Some fraction of the population is naturally poly, some naturally mono, some can go either way depending on circumstances. In the general population many naturally poly people are 'conformed' into being mono the same way they might be conformed into being religious. Thus 'people who want to be poly can be' would reasonably be expected to correlate with elements of the Correct Contrarian Cluster, and you would expect to find more polyamorous atheists or (he predicted more boldly) polyamorous endorsers of no-collapse quantum mechanics than in the general population, even outside LW. There are also specifically cognitive-rationality skills like 'resist Asch's conformity' and 'be Munchkin', and community effects like 'Be around people who will listen with interest to long chains of reasoning instead of immediately shunning you.'