Benevolent social behaviours, such as parental care, are predicted to relax selection against deleterious mutations, enabling them to persist. We tested this prediction experimentally using burying beetles, Nicrophorus vespilloides, which make an edible nest for their larvae, whom they nourish and defend. For 20 generations, we allowed replicate experimental burying beetle populations to evolve either with post-hatching care (‘Full Care’ populations) or without it (‘No Care’ populations). Lineages were seeded from these experimental populations and then inbred to expose differences in their mutation load. Outbred lineages served as controls. Half the lineages received post-hatching care, half did not. We found that inbred lineages derived from the Full Care populations had lower breeding success and went extinct more quickly than lineages derived from the No Care populations – but only when offspring received no post-hatching care. We infer that Full Care lineages carried more recessive deleterious mutations. When parents provided care, the developmental environment was sufficiently benign that broods had higher survival, whether the population had a high mutation load or not. We suggest that the increased mutation load caused by parental care increases a population’s dependence upon care. This could explain why care is seldom lost once it has evolved.