The crux of socialism is that central government planners believe they know better than individual people how to allocate resources across an economy.

That is why the newly elected New York City mayor, self-proclaimed democratic socialist Zohran Mamdani, believes the wealthiest people should pay more than half of what they earn to the government. While defending his proposed 54% top marginal income tax rate in a recent Fox News interview, Mr. Mamdani said: “My point is this. If you are making $1 million in New York City, or more than that, you can afford to pay 2% more, and the reasons you can afford to do so is because that money will be used to better your quality of life as well.”

In other words, would-be central planner Mr. Mamdani believes that he can improve overall societal well-being through redistribution and that he can improve individual people’s lives by taking more of their money and spending it better than they could.

That notion flies in the face of Milton Friedman’s commonsense observation, “Nobody spends somebody else’s money as carefully as he spends his own.”

It’s also a particularly audacious claim considering that Mr. Mamdani is talking about some of the most capable and innovative people who have demonstrated their productivity by earning more than $1 million annually.

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Mr. Mamdani can’t know how thousands of wealthy New Yorkers would have spent the extra taxes he wants to take from them.

Suppose one high-income earner would have used that money to expand her business and create 10 new jobs, and another would have given all his newly taxed earnings to a city charity that helps lift people out of cycles of poverty. Or yet another who would have invested in a biotech startup developing gene therapies to extend human lifespans?

Can Mr. Mamdani make every one of them, or even society as a whole, better off through more rent controls, government-subsidized child care and government-run grocery stores?

Mr. Mamdani’s popularity rests on his promises of “free” government handouts for far more people than he plans to tax, but that promise is a pipe dream.

Mr. Mamdani’s vision of a supersized European welfare state would require supersized European taxes. Already, middle-class Europeans pay tax rates comparable to what Mr. Mamdani wants to impose on the millionaires of the Big Apple. They also pay almost twice as much in taxes, $12,000 more per year, than the same middle-class Americans. European families also pay roughly 50% higher taxes than American families.

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Inevitably, attempts to soak the rich also sink the middle class.

That’s particularly true when talking about a single city that constitutes just 2.5% of the U.S. population and less than 0.1% of America’s land mass. Those who are charged with paying for all of Mamdani’s “free” handouts can just leave the city or the state. New York City already leads the nation in out-migration, having lost hundreds of thousands of residents and tens of billions of dollars in tax revenue over the past 12 years as people have fled to states such as Florida, which offer sunnier skies and zero income tax.

Although the freedom to flee New York City is welcome relief for those who foresee the folly of Mr. Mamdani’s promises, it will be of little recompense to those whose dreams of a socialist utopia instead come at the expense of their freedom, opportunity and material well-being.

What Ronald Reagan said four decades ago rings true today: “A government big enough to give you everything you want is big enough to take away everything you’ve got.”