Three main methodological practices have been employed in psychotherapy science: the empirical-quantitative, empirical-qualitative, and theoretical one. Some scholars have argued for a monopoly of the empirical-quantitative methodological practice, sustained by scientific monism. This systematic review aims at exploring the contribution of each methodological practice to the field. Fifteen journals were searched from 2003 to 2013. A total of 9796 publications and 9915 studies met inclusion criteria and were coded for the methodological practice employed. Empirical-quantitative studies were the most published overall, over time, and in most of the journals considered. This overwhelming prevalence increased when considering the citation rates of the publications. We argue that these results are indicative of a quantitative monopoly, which in turn is due to a lack of critical thinking sustaining scientific monism. We call for disciplinary critical thinking and the consequent scientific pluralism valuing the plurality, diversity, and multiplicity of all the existing methodological practices.