Every atom the functions as part of our biological system craves specific relationships with other atoms. There are systems that are geared to satisfy those hungers and other systems, like our immune system, that are geared to reject and expel any elements the “do not belong to the in-group” so to speak. It is this complex social dynamic between physical elements that forms and maintains our biological structure.
Like the relationship dynamic that happens on a micro scale, as a whole, our biological system has specific hungers that must be met as well. From a certain perspective, our own behavioral and social actions are, in essence, a reflection of that from which we are physically composed. This can be found clearly echoed in scientific disciplines such as sociology. The following is one example:
According to Social Identity Theory, comparison with an outgroup is the main engine by which positive ingroup distinctiveness is formed.
Experiments conducted by Henri Tajfel and others into the so-called Minimal Group Paradigm illustrate this point well.
In the experiments to see what the minimum was to establish an in-group, a number of assumptions, concepts, values or practices were accepted in order to better allow a view of the onset of human group formation and of the appearance of discriminatory behaviours toward out-group.
From the article:
“Intergroup behaviour was analyzed in a situation of “mere categorization” such as where people involved as subjects in this research were told that they were individually “overestimators” or “underestimators” of the number of dots in a display. It was found that even under very flimsy and apparently baseless assigned social categorisation into two distinct, and previously “unheard of” social categories, in-group favoritism and out-group derogation occurred in the distribution, by the research subjects, of “rewards for participation” in the study.”
This is more evidence that shows how hard wired we are to cling to a group and reject anything perceived as out-group. For a more detailed look Click Here