I've never been convinced by this one. I do think overpopulation causes issues, but humans adapt to it well.

Look at the differences in human etiquette in New York compared to a small rural town in the South, for example.

In small towns it is rude not to talk to the cashier. In New York it is rude TO talk to the cashier...because there's almost certainly a line.

Manners and etiquette are how we adapt to different social situations, including population density.