Gothenburg, Sweden

What is socialism? Some of its advocates have trouble defining the ideology. “Being a socialist means different things to different people,” a leftist podcast host told actress Cynthia Nixon last year. “For some, it is a Nordic-style welfare state. For others, it is the liquidation of the capitalist class and the democratization of the means of production.” The host asked a flustered Ms. Nixon, then a candidate for New York governor, where she came down.

“Being...