What link between urban density and social ills?

I was browsing a major sustainability textbook today:

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When I see this on p. 31:

In support, a single citation from a 1983(!) article, which I couldn’t read because it’s in German.

I’m not even sure how one would get around understanding the link between density itself (and not social ills correlated with density) and birth rates, mortality, psychological distress etc. It seems like an absolute methodological nightmare, starting with data collection (you’d have to include lots of cities from lots of countries, and you’d have to have good quality data on dozens of variables on each of them, going back many years).

Apart from this, what would be the mechanism? And how would one explain away high-fertility, high-density slums? There are definitely quite a few of those throughout Africa and the Middle East.

Strange.