Okay, I was assuming you had at least a little bit of familiarity with the field. There are a lot of papers which cover this. Starting with a Google search on

Okay, I was assuming you had at least a little bit of familiarity with the field. There are a lot of papers which cover this. Starting with a Google search on population density and fertility rate, without including "causes" in the search, the top academic results I get are: https://www.sciencedirect.com/science/article/pii/S0304387817300135 https://www.sciencedirect.com/science/article/pii/S030438781... https://www.jstor.org/stable/27503981 https://www.jstor.org/stable/27503981 http://paa2004.princeton.edu/papers/41719 http://paa2004.princeton.edu/papers/41719 The population density-fertility rate assumption was not made without evidence.