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“Birds of a feather flock together” and “Opposites attract” are a couple of old sayings that together suggest you can find an old saying to support any prejudice you choose.  The bottom line, of course, is that before you make up your mind about anything that matters, consider the evidence.  Some more evidence (there was really no shortage before) has just emerged (BJ, I liked you from the start SCIENCE vol. 335 no. 6068 February 3, 2012 page 504 reviewing Proc. Natl. Acad. Sci, U.S.A. 109, 68 (2912)) in support of the feather hypothesis. 
They looked at Facebook entries for a number of college students over a period of time, and found that indeed those with similar tastes tended to be friends.  That does not necessarily mean cousins always like each other better than non cousins, but it’s hard to think otherwise.  I mean surely cousins have to resemble each other in most respects more than they resemble random members of the community.  So marrying kin is not just a biological imperative.  It is, so more than a trivial degree, obeying the dictates of the heart.  That, if true, is good news.  One would hate to give up love simply in order to survive. 
Rather more riveting is the observation that friends did not in fact gravitate toward having the same tastes.  Taste, within the parameters of the study, was not contagious.  I do find that a trifle odd.  What in the world is taste except liking something somebody else likes?  But there it is.  By the time they reached college these young folk were confident of their own opinions enough so that even with friends who were different, they kept on their own.  I suppose cupid should take note.  Don’t form a relationship and hope differences are going to go away.  Some wit once remarked, “A man marries a woman thinking she’s not going to change and she does; a woman marries a man thinking he’s going to change and he doesn’t.”  There you go.  An old saying for any season, eh? 
Well courtship is a complex issue, and with my spectacularly bad record at it I suppose I should not talk.  But pulling up evidence should be all right.

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