What is the primary mechanism behind nationalism, sports-team-ism, product-ism etc.? I'm interested in doing research on the subject of nationalism, but I believe that what I'm looking for is more generic than that. I observe that nationalism looks similar to the way that people feel about their sports team or their favorite product or their religion or their race. People proselytize for these things, and people sometimes kill over them (e.g. -- soccer riots). I note also that this kind of devotion can also produce strong negative feelings towards the other teams, countries, etc.

In each case, a person exhibiting this behavior with respect to a country, sports team, or product has at least one of the following characteristics (maybe others?).


Person identifies personally with X.
Person feels a sense of belonging with others who also identify with X.
Person is inclined to convince others that they should also follow/admire/believe X.


Is there an underlying mechanism behind these symptoms or term that describes this behavior? What is the body of study that examines these types of behaviors (causes, definitions, and effects)? If there isn't a particular term for this as a generic subject of study, are there at least some studies that examine the common features of these behaviors?

What I have researched so far: This question helped a little bit in thinking about this issue, but I'm not sure it answers my question. Also, Maslow's hierarchy of needs points to "respect by others" and "belonging" as posibilities, but I don't know much beyond that or where to look.



Update

I'm stuck. I'm just not sure that ingroups and outgroups really characterize the fundamental mechanism behind why people hold some beliefs with evangelistic or violent fervor. That feature of human nature is not dependent on the existence of a group. If I try to convert someone to my way of thinking, I am not doing so because of a group to which I belong. Certainly group behaviors that @PEEJWEEJ mentioned can feed such sentiments, but like I mentioned to @Ana, I'm not convinced that the group itself is the cause. Perhaps I could be persuaded otherwise, but I think there's something else going on here.
The terms you are looking for are ingroups and outgroups. From wikipedia:


  In sociology and social psychology, an ingroup, is a social group to
  which a person psychologically identifies as being a member. By
  contrast, an outgroup is a social group to which an individual does
  not identify. For example, people may find it psychologically
  meaningful to view themselves according to their race, culture, gender
  or religion. It has been found that the psychological membership of
  social groups and categories is associated with a wide variety of
  phenomena.


Googling got me to this book chapter on the topic, but there's plenty more information out there.