I didn't read the article but the headline makes me postulate that this is because the optimal individual strategy in Prisoner's Dilemma is tit-for-tat. Respond in kind to every action taken against you. If someone offends, attack, if they offer peace, be peaceful. Nice people will more often offer the olive branch (sub-optimally), so anyone else playing the optimal strategy will offer it back. Over time this probabl…
( * I think it's this: https://sciencehouse.wordpress.com/2012/09/04/a-new-strategy... )