 It might be noted that what is here called “PD between replicas” is usually called “PD with twins” in the literature. One reason for the present nomenclature is to distinguish these ideas from an experimental literature reporting on PD games played with real (identical or fraternal) twins. (See, for example, Segal and Hershberger.) It turns out that twins are more likely to cooperate in a PD than strangers, but there seems to be no suggestion that the reasoning that leads them to do so follows the controversial arguments presented above.