The Evolution of Trust

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During World War I, peace broke out.
 
 It was Christmas 1914 on the Western Front.
 Despite strict orders not to chillax with the enemy, British and German soldiers left their trenches, crossed No Man’s Land,
 and gathered to bury their dead, exchange gifts, and play games.
 
 Meanwhile: it’s 2017, the West has been at peace for decades, and wow, we suck at trust. Surveys show that, over the past forty years, fewer and fewer people say they trust each other. So here’s our puzzle: