"anyone who prefers to act according to the Old Testament motto of 'An eye for an eye, and a tooth for a tooth' has on average less friends – and is clearly less than satisfied with his or her life."

Makes it sounds as though the only options are retaliation in kind or non-retaliation, which complete ignores the purpose of the "motto": to limit revenge. If you really want to see someone without many friends, try someone who would literally kill someone who knocked out his tooth.