introduction
In humans fairness has been studied extensively using games
played for money1. The paradigmatic example is the Ultimatum Game (UG)2 where
one player (the Proposer) is given an endowment (e.g. £10) and proposes a
division (e.g. keep £6/offer £4) to a second player (the Responder), who then
accepts (both get the proposed split) or rejects (both get nothing) the offer.
In the UG with money humans typically reject low, “unfair”, offers even at cost
to themselves1 Here, we asked if thirsty humans in an UG instead make
self-interest maximising responses to unfair offers with a primary reward of
water, and attempted to maximise our power to induce such self-interested
behaviour.