In group decision making problems, it is almost impossible to have a homogeneous group of decision makers whose experiences, attitudes, knowledge are the same or similar. Therefore, it is required to determine the weights of decision makers to reflect their relative importance or contribution to the problem. Decision maker weights show the importance or reliability of decision makers in solving the particular problem. The studies on determining the weights of the decision makers are limited. Besides, there is no comprehensive literature review or survey related to the determination of decision makers' weight among the limited numbers of studies. Therefore, in this study, the literature on deriving decision makers' weights is reviewed to present the state-of-the-art in the group decision making environment. Subsequently, a new classification system is proposed. Objective methods for deriving decision makers' weights are classified into five categories: Similarity-based approaches, index-based approaches, clustering-based approaches, integrated approaches, and other approaches. The literature review and analysis of the studies are conducted based on these categories; moreover, challenges and potential research directions are identified. According to the analysis of fifty-five papers, the interest in the topic increases dramatically after 2011. The highest percentage of the studies fell into the similarity-based approaches.