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 probably results in a lower violence rate. Even mean people (also sub-optimal) will get dragged into niceness over time since nice/mean probably changes dynamically.
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 probably results in a lower violence rate. Even mean people (also sub-optimal) will get dragged into niceness over time since nice/mean probably changes dynamically.