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New research from Shanto Iyengar, a political scientist at Stanford University, suggests that politics is the single most divisive force in American society. While differences like race and religion are to some extent constrained by norms against discrimination, Iyengar says, "there are no corresponding pressures to temper disapproval of political opponents."

"In fact, political hostility toward the opposition is acceptable, even appropriate," write Iyengar and his research partner, Sean J. Westwood of Princeton University. "Partisans therefore feel free to express animus and engage in discriminatory behavior toward opposing partisans."

The disintegration of society this contention produces shows up in some pretty concrete ways: Neighborhoods are increasingly politically homogenous, and bipartisan marriages like that of James Carville and Mary Matelin are rare. And if you're applying for college scholarships and putting anything political on your resume, be warned: Scholarship reviewers awarded students who matched their party affiliation "some 80 percent of the time — even if academic credentials were weaker."