The Bruce Effect

Professor Bill Mollison, who incidentally is also the inventor of permaculture gardening, demonstrates how humans in sealed environments above the 5th floor of a high rise building are effectively sterilized. 

The implications this has for high density populations packed into urban sprawl are far reaching and chilling. 

Here is an article proving this happens in humans:So packing people into cities has even further depopulation effects than what is usually noted.