Computational folkloristics, which is rooted in the movement to make folklore studies more scientific, has transformed the way researchers in humanities detect patterns of cultural transmission in large folklore collections. This interdisciplinary study contributes to the literature through its application of Bayesian statistics in analyzing Vietnamese folklore. By breaking down 307 stories in popular Vietnamese folktales and major story collections and categorizing their core messages under the values or anti-values of Confucianism, Buddhism, and Taoism, the study shows how the Bayesian method helps discover an underlying behavioural phenomenon called "cultural additivity." The term, which is inspired by the principle of additivity in probability, adds to the voluminous works on syncretism, creolization and hybridity in its technical dimension. Here, to evaluate how the values and norms of the aforementioned three religions ("tam giáo" 三教) co-exist, interact, and influence Vietnamese society, the study proposes three models of additivity for religious faiths: (a) no additivity, (b) simple additivity, and (c) complex additivity. The empirical results confirm the existence of "cultural additivity" : not only is there an isolation of Buddhism in the folktales, there is also a higher possibility of interaction or addition of Confucian and Taoist values even when these two religions hold different value systems (β {VT.VC} = 0.86). The arbitrary blend of the three religions is an example of the observed phenomenon of Vietnamese people selecting and adding ideas, beliefs, or artefacts-which may sometimes appear contradictory to principles of their existing beliefs-to their culture. The behavioural pattern is omnipresent in the sense that it can also be seen in Vietnamese arts, architecture, or adoption of new ideas and religions, among others. The "cultural additivity" concept, backed by robust statistical analysis, is an attempt to fill in the cultural core pointed out by syncretism and account for the rising complexity of modern societies.