Wikis have attracted attention as a powerful technological platform on which to harness the potential benefits of collective knowledge. Current literature identifies different behavioral factors that modulate the interaction between contributors and wikis. Some inhibit growth while others enhance it. However, while these individual factors have been identified in the literature, their collective effects have not yet been identified. In this paper, we use the system dynamics methodology, and a survey of Wikipedia users, to propose a holistic model of the interaction among different factors and their collective impact on Wikipedia growth. The model is simulated to examine its ability to replicate observed growth patterns of Wikipedia metrics. Results indicate that the model is a reasonable starting point for understanding observed Wiki growth patterns. To the best of our knowledge, this is the first attempt in the literature to synthesize a holistic model of the forces underlying Wiki growth.