The Web form is the primary mechanism to collect personal data from individuals on the Web. Privacy concerns, time spent, and typing effort act as a major deterrent to completing Web forms. Yet consumers regularly provide more data than required. In a field experiment, we recruited 1500 Web users to complete a form asking for ten items of identity and profile information of varying levels of sensitivity. We manipulated the number of mandatory fields (none vs. two) and the compensation for participation ($0.25 vs. $0.50) to quantify the extent of over-disclosure, the motives behind it, and the resulting costs and privacy invasion. We benchmarked the efficiency of compulsion and incentives in soliciting data against voluntary disclosure alone. We observed a high prevalence of deliberate and unpaid over-disclosure of data. Participants regularly completed more form fields than required, or provided more details than requested. Through careful experimental design, we verified that participants understood that additional data disclosure was voluntary, and the information provided was considered sensitive. In our experiment, we found that making some fields mandatory jeopardised voluntary disclosure for the remaining optional fields. Conversely, monetary incentives for disclosing those same fields yielded positive spillover by increasing revelation ratios for other optional fields. We discuss the implications for commercial Website operators, regulators, privacy-enhancing browser standards, and further experimental research in privacy economics.