Nonreproductive workers of many eusocial Hymenoptera 'police' the colony, that is, they attack reproductive sister workers or destroy their eggs (unfertilized; developing into haploid males). Several ultimate causes of policing have been proposed, including (1) an increase in colony productivity, applicable if reproductive workers work less, or (2) an increase in worker-to-male relatedness, applicable if within-colony relatedness is low. To explain the distribution of policing across taxa, the explanatory power of these and other potential ultimate causes should be assessed separately. One of the few species for which this can be done is the leafcutter ant Acromyrmex echinatior. We previously demonstrated that colony productivity incentives (and sex ratio incentives) are minimal here, while relatedness incentives are strong because queens are highly multiply mated. Overcoming technical difficulties peculiar to leafcutter ants, we introduced reproductive versus nonreproductive workers and batches of queen-laid versus worker-laid eggs into experimental colony fragments and observed their fate. Our main finding was that workers policed by selectively destroying worker-laid eggs, but without attacking reproductive workers. We infer that relatedness incentives are the most likely ultimate cause of the evolutionary maintenance of worker-egg policing in A. echinatior. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved