Under a coercion (or negative reciprocity) framework, threats of punishment lead to subordinates refraining from imposing costs on others (self-restraint) or providing help.  Theoretical models predict that self-restraint by subordinate group members can be evolutionarily stable when subordinates are costly to dominants, restraint is credible, dominants can inflict punishment and relatedness is low.  In the cooperatively breeding cichlid, Neolamprologus pulcher, dominance hierarchies are size-structured, and small size differences between adjacently ranked fish are associated with high costs to dominants.  We tested whether subordinate fish would restrain growth when size differences were small by manipulating the size structure of experimentally-created groups of N pulcher.  We assembled groups with small and large size differences between 1st and 2nd ranked males.  Second-ranked males grew more slowly when size differences were small, as predicted.  Subordinates were also more submissive and received more aggression when size differences were small.  Behavioral and hormonal data were inconsistent with slow growth resulting from competition for food, stress or direct dominant aggression.  In a similar experiment, manipulation of size differences among females did not result in changes in the growth of 2nd ranked females.  This was unexpected, as size is generally a more credible cue of reproductive ability for female fish than for males.  These differences with males may result because relatedness among female group members is typically higher than that among males.  Dominant females also may have low power to impose costs on subordinates because of male interference.