In the early twentieth century, a significant movement emerged in response to the unilineal evolutionary theory, led by U.S. anthropologist Franz Boas. This movement offered an alternative explanation for the existence of societal similarities and differences.

Boas became a vocal critic of unilineal evolutionists, challenging their attempts to classify societies into stages of evolution. He also took issue with their comparative methods and the arbitrary way they structured data to support their evolutionary theories. Boas argued that these nineteenth-century evolutionary models lacked sufficient empirical evidence. He called for an end to what he termed “armchair anthropology,” a practice where scholars relied on secondary data from travelers, traders, and missionaries to develop speculative evolutionary models. Instead, he advocated for rigorous, scientifically grounded fieldwork to collect foundational ethnographic data.

His fieldwork experiences and academic training  led Boas that each society has its own unique historical development. This theory, known as historical particularism, maintains that each society must be understood as a product of its own history.. This viewpoint also gave rise to cultural relativism, the idea that each society should be evaluated based on its own cultural practices and values. Boas argued that societies should not be ranked hierarchically in terms of savagery, barbarity, or civility, and he called for the abandonment of ethnocentric terminology.

Boas’s perspectives became the prevailing theoretical approach in anthropology during the first half of the twentieth century. As a result, anthropologists began to engage in rigorous ethnographic fieldwork to collect empirical evidence. Boas introduced the participant observation method as a fundamental strategy for ethnographic research, allowing researchers to gather valid data to better understand human behavior. He also encouraged his students to enhance their linguistic skills to communicate effectively with the communities they studied.

A notable offshoot of the Boasian approach was the development of culture-and-personality theory within American anthropology. Boas and his students highlighted the vital role of culture in shaping both the conscious and unconscious aspects of individuals within specific regions. They underscored the importance of cultural differences and critiqued earlier racialistic justifications for behaviour and cultural practices.

However, one limitation of the historical particularism approach was its tendency to avoid cross-cultural explanations for human behaviour. Today, many anthropologists recognise that both cross-cultural regularities and cultural variations exist within humanity, marking an evolution in anthropological understanding.