Pictures and movies of times prior to WW2 show men wearing hats. What killed the hat as something you would not leave home without? I'm guessing WW2 and not hat head.
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"[J. F.] Kennedy despised how he looked in hats and usually refused to wear them - despite howls of protest from habderdashers whose hat sales took a precipitous downturn in the 1960s" (Barbara Perry). What factors led to the downturn in menswear hat sales in the 1960s?