Kevin Hurley and I have been debating the ethics of having children over at his blog. I am trying to make the argument that having children benefits existing people (at the margin). The point I have been trying to make is basically that ideas are the real scarce resource of the modern age, and more children means more ideas.
On a somewhat related note, I used Wikipedia's timeline of historic inventions to tally up a rude measure of historic inventions per year. I broke time into segments the same way as Wikipedia and averaged. The x-axis is the year, from 10,000 BCE to the present, and the y-axis is historic inventions per year on a log scale. To give an idea of the magnitudes involved, there were about 1,500 times more historic inventions per year in the 1950's than there were in the 9th century BCE (the lowest and highest points on the graph).