The answer to your question is: Since a few million years. Nevertheless, I understand your point. My point was more along the lines of criticizing 'correlation

The answer to your question is: Since a few million years. Nevertheless, I understand your point. My point was more along the lines of criticizing 'correlation vs. cause' and providing equally weak arguments. What I believe is that there are multiple factors at work here. So higher home prices are probably one reason why people are deciding against children, but most certainly not the only one. And 'deciding against children' doesn't mean they will never have any children, maybe they just wait a few more years before they get the first and in the end there will be one less for some couples.