After graduating from school I despaired of finding a job and applied instead for unpaid internships, landing at The Paris Review, a literary magazine in Manhattan. I read through the slush pile, learned how to fact-check and performed light janitorial tasks, like emptying the dishwasher and taking out the trash. It was a fine four months. My parents lived in the city, so I didn’t have to pay rent; the editors were kind, smart people who seemed appropriately embarrassed by the trash situation.

Such experiences are common. Ross Perlin, the author of “Intern Nation,” estimated in 2011 that between one and two million people participate in internships each year in the United States, and that as many as half of internships are unpaid or paid below the minimum wage.

Employers see nothing wrong with soliciting free labor on public forums. This month, a high-level editor at Lean In, the foundation Sheryl Sandberg started to help women “pursue their ambitions,” tried using Facebook to find a “part-time, unpaid” intern “with editorial and social chops” as well as “Web skills.” After an uproar — how could Lean In, of all places, lean on interns? — the foundation president promised to set up a more formal internship program, with compensation.

Unpaid internships are, at best, ethically iffy. A necessary precursor to jobs in certain fields, they act as both a gateway and a barrier to entry. Young people believe they have no choice. Anyone unable to forgo pay risks being shut out.