Asked in the January/February issue of the Stanford alumni magazine, "What longtime belief have you changed your mind about?", Professor Paul Ehrlich answers:

In the 1960s, I believed that lowering human birthrates would be teh most difficult task facing those who wished to acheive a sustainable society--because having fewer children was basically going "against biology." In fact, lowering birthrates has proven easier than I expected, and substantial (but not enough) progress has been made since then. Reducing overconsumption has proven very much more difficult.