  Traditional Indigenous marriage rules have been studied extensively since the
mid 1800s. Despite this, they have historically been cast aside as having very
little utility. This is, in large part, due to a focus on trying to understand
broad-stroke marriage restrictions or how they may evolve. Here, taking the
Gamilaraay system as a case study, we instead ask how relatedness may be
distributed under such a system. We show, remarkably, that this system
dynamically trades off kin avoidance to minimise incidence of recessive
diseases against expected levels of cooperation, as understood formally through
Hamilton's rule.
