Homophily (i.e., "love of the same") is the tendency of individuals to associate and bond with similar others. The presence of homophily has been discovered in a vast array of network studies. More than 100 studies that have observed homophily in some form or another and they establish that similarity breeds connection.[1] These include age, gender, class, and organizational role. http://en.wikipedia.org/wiki/Homophily

cf Social Capital, Assortative Mating, Tribe

Unfortunately the flip side of this is often Prejudice: Racism, Sexism, Ageism, etc.

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