The invention relates to a system and method of automatically distinguishing between computers and human based on responses to enhanced Completely Automated Public Turing test to tell Computers and Humans Apart (“e-captcha”) challenges that do not merely challenge the user to recognize skewed or stylized text. A given e-captcha challenge may be specific to a particular knowledge domain. Accordingly, e-captchas may be used not only to distinguish between computers and humans, but also determine whether a respondent has demonstrated knowledge in the particular knowledge domain. For instance, participants in crowd-sourced tasks, in which unmanaged crowds are asked to perform tasks, may be screened using an e-captcha challenge. This not only validates that a participant is a human (and not a bot, for example, attempting to game the crowd-source task), but also screens the participant based on whether they can successfully respond to the e-captcha challenge.