Assuming individuals rationally decide whether to participate or not to participate in lab experiments, we hypothesize several non-representative biases in the characteristics of lab participants. We test the hypotheses by first collecting survey and experimental data on a typical recruitment population and then inviting them to participate in a lab experiment. The results indicate that lab participants are not representative of the target population on almost all the hypothesized characteristics, including having lower income, working fewer hours, volunteering more often, and exhibiting behaviors correlated with interest in experiments and economics. We discuss the implications and various methods for addressing non-representative biases.