Well, to start off, the reason I picked 50% is the belief that most people, had they not been born, would have been replaced by someone else. Slightly <50% of the world could expect, on average, to be replaced by someone better. (specifically, 0.5*[the proportion that would be replaced])
The reason I chose >50% is that I believe that the population at present is larger than the optimum population. I'm not sure by how much, but I suspect the optimum for our current technology level would be less than 1/3rd of our current population.
Would you like me to explain why I believe the population is above optimum?
I would. Is it just the number, or is it the composition, or maybe the geographical distribution?
To break up the awkward silence at the start of a recent Overcoming Bias meetup, I asked everyone present to tell their rationalist origin story - a key event or fact that played a role in their first beginning to aspire to rationality. This worked surprisingly well (and I would recommend it for future meetups).
I think I've already told enough of my own origin story on Overcoming Bias: how I was digging in my parents' yard as a kid and found a tarnished silver amulet inscribed with Bayes's Theorem, and how I wore it to bed that night and dreamed of a woman in white, holding an ancient leather-bound book called Judgment Under Uncertainty: Heuristics and Biases (eds. D. Kahneman, P. Slovic, and A. Tversky, 1982)... but there's no need to go into that again.
So, seriously... how did you originally go down that road?
Added: For some odd reason, many of the commenters here seem to have had a single experience in common - namely, at some point, encountering Overcoming Bias... But I'm especially interested in what it takes to get the transition started - crossing the first divide. This would be very valuable knowledge if it can be generalized. If that did happen at OB, please try to specify what was the crucial "Aha!" insight (down to the specific post if possible).