By using the verb "to defect" I am assuming you are familiar with the button-pressing tit-for-tat game-theory stuff. AFAIK one simple yet efficient algorithm is "reciprocate what the other player does, but when you both are stuck in mutual non-cooperative loops, offer "forgiveness" by pressing the cooperate button once and see if it is reciprocated and you both can enter a mutually cooperative loop. This "forgiving" press is clearly unearned trust!