Perhaps nothing has been more responsible for the cratering presidential campaign of Vermont Sen. Bernie Sanders, the self-described “democratic socialist,” than his clumsy attempt last month to defend Cuba’s socialist regime.

Mr. Sanders maintained that he admires the regime’s accomplishments but opposes its authoritarian nature. Yet, the very things he admires are direct outcomes of this authoritarianism.

Defending his remarks, Mr. Sanders told CBS’ “60 Minutes,” “We’re very opposed to the authoritarian nature of Cuba but you know, it’s unfair to simply say everything is bad. You know, when Fidel Castro came into office, you know what he did? He had a massive literacy program. Is that a bad thing?”

Most indicators of well-being tend to improve as a country’s economy grows. As incomes rise, so do literacy, life expectancy and a host of other measures. It doesn’t take a Ph.D. in economics to understand why: As people become wealthier they’re able to afford more of everything, including health care and education.

Cuba is one of the poorest countries in the hemisphere, but it has some of the highest literacy rates and longest life expectancies. Democratic socialists such as Mr. Sanders like to praise these outcomes as accomplishments of the socialist economic system, while distancing themselves from the authoritarian nature of the Cuban government. Yet, the two are inseparable.

Cuba was a relatively wealthy Latin American country prior to the 1959 revolution. The socialist economic system that Castro imposed after the revolution stagnated the economy; as a result, Cuba’s per capita income has fallen farther and farther behind most other Latin American countries ever since. (For a notable exception, see socialist Venezuela.) At the same time, however, literacy and life expectancies have improved relative to the rest of Latin America.

The reason Cuba’s statistics are an anomaly is precisely because it has a socialist economic system. Socialism requires authoritarianism: Abolishing private property and replacing it with state ownership and control. It substitutes government economic planning for competitive markets and individual decision-making. Once state planners have control of the major resources in a country, they can direct those resources to any ends government officials choose with little regard for the desires of the citizens.

The former Soviet Union, for example, had an impressive space program and a world-threatening military. At the same time, the average Soviet citizen faced shortages of consumer goods, from food to toilet tissue —mostly all of poor quality—because socialist planners prioritized “guns” over “butter,” a classic economic trade off.

Similarly, Cuban socialist planners have chosen to funnel resources to education and health care—while buildings crumble, food choices are limited, and there are so few cars (many of them 1950s U.S. classics) that horseback travel is common.

When people are free to choose for themselves they don’t spend all of their money on just one or two things. That’s why economic growth and prosperity typically leads to progress in many aspects of life.

When Mr. Sanders promises “free” education and health care in the United States, he ignores the fact that the costs must be borne by someone. Similarly, when he applauds Cuban health care and literacy he ignores the fact that to pay for this the Cuban people have been denied other goods and services.

Socialist economic systems necessarily concentrate power in the hands of government authorities, who not only dictate what one can buy, and at what price, but can punish dissent through their economic edicts. Cuba has literacy, but no accompanying freedom of speech, freedom of the press, Baskin & Robbins, Walmart or Ford dealerships.

It’s no accident that every socialist country has been ruled by an authoritarian regime. Political freedom cannot survive without a large degree of economic freedom.

Bernie Sanders naively was trying to claim the so-called successes of the Cuban regime as achievements while distancing himself from the authoritarian aspect of it. Unfortunately for him, socialism is inseparable from authoritarianism. Democratic voters apparently understand that.