Altruistic punishment increases cooperation in public goods games, but the mechanisms by which it exercises its impact on behavior are yet to be studied in detail. We show experimentally that in the presence of sanctioning, subjects adjust their behavior so that to avoid the freerider’s status. They do that in the STANDARD sanctions treatment, where they directly experience any sanctions assigned to them, but also in the SECRET sanctions treatment, where no information on sanctions received is available until the end of the experiment. We observe no such freeriding avoidance in the treatment without sanctioning. We conclude that the mere knowledge that sanctions might be assigned increases cooperation: subjects expect that altruistic sanctioning takes place against freeriders. This supports the hypothesis that altruistic sanctioning is a social norm, and our subjects shared beliefs in altruistic punishment. Moreover, these expectations are correct as we observe similar pattern and extent of sanctioning in treatments with STANDARD and with SECRET sanctions. (With Yukihiko Funaki and Ai Takeuchi.)