Humanitarian aid can be seen as a political investment motivated by altruism or by economic benefits for the donor. Uncertainty in the returns to this investment may generate hysteresis effects and inertia in aid allocations. I model the allocation decisions of the three largest humanitarian aid donors: the US government, the UK government and the European Commission, finding evidence that allocations depend on both recipient need and donor economic interest. Some donors exhibit more inertia than others, and some are more influenced by the decisions of other donors. Despite being a relatively small donor, the UK is particularly influential.