Solver participation plays a critical role in the sustained development of knowledge-intensive crowdsourcing (KI-C) systems. Extant theory has highlighted numerous factors that influence solvers' participation behaviors in KI-C. However, a structured investigation and integration of significant influential factors is still lacking. This study consolidated the state of academic research on factors that affect solver behaviors in KI-C. Based on a systematic review of the literature published from 2006 to 2021, this study identified five major solver behaviors in KI-C. Subsequently, eight solver motives and seventeen factors under four categories, i.e., task attributes, solver characteristics, requester behaviors, and platform designs, were identified to affect each of the solver behaviors. Moreover, the roles of solver motives and the identified factors in affecting solver behaviors were demonstrated as well. We also suggested a number of areas meriting future research in this study.