This work examines the influence of personality factors on willingness to participate in studies. Participants were recruited either via a market research firm or via a face-to-face interception technique. In addition to completing their required tasks, all 256 participants subsequently completed the Myers-Briggs Type Indicator (MBTI). The type distributions from the market research task and the interception task were compared to that of the normative United State National Representative Sample (US NRS). Personality type distributions from the market research recruited participants and the interception recruited participants were found to be significantly different to the US NRS. Further, all over-represented personality types were either Intuitive-Feeling (NF) or Intuitive-Thinking types (NT) and so shared the common trait of “Intuition” whereas all underrepresented types shared the opposing trait of “Sensing” and were either Sensing-Thinking (SN) or Sensing-Feeling (NF) types. Results suggest that personality factors affect a person’s decision to participate in a study. Importantly, since personality type has not usually been part of selection criteria in past studies it may be that a systematic non-response bias may unknowingly have always existed. The implications of such a bias on the true state of knowledge regarding human behavior are potentially profound.