Natural selection used to be about the survival of the fittest.
Now being fitter or better isn’t a prerequisite for passing your seed onto the next generation.
If anything the opposite is true. The welfare state encourages the laziest and most dependant segment of society to breed like rabbits.
The argument in this paper isn’t limited to the welfare state.
Basic subsistence agriculture seems to have been the main culprit.
A hunter has to think. He who thinks best, survives best.
Historically, once agriculture came in, populations exploded and filled the land. And then population growth levelled off, limited by disease and other natural controls. These new natural controls applied to stupid and intelligent alike. That sort of agriculture is simple enough that sons can follow fathers, going through the same motions without much thought required. One could be just smart enough, and no more.