Humans have always lived together in societies: organized entities that are kept together and made functioning through formal rules and social codes. Throughout almost the entire history of our species, these societies have included a few tenths, or maybe hundreds, of individuals. Today, they have grown to include millions. This change has required new methods for keeping societies together, and for making all the people who constitute a society feel that they belong to and want to contribute to it although they know very few of its other members.

Different societies have had different degrees of success with this and in different ways. There are several theories about which factors are most conducive to success. Among these, two in particular may be affected by immigration: shared identity and perceived justice.

It is a quite basic psychological insight that shared identity is conducive to cohesion and collaboration. The more we recognize and can identify with in each other, the greater the feeling of solidarity with each other becomes and the more natural it feels that we are a unit that can or should work together for our common good, combining our strengths and sharing our risks. When we speak about identity in this context, we often use the word ethnicity, which approximately denotes a common identity with which a group of people identify. (Ethnicity does not mean biological ancestry, although many of these group identities are to some extent founded on shared biological ancestry.) Closely related to this is the word culture, which we can define loosely as shared elements of worldview, attitudes and behaviors among a group of people. Common cultural elements are often important ingredients in strengthening the identification with an ethnicity.

The perception of justice in a society depends both on formal rules and institutions and on how these are implemented in practice. This is both about the social contract— that is, what society formally offers to its members and what it requires from them in return— being perceived as a decent deal, and about these formal rules being perceived as equal for all, and not undermined in practice, for example, by discrimination or corruption.