Core costs, and how to fund them, are important issues for many charities, as donors and other funders seek to get the best ‘bang for their buck’ in a marketplace for social good where quantity and quality are very often hard to determine. We believe that to some extent, funders’ behavioural biases – the facets of human psychology that lead us to make decisions that are not always economically rational – are driving their aversion to core costs. The rise, over the last forty years, of behavioural science and its applied cousin behavioural insights, offer suggestions for how charities might, either individually or collectively, go about overcoming these biases.