Tribalism

Tribalism is the state of being organized by, or advocating for, tribes or tribal lifestyles. Human evolution has primarily occurred in small groups, as opposed to people's cooperation in society as a whole. With a negative connotation and in a political context, tribalism can also mean discriminatory behavior or attitudes, based on loyalty.
Definition.
The word "tribe" can be defined to mean an extended kin group or clan with a common ancestor, or can also be described as a group with shared interests, lifestyles and habits. The proverb "birds of a feather flock together" describes homophily, the human tendency to form friendship networks with people of similar occupations, interests, and habits. Some tribes can be located in geographically proximate areas, like villages or bands, though telecommunications enables groups of people to form digital tribes using tools like social networking websites.
In terms of conformity,
tribalism has been defined as a "subjectivity" or "way of being" social frame in which communities are bound socially beyond immediate birth ties by the dominance of various modalities of face-to-face and object integration. Ontologically, tribalism is oriented around the valences of analogy, genealogy and mythology. That means that customary tribes have their social foundations in some variation of these tribal orientations, while often taking on traditional practices (e.g. Abrahamic religions such as Christianity, Judaism, and Islam), and modern practices, including monetary exchange, mobile communications, and modern education.
Social structure.
The social structure of a tribe can vary greatly from case to case, but the relatively small size of customary tribes makes social life of such tribes usually involve a relatively undifferentiated role structure, with few significant political or economic distinctions between individuals.
A tribe often refers to itself using its own language's word for "people", and refers to other, neighboring tribes with various epithets. For example, the term "Inuit" translates to "people".
Types.
Tribalism implies the possession of a strong cultural or ethnic identity that separates one member of a group from the members of another group. Based on strong relations of proximity and kinship, members of a tribe tend to possess a strong feeling of identity. Objectively, for a customary tribal society to form there needs to be ongoing customary organization, enquiry and exchange. However, intense feelings of common identity can lead people to feel tribally connected.
The distinction between these two definitions for "tribalism", objective and subjective, is an important one because while "tribal societies" have been pushed to the edges of the Western world, "tribalism", by the second definition, is arguably undiminished. A few writers have postulated that the human brain is hard-wired towards tribalism by its evolutionary advantages, but that claim is usually linked to equating original questions of sociality with tribalism.
Concept evolution.
Tribalism has a very adaptive effect in human evolution. Humans are social animals and ill-equipped to live on their own. Tribalism and social bonding help to keep individuals committed to the group, even when personal relations may fray. That keeps individuals from wandering off or joining other groups. It also leads to bullying when a tribal member is unwilling to conform to the politics of the collective.
Some scholars argue that inclusive fitness in humans involves kin selection and kin altruism, in which groups of an extended family with shared genes help others with similar genes, based on their coefficient of relationship (the amount of genes they have in common). Other scholars argue that fictive kinship is common in human organizations, allowing non-kin members to collaborate in groups like fraternities.
Socially, divisions between groups fosters specialized interactions with others, based on association: altruism (positive interactions with unrelated members), kin-selectivity (positive interactions with related members) and violence (negative interactions). Thus, groups with a strong sense of unity and identity can benefit from kin selection behaviour such as common property and shared resources. The tendencies of members to unite against an outside tribe and the ability to act violently and prejudicially against that outside tribe likely boosted the chances of survival in genocidal conflicts.
Modern examples of tribal genocide rarely reflect the defining characteristics of tribes existing prior to the Neolithic Revolution; for example, small population and close-relatedness.
According to a study by Robin Dunbar at the University of Liverpool, social group size is determined by primate brain size. Dunbar's conclusion was that most human brains can really understand only an average of 150 individuals as fully developed, complex people. That is known as Dunbar's number. In contrast, anthropologist H. Russell Bernard and Peter Killworth have done a variety of field studies in the United States that came up with an estimated mean number of ties, 290, roughly double Dunbar's estimate. The Bernard–Killworth median of 231 is lower because of upward straggle in the distribution, but it is still appreciably larger than Dunbar's estimate.
Malcolm Gladwell expanded on this conclusion sociologically in his book, "The Tipping Point", where members of one of his types, "Connectors", were successful by their larger-than-average number of close friendships and capacity for maintaining them, which tie together otherwise-unconnected social groups. According to such studies, then, "tribalism" is hard to escape fact of human neurology simply because many human brains are not adapted to working with large populations. Once a person's limit for connection is reached, the human brain resorts to some combination of hierarchical schemes, stereotypes and other simplified models to understand so many people.
Negative outcomes.
Anthropologists engage in ongoing debate on the phenomenon of warfare among tribes. While fighting typically and certainly occurs among horticultural tribes, an open question remains whether such warfare is a typical feature of hunter-gatherer life or is an anomaly found only in certain circumstances, such as scarce resources (as with the Inuit or Arabs) or only among food-producing societies.
Tribes use forms of subsistence such as horticulture and foraging that cannot yield the same number of absolute calories as agriculture. That limits tribal populations significantly, especially when compared to agricultural populations. Jesse Mathis writes in "War Before Civilization" that examples exist with low percentage rates of casualties in tribal battle, and some tribal battles were much more lethal as a percentage of population than, for example, the Battle of Gettysburg. He concludes that no evidence consistently indicates that primitive battles are proportionately less lethal than civilized ones.
The realistic conflict theory is a model of intergroup conflict, arguing that in a real or perceived zero-sum system, conflicts arise over shared interests for finite resources. The 1954 Robbers Cave Experiment involved researchers putting 12-year old boys into groups, where they formed their own ingroups, before then developing hostility and negativity towards the other group during simulated conflict over finite resources in a zero-sum game.