  If group selection is not the correct explanation for how the altruistic behaviours found in nature evolved, then what is? In the 1960s and 1970s a rival theory emerged: kin selection or ‘inclusive fitness’ theory, due originally to Hamilton (1964). This theory, discussed in detail below, apparently showed how altruistic behaviour could evolve without the need for group-level selection, and quickly gained prominence among biologists interested in the evolution of social behaviour; the empirical success of kin selection theory contributed to the demise of the group selection concept. However, the precise relation between kin and group selection is a source of ongoing controversy (see for example the recent exchange in Nature between Nowak, Tarnita and Wilson 2010 and Abbot et. al. 2011). Since the 1990s, proponents of ‘multi-level selection theory’ have resuscitated a form of group-level selection—sometimes called ‘new’ group selection—and shown that it can permit altruism to evolve (cf. Sober and Wilson 1998). But ‘new’ group selection turns out to be mathematically equivalent to kin selection in most if not all cases, as a number of authors have emphasized (Grafen 1984, Frank 1998, West et al. 2007, Lehmann et al. 2007, Marshall 2011); this point was already appreciated by Hamilton (1975). Since the relation between ‘old’ and ‘new’ group selection is itself a point of controversy, this explains why disagreement about the relation between kin and group selection should persist.