Research participants recruited using online labor markets may feign medical conditions and overreport symptoms: Caveat emptor

•A high proportion of online survey respondents appear to feign illness.•Respondents who appear to feign illness score more poorly on related health measures.•Uploading a photo of medication may help improve online health survey quality.•Relative to those who are genuine about their condition, online respondents who appear to feign clinical pain or diabetes score more poorly on related health measures.•A novel health verification procedure helps to differentiate authentic from inauthentic responders.