[Kuncher]

tl;dr: We look for reasons to trust others since humans are both social creatures with group identities as well as egotistical creatures who love to boost their own ego. As a result, we look for ways to boost our ego via group identity subconsciously (sometimes consciously as well). The occurrence of trust based on arbitrary features such as lineage, religion, etc, is simply an expression of us boosting our ego via group identity.

And now onto the actual post...

Ultimately, I think the relationship between trust and group identity is a question of in-groups and out-groups.

The classifications of groups you pointed out for the basis of trust are just that: groups. As such, we can say that all non-reclusive humans belong to at least one group. Correct? That will give that individual distinction between those who he knows and feels affinity with based on group membership, and those he does not know and feel affinity towards. Naturally, he begins to view himself as a member as that group which will solidify into definite groups in the individual's mind. He then begins to view the group as a faucet of himself.

For a real world example, take the goths. If a person hangs out with a goth clique long enough, he will begin to associate himself with goth culture. Eventually, if the interaction with the goth clique continues, he will begin to take on goth values, clothing styles, thought patterns, etc. He himself might not say the words, "I am a goth," but he will think it on an unconscious level, that he is apart of the goth group, and thus view the group as a representation of himself.

Anyways, back to the point...

Herein lines the secret of in-groups and out-groups. Our hypothetical man has created a group identity for himself. For the purpose of this post, it is the goth group. For this next part, we need to agree that man is an egotistical creature. He enjoys that which boosts his view of himself, and that which increases his view of himself relative to others. In other words, he likes personal success (winning contests, compliments, praise, getting raises, etc) as well as seeing others fail in areas our hypothetical person prides himself in.

If we keep this quirk of human nature as well as our understanding of how humans see the groups they belong to as a faucet of himself, we begin to understand where this blind trust from group membership comes from. Because man is egotistical, he wants to see the best in himself. Because he is apart of a group and sees the group as a faucet of himself, he wants to see the best from his group because he will be seeing the best in himself by proxy, whether consciously or subconsciously.

From here, a known phenomena known as "confirmation bias" takes over to reinforce these mechanisms in play. Confirmation bias is just the tendency for people to look for situations which confirm their views of the world. Subconscious cherry picking of facts, you can say.

As a result of all of these mechanisms, our individual begins to trust those in his in-group more than those in any out-group, even if there is no statistical basis which says that his group is, in fact, more trustworthy than anyone outside of his group.

If you'd like to read more on in-groups and out-groups, I'd highly recommend digging around. The concept of in- and out- groups is of importance to sociology and psychology, so there's been a lot written on the subject as well as related subjects.