OAKLAND, Calif.—On every NBA court, about 24 feet from the basket, there is a thin stripe of colored paint. The flat-sided semicircle it forms is the boundary between shots that count for two points and shots that are worth three.

When the NBA added the lines in 1979, the players weren’t sure what to think. They sniffed and pawed at them like cats with a new toy. Only 3% of the shots they put up that season were 3-pointers.

Over...