Massive Open Online Courses (MOOCs) have spread incredibly fast over the past two years, triggering a worldwide debate over the impact of online learning on the democratization of higher education. Given the increasing number of students registering to MOOCs for free, course instructors who want to go beyond automated evaluation have no choice but to use peer assessment. In response to recurrent criticism over the unreliability of peer evaluation, techniques such as calibrated peer review have been developed, but still very little is known about the factors that influence assessors' behavior. Based on two editions of Introduction to Project Management, the first French xMOOC, we explored the impact of learners' background on their engagement in peer assessment and on the grades they gave. We observed that registrants that took part in peer evaluation differed significantly from other participants in regards to time constraints and demographic variables such as geographical origin. As far as grades were concerned, students tended to give higher grades than other registrants, especially when they could get credentials by completing the course.