There's an excellent article on this issue in the current edition of the
IPA review that arrived in my post box this morning.
1) GDP per capita of the world over time - see
http://www.ggdc.net/maddison/
2) Simply measuring human density is a crap way of measuring so-called "overpopulation." Monaco, at 23,660 people per square kilometre, is the worlds highest density country (
source). I doubt that the people of Monaco are deeply aggrieved by this situation.
3) The problem, then, is not an issue of density - it's an issue of living standards. If people can live happily and healthily in the high densities of Monaco, or the low average densities of Australia (2.6people per square kilometre,
Source) it seems that density alone is not very well correlated to happiness/healthiness/etc.
4) If the issue is living standards, then the obvious solution, to me, seems to be to improve living standards. Things like free trade, ending the US/European/Japanese farm subsidies, would all go a long way towards fixing this stuff.
5) Typical European cows receive larger subsidies to live per day than African humans (
source). If you're a European your taxes are currently being used (indirectly, admittedly) to impose poverty on the least developed nations in the world.
6) The Americans, though not nearly as bad as the European poverty makers, aren't far behind. At least George Bush has the courage to propose massively cutting these subsidies (
Source)