THE rIGHT IS THE lEFT:

A new study could shed light: Published in Proceedings of the National Academy of Sciences of the United States of America, it suggests political polarization increases as people retreat into their party identities. But more worryingly, it also suggests there exists a political "tipping point, beyond which extreme polarization becomes irreversible." If warring factions reach that point--even if the whole country were attacked by some foreign power--teamwork would be a lost cause. "Instead of uniting against a common threat," said Michael Macy, a Cornell professor and the study's lead author, "the threat itself becomes yet another polarizing issue."

The researchers conducted their experiment on a predictive model similar to the U.S. Senate, a partisan group. The model was trained to behave like a legislature with 100 members of varying stances on 10 divisive topics like gun control and abortion; over time, their stances shifted based on the push and pull of allies and opponents. In previous research, the model accurately predicted polarization trends in 28 of the past 30 actual U.S. Congresses.

When Macy's team used it to test the boundaries of ideological extremism, they came to a disturbing conclusion. "We found that polarization increases incrementally only up to a point," said Macy. "Above this point, there is a sudden change in the very fabric of the institution, like the change from water to steam when the temperature exceeds the boiling point." The dynamics of the group, the team reported, were similar to what physicists call "hysteresis loops," which trace the magnetization of an object when a magnetic field is applied. In Macy's experiment, the forces were instead "party identity" and "political intolerance"--and when they reached a certain strength, the polarization became irreversible.

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