etc - fall, and fertility drops, but only with an important lag. And this I think is the point, leads and lags in demography work on a different time scale to, for example, those to be found in the garden variety business cycle. Such lags could easily be of a century or more, Meantime what we have is overshoot, while the eco-system (in this case the eco-fertility system is 'struggling' to adapt and adjust).

Of course here there is more. There is an increase in longevity, there is an increase in child rearing time, there is an increase in child bearing age. But maybe, just maybe, the underlying problem is simply one of system overshoot. The problem meanwhile is the overshoot is sending us into a nosedive, and we don't know how to handle it.

By way of illustration of the kinds of problems such processes can produce, here is an extract from the summary of a science article by Austrian demographer Wolfgang Lutz (and colleagues):

The age composition of Europe’s population has been modified by past low birth rates to such a degree that there are fewer children today than adults of reproductive age. This phenomenon – called “negative momentum” by demographers – implies that even if women in the future should have an unexpected fertility increase to two children on average the population would (in the absence of migration and mortality changes) be destined to shrink. This new force toward population shrinking appeared in Europe around the year 2000 and will become more and more powerful the longer fertility stays at its current low levels. And these current low fertility rates are partly a result of continued delays in childbearing.

“Negative momentum has not been experienced on a large scale in world history so far. It is now like sailing against a current running toward population shrinkage and aging,” Lutz said. Two factors are responsible for Europe’s negative population momentum. The first is well known: that women are having fewer than two children, on average. The second factor, whose future impact hasn’t been addressed directly until now, the authors say, is that women’s average age at childbirth has been increasing over time. This so-called “tempo effect” matters because it reduces the number of children born in a given year, boosting the average age at which women have children. The researchers estimated how these two factors might affect Europe’s population in future decades. They found that approximately 40 percent of potential future population declines caused by low fertility were related to the postponing of births.

According to the researchers’ calculations, if women’s average age at childbirth continues to increase for another 10 to 40 years:

There will be a built-in tendency for population size to decline by 55 million to 144
million by 2100;

An additional 500 to 1500 million person-years of workers will be needed to maintain
the support ratio for the elderly population over the rest of the century.
Source: IIAS Press Release
LINK