Final answer:
Xenophobia is the irrational fear or hatred of foreigners, leading to discrimination and calls for immigration restrictions or violence toward outsiders.
Step-by-step explanation:
The factor that has led to calls for immigration restrictions or even violence toward outsiders is xenophobia. Xenophobia is the irrational fear or hatred of people from other countries or cultures. It is often manifested in discriminatory attitudes, which can lead to exclusionary policies and acts of violence against those perceived as foreign. This behavior is rooted in the false belief that outsiders pose a social or economic threat to the nation's way of life. In contrast, the concepts of ethnoreligion, pluralism, and assimilation approach cultural differences in various ways. Ethnoreligion refers to a religion associated with a particular ethnic group, pluralism denotes a societal structure where multiple groups maintain their identity, and assimilation occurs when minority groups conform to the dominant culture.