I think it is not a just-so story, it largely predicts everything it should and fails to predict everything it shouldn't. Runaway processes require feedback, I think this is the key. Look for the thing that intelligence made harder. That thing is birth and babycare. Intelligence makes it hard, this causes X to be stronger, and X causes more intelligence, that is the feedback process. What could X be? Sexual competition. More: http://lesswrong.com/lw/mcj/open_thread_jun_15_jun_21_2015/chju