Evolutionary societal changes often prompt a debate. The positions of the two major political parties in the United States on civil rights issues underwent a reversal in the 20th century. The conventional view holds that this shift was a structural break in the 1960s, driven by party elites, while recent studies argue that the change was a more gradual process that began as early as the 1930s, driven by local rank-and-file party members. Motivated by this controversy, this paper develops a nonparametric Bayesian model that incorporates a hidden Markov model into the Dirichlet process mixture model. A distinctive feature of the proposed approach is that it models a process in which multiple latent clusters emerge and diminish as a continuing process so that it uncovers any of steady, sudden, and repeated shifts in analysing longitudinal data. Our model estimates each party's positions on civil rights in each state based on the legislative activities of their Congressional members, identifying cross- and within-party coalitions over time. We find evidence of gradual racial realignment in the 20th century, with two periods of fast changes during the 1948 election and the Civil Rights Movement.

翻译：社会演变常引发学术争议。美国两大政党在民权议题上的立场于20世纪发生倒转。传统观点认为这一转变是1960年代由政党精英推动的结构性断裂，而近期研究主张该变化是始于1930年代、由地方基层党员推动的更渐进过程。基于此争议，本文提出一种非参数贝叶斯模型，将隐马尔可夫模型融入狄利克雷过程混合模型。该方法的显著特点是：将多个潜在类簇的涌现与消退建模为连续过程，从而在分析纵向数据时能识别稳态、突变及重复性转变等多种模式。基于各州国会议员的立法活动数据，本模型估计了各政党在各历史时期的民权立场，并识别出跨党派及党内的动态联盟关系。研究发现20世纪存在渐进的种族立场重组证据，并在1948年大选与民权运动期间出现两次快速转变期。