Making a costly apology or inflicting self-punishment after an unintentional transgression can serve as a costly signal of the transgressor's benign intention. In the present research, after an unintentional transgression (i.e., unequal resource allocation between themselves and a partner), participants were provided with an opportunity to send an apology message to their partner (in Experiments 1 and 2) or to privately deduct some amount from their own reward (in Experiments 2 and 3). Across these experiments, approximately half of the participants indicated their willingness to incur some cost to produce these costly signals. In Experiment 1, neither history nor expectation of interaction with a partner altered the frequency of a costly apology. In Experiment 2, despite explicit instructions that their partner would not be informed whether they had inflicted the self-punishment, the frequency of self-punishment was approximately equal to that of a costly apology. These results suggest that the two types of costly signal were not solely directed at the victim. Experiment 3 revealed that these costly signallers endorsed the equality principle more than the non-signallers. This result is consistent with the idea that the two forms of costly signals serve to protect the signaller's reputation as a fair person.