This tendency seems to extend beyond the family, the clan, the tribe, the gang, the team, or the nation, even into the relatively abstract realm of bogus pseudo-science and role playing games.
Let me explain. I have no scientific evidence for this - I'm just a humble lawyer - but it seems relatively clear that one of the reasons why astrology is so popular is that it gives you a sense of belonging to a certain group with certain characteristics, which become desirable and superior to the characteristics of other groups simply by dint of the fact that you have them. So, being a Leo, I read in wikipedia that Leos are:
[A] natural star and leader. They are outgoing, warm-hearted, and wish to excel in all they do. Leo personalities are known to love those they are close too and they wish to protect and defend all those that need it. Leos enjoy flattery and attention from others, they are also sensitive to any form of criticism. People born under the Leo star are invariably materialistic they set a great value to material boons and strive to financial success. Leo individuals need love and recognization from others to boost their ego and they demand it like something for granted. They are known to have plenty of worshippers but also many hidden enemies.
And I immediately begin to kid myself that these are the best characteristics that any person could have, much better than those of those stinky Taurans, Sagittarians and Pisceans, not to mention those hideous fucking Virgos, and I lie back satisfied with the world and my place in it, as a proud, outgoing, warm-hearted Leo. Never mind that I don't actually know any other Leos; I still feel as if I am part of this supranational, supracultural in-group, and that we are the best of the star signs.
In the same way, people who play role playing games like to organise themselves into abstract groupings. I was reminded about this when reading this thread on rpg.net recently. It's about Werewolf: The Apocalypse, a ridiculous game in which all of the world's werewolves belonged to one of a number of 'tribes', each with its own unique special talents, power levels, characteristics, and abilities. Just like with astrology, the notion that you can divide all the world's population into a dozen or so groups with similar characteristics is utter nonsense, yet people (including myself) love this element of the game: we like thinking of ourselves as part of the in-group, with special characteristics better than the others. And again, just like with astrology, it doesn't matter that you do not know anybody else whose werewolf is a Bone Gnawer; still it gives you a bogus sense of belonging to a group of a kind, which is known for being highly adaptable, tough, intelligent and resourceful, and this gives you warm fuzzy feelings of a most basic and instinctive kind. For the short time in which you inhabit this character, you are thinking to yourself, Bone Gnawers are just plain better than the other tribes - although undoubtedly it happens on a mostly subconscious level.
Of course, the difference between astrology and Werewolf: The Apocalypse is that when your Bone Gnawer character dies you can choose to be a werewolf of another tribe (whereupon you'll likely start to think that, actually, this new character is a bit better and his tribe is kewler, because you're a human being and that's what you do), whereas you can't choose a new star sign: you'll be a Leo for life. Not a bad thing, because Leos are the greatest and if you don't agree with me it's probably because you're some idiotic Cancerian or Fire Horse.