The
conditions that propel humans to make sacrifices for groups of
unrelated, and often unknown, individuals has received considerable
attention across scientific disciplines. Evolutionary explanations for
this type of sacrifice have focused on how men form strategic coalitions
organized around kin networks and reciprocity when faced with out-group
threats. Few studies, however, have analyzed how women respond to
external threats. Using data from one of the largest female paramilitary
organizations in history we show that women who have more brothers,
women whose husbands serve in the military and women without children
are more likely to volunteer. These results provide qualified support
for the hypothesis that women are more likely to sacrifice for their
country when members of their family are at risk. Overall, our analysis
suggests that self-sacrifice and intense bonding with an imagined
community of unknown individuals, such as the nation state, may arise
out of a suite of psychological adaptations designed to facilitate
cooperation among kin (i.e. kin psychology). These results can be
interpreted within the framework of kin selection showing how
individuals come to view unrelated group members as family. They may
also shed light on various theories of group alignment, such as
‘identity fusion’ – whereby individuals align their personal identity
and interests with those of the group – and on our understanding of
evolutionary adaptations that cause women to direct altruism toward
in-groups.