The Most Neglected Economist
Thomas Malthus was one of the first to write on economics, but he is also almost invisible in conventional economic discussions. I am inclined to think that this is because his idea is both simple and uncongenial to both Marxist and most Capitalist theories of the economy. Reduced to its essence, his idea is that humans can reproduce faster than our means of sustenance can increase. Darwin took this idea and made it the core of his theory of evolution.
Neglect of Malthus cannot be explained by lack of confirmatory data. The Chinese adoption of the "one child" law was draconian, but it is probably the most successful economic policy in human history. It took decades, but China went from being a high fertility and desperately poor country to an economic superpower. Many advanced countries achieved the same things by less drastic measures, but the data clearly shows that high fertility destroys economic progress and low fertility promotes it.
Low fertility promotes economic progress by freeing women to work and by decreasing the number of non-producing consumers. Of course in the longer term, it also produces an older and less productive population in the longer term, but experience shows that this is good tradeoff. In the longer term, it produces a shrinking population, but in my opinion this is also a good deal - the human population of Earth is much too large for all of us to live well.