Score contribution per author:
α: calibrated so average coauthorship-adjusted count equals average raw count
We let subjects in a voluntary contribution experiment make non-binding numerical announcements about their "possible" contributions and, in some treatments, send written promises to contribute specific amounts. We find that announcements were responded to both by others' announcements and by real play, for example announcements led to costly punishment when found to be misleading. We also find that adding pre-play announcements to treatments with punishment can increase efficiency by letting cost-free warnings substitute for costly punishment. The threat of punishing false announcements and promises helps reduce false signals, but only when promise statements can be sent is the effect sufficient for achieving greater efficiency.