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Research suggests that people seek romantic partnerships, close family involvements, friendships, and coworker relationships with those whom they see as similar to themselves (Miller, Hefner, & Scott, 2007). This is known as the birds-of-a-feather effect (from the saying “Birds of a feather flock together”). Say that you have two sisters. Both of them have lived with you since birth (proximity), and both have many attributes that you appreciate (resources). But one sister has interests and personality traits very similar to yours. The birds-of-a-feather effect suggests that you’ll feel closer to this sister than you will to your other sister.

At the same time, differences in surface-level tastes and preferences, such as foods, music, and movies, won’t hurt your relationship as long as you and the other person are similar in other, more important ways (Neimeyer & Mitchell, 1988). For example, Steve loves the band Radiohead; he even has a RADIOHD vanity plate on his car. His close friend Mac doesn’t like the band. However, they have other musical tastes in common (John Coltrane, Dave Brubeck, Miles Davis), and—more important—they have very similar personalities, senses of humor, and political views; thus, their friendship endures.