It's an interesting metric, you are mis understanding it.
As used it is an indicator of physical health,
but we both agree it is not useful in this case as western atheists generally choose their own fertility rate rather than let nature take its course.
A neutral 3rd party observer might have difficulty distinguishing between the voluntary adoption of an 'intellectual structure' whose adoption drove total fertility to zero and a spreadable physical ailment that did the same.
Voluntarily choosing sub-replacement fertility is a rational choice when resources are insufficient to properly raise a additional child. It is also a rational choice if one foresees such a shortage in the future and acts preemptively with a discrete plan for ones descendants to return to at least replacement level fertility. Finally it is a rational choice perhaps a heroic one when an individual is a known carrier of severe genetic disease and chooses not to pass that to the next generation.
Absent these conditions voluntarily choosing sub-replacement fertility is not rational as it is not synonymous with sustained existence.
So you're claiming fertility rate is also a sign of psychological incompetence? Only within a limited framework.
You seem to be basing the desired fertility to be based on survival. We are intellectual beings and survival is not be our raison d'etre.
If you want to be an animal sure.
We in the Western World live in a time of plenty, there is no threat to our sustained existence, only to our personal set of genes.
One reason you might consider it a necessary in this time of plenty, is if you think your particular cult/tribe needs to outbreed another cult/tribe. (as was more important in the stories of the Old Testament)