There is actually a human algorithm in the cooperate/defect space - which is to distrust anyone who seems to spend a long time making a cooperate decision. Anyone who has a really quick, simple way of deciding to cooperate is OK - anyone who takes a long time is thinking about whether you will defect, or whether they should defect - and you should probably find someone else to trust. This is one of the good things about Tit for tat - it passes this test.