Despite several criticisms surrounding the DSM classification in psychiatry, a significant bulk of research on mental conditions still operates according to two core assumptions: a) homogeneity, that is the idea that mental conditions are sufficiently homogeneous to justify generalization; b) additive comorbidity, that is the idea that the coexistence of multiple conditions in the same individual can be interpreted as additive. In this paper we take autism research as a case study to show that, despite a plethora of criticism, psychiatric (...) research often continues to operate in accordance with this model. Then we argue that such a model runs into problems once facts about comorbidity are taken into account. Finally, we offer some suggestions on how to tackle the challenge raised by comorbidity and its impact on heterogeneity. To do so, we explore transdiagnostic stratification accounts and network models to show that combining these approaches can move us in the right direction. (shrink)