 Within the philosophy of biology, the units of selection debate (Sober 2000 [2018: ch. 4], 2014; Williams 1966) concerns whether natural selection operates only at the level of the individual or also on groups (where the individual is typically conceived either as the organism or the gene). This debate is especially important for understanding the evolution of altruism (Sober &amp; Wilson 1999). Since altruistic individuals harm their own chances of survival and reproduction, they are less fit, and it is thus unclear how altruism could evolve as a result of natural selection. If, however, groups with more altruists are fitter than groups with fewer, and selection can act on groups, this could potentially explain how altruism could still evolve. Within the units of selection debate, Simpson reversals have played an important role in explaining the possibility of group-level selection.