Obtaining informed consent from individuals to participate in voluntary research studies is widely considered to be an ethical research practice. However, there is considerable debate over how consent should be obtained from subjects. Many researchers argue that active (opt-in) consent is the only type of consent that accurately reflects the true wishes of the subject and is closer to the informed consent ideal than passive (opt-out) consent procedures. Opponents of active consent procedures argue that such procedures harm study participation rates and increase the risk of self-selection bias to a greater extent than passive consent procedures. Empirical evaluations of these claims are rare, given the lack of studies that experimentally assign subjects to different consent procedures and utilize a control group (in which no consent is sought) to facilitate comparison. We report on an experiment that overcomes these issues in an study of consent to transfer contact data from a federal register to a third-party data collector for purposes of carrying out a telephone survey. Specifically, we evaluate the impact of requiring consent on survey participation rates, self-selection bias, and the resulting survey estimates. We find that the passive consent procedure does a better job of minimizing self-selection bias and maximizing the validity of the survey estimates (relative to the control group) compared with the active consent procedure. However, neither procedure is ideal: Both consent procedures increase the total self-selection bias and reduce the sample size. We conclude with a general discussion of the main findings and their practical implications.