Why do population projections keep underestimating
the size at which world population is supposed to level-off, so that maximum population keeps getting revised upwards?
The answer is that demographic projections are typical of the social sciences in regarding all humans everywhere as interchangeable units (after controlling for age and sex differences) - and therefore the population projections have two false built-in assumptions:
1. That all populations in the world are the same in terms of psychology - and will therefore behave the same way given the same environmental stimuli.
This is false with respect to races, and also religions/ no religion.
That human psychology remains constant over time - that the
population-relevant behavioural characteristics are not changing over time - not
undergoing 'secular changes'.
This is also false. Since both are highly heritable general
intelligence is declining while heritable personality is changing due to differential fertility (only slightly modified by differential childhood mortality - which is nowadays very low) - in ways that will reduce economic productivity.
Indeed, I believe (based on historical changes in simple reaction time data) that the rate of change in human psychology is very much faster (about
twice as fast) as most people have so far assumed.
But as well
as applying biologically due to natural selection, secular change in human psychology also applies at a societal level due to cultural selection - for example, secularization and apostasy have been a major reason for the collapse of fertility below replacement level in all developed countries.
Furthermore, the work of Eric Kauffmann shows that those religions, and
especially traditional (anti-modern) denominations within religions, that support larger families are
growing in their differential representation - which amplifies
future population growth when offspring are retained within fertile denominations.
So we can be reasonably confident that the
world population will keep growing until something stops it - famine and
disease probably, backed by lethal violence, and perhaps (in the West)
the novel phenomenon of mass suicide.
Or maybe the ruling elites will escape mass media influence, snap-out-of their Leftist psychosis, embrace real Christianity, and take sensible action to prevent the mega-horrors to come?
Ha! Only kidding...