This study investigates the effect of social rewards and sanctions on cooperation by examining a voluntary contribution mechanism used in conjunction with ex post communication. The experiment consists of two stages. In the first stage, paired subjects play a standard public-good game; in the second, each of them evaluates his or her partner’s contribution in the first stage and sends a free-form written message to him or her. The experimental results show that the mere presence of an opportunity to praise or blame is not enough to promote cooperation. However, once subjects actually experience being blamed, they contribute significantly more in the following round. When disapproval is expressed in the subject’s own words, unlike in previous studies, social sanctions were found to have a stronger impact on cooperation. The experience of being praised, in contrast, does not have such an effect. These results suggest that subjects may be more sensitive to the disutility of being blamed than to the utility of being praised.