  Joint commitment was argued to "make our social world" (Gilbert, 2014) and to
separate us from other primates. 'Joint' entails that neither of us promises
anything, unless the other promises as well. When we need to coordinate for the
best mutual outcome, any commitment is beneficial. However, when we are tempted
to free-ride (i.e. in social dilemmas), commitment serves no obvious purpose.
We show that a reputation system, which judges action in social dilemmas only
after joint commitment, can prevent free-riding. Keeping commitments builds
trust. We can selectively enter joint commitments with trustworthy individuals
to ensure their cooperation (since they will now be judged). We simply do not
commit to cooperate with those we do not trust, and hence can freely defect
without losing the trust of others. This principle might be the reason for
pointedly public joint commitments, such as marriage. It is especially relevant
to our evolutionary past, in which no mechanisms existed to enforce commitments
reliably and impartially (e.g. via a powerful and accountable government). Much
research from anthropology, philosophy and psychology made the assumption that
past collaborations were mutually beneficial and had little possibilities to
free-ride, for which there is little support. Our evolutionary game theory
approach proves that this assumption is not necessary, because free-riding
could have been dealt with joint commitments and reputation.
