The world has been conducting an inadvertent socioeconomic experiment on young people, one that would in normal circumstances be prohibited for being cruel and inhuman.

As part of their efforts to control the pandemic, many countries have for parts of the past two years closed schools. Some resorted to this measure more, others less, and they all did it in different ways. It’s as though they had set out to test what happens in the long term to individuals, societies and nations when you deprive some children of education.

It should be obvious what’s cruel and inhuman about this. The study’s subjects, namely the kids, never had a chance to opt in or out. They just found themselves in a given country, school district, household and family — and then lived with the consequences. For many, those will be grave.

Among those now laying the groundwork for future research are Vera Freundl, Clara Stiegler and Larissa Zierow at the Ifo Institute in Munich, an economic think tank. In a new paper, they compare seven representative countries in the European Union: Poland, Germany, France, Spain, Sweden, Austria and the Netherlands.

At one extreme is Sweden, which became famous (or notorious, depending on your point of view) for refraining from harsh coronavirus restrictions and largely kept schools open. France and Spain, even though they had horrendous outbreaks, also tried hard to keep kids in school: They instead restricted the activities of adults more severely. Austria and the Netherlands muddled through, the latter opting more for partial closures — holding classes in shifts, for example. Germany and Poland kept kids at home longest.

In surveys, parents have confirmed that the more time children spent at home, the less time they spent learning.

The quality varied just as much, and that depended on the digital sophistication of countries as they went into the pandemic. In this group, Sweden topped the list, with 80% of school principals in surveys saying that they were immediately ready with an online-learning platform. Germany was the laggard, with only one in three principals claiming the same. Students reported similar differences. German children, in short, lost twice: in the quantity and quality of teaching.

Comparing actual learning is harder. But it’s obvious that children have huge gaps, not just in reading and math but also in social and creative skills.

A Dutch study showed that the closures in the Netherlands in the spring of 2020 led to a learning loss equivalent to 20% of the academic year, or exactly as long as children stayed home. You can extrapolate the loss in the countries that closed schools longer and weren’t as good at teaching online.

Since this pandemic is far from over, let’s stipulate the following: Education is the greatest boon available to individuals and societies alike, so never again squander it. And if you ever need to run an inadvertent socioeconomic experiment again, do it with adults, not children.

Andreas Kluth is a columnist for Bloomberg Opinion. He was previously editor in chief of Handelsblatt Global and a writer for the Economist.