He maketh wars to cease unto the end of the earth;he breaketh the bow, and cutteth the spear insunder; he burneth the chariot in the fire.

—Psalm 46: 8–10

WHEN I WAS IN THE THIRD GRADE, MY YOUNGER SISTER broke my favorite toy. I yelled at her, she screamed back, and then, to my surprise, she launched herself at me in a fury, scratching me hard on the arm. My reaction was blind, unthinking; I raked my own nails down her forearm, making furrows that, to my shock, began to ooze blood. I was punished, but nothing cut so deeply as the guilt I’d felt at her pain.

I’ve wondered from time to time what happened in this primitive, instinctual tit for tat, a variant of any playground fight. Some kid pinches you, and you pinch ...

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