Classic, segmented, and neo-assimilation theories delineate immigrants’ assimilation trajectories. Classic assimilation, since the early twentieth century, treats newcomers’ and established groups’ interactions as leading to ethnoracial pluralism within one culture. Segmented-assimilation, since the 1990s, examines non-European immigrants’ experiences, and considers how factors such as ethnic capital, immigration policies, and racial discrimination cause assimilation into upper, middle, and underclasses. Neo-assimilation, since the early 2000s, posits that assimilation compels upward mobility into a diverse mainstream. While the progenitors of each theory have pointed out the other's deficiencies, in this paper we simultaneously compare the three theories in light of the scientific method's criteria of deductive-inductive hypothesizing and falsification. We find that classic theory follows the scientific method and is falsifiable. In comparison, we find that because segmented- and neo-assimilation each depart from the scientific method in three ways they are not readily testable or falsifiable. We discuss the implications for migration and assimilation research.