The long-standing view has been that the first several years of elementary school should be devoted to basic reading skills. History, science, and the arts can wait. After all, the argument goes, if kids haven’t learned to read—a task that is theoretically accomplished by third grade—how will they be able to gain knowledge about those subjects through their own reading?

The federal No Child Left Behind legislation, enacted in 2001, only intensified the focus on reading. The statute required states to administer annual reading and math tests to students in grades three through eight and once in high school, and attached hefty consequences if schools failed to boost scores. The law that replaced No Child Left Behind—the Every Student Succeeds Act, passed in 2015—has eased the consequences but has hardly weakened the emphasis on testing.

What is tested, some educators say, gets taught—and what isn’t doesn’t. Since 2001, the curriculum in many elementary schools has narrowed to little more than a steady diet of reading and math. And when test scores fail to rise after third grade—as they often do, especially in high-poverty schools—subjects such as history and science may continue to be relegated to the far back burner through middle school.

To some extent, it does make sense to focus on reading skills in the early years. One component of reading is, like math, primarily a set of skills: the part that involves decoding, or making connections between sounds and the letters that represent them.

But educators have also treated the other component of reading—comprehension—as a set of skills, when in fact it depends primarily on what readers already know. In countries that specify the content to be taught at each grade level, standardized tests can test students on what they’ve learned in school. But in the United States, where schools are all teaching different content, test designers give students passages on a variety of topics that may have nothing to do with what they’ve learned in school—life in the Arctic, for example, or the disappearance of Amelia Earhart. The tests then ask questions designed to assess comprehension: What’s the main idea of the passage? What inferences can you make?

On a daily basis, teachers have their students practice skills and strategies such as “finding the main idea” or “making inferences.” And teachers select books that match the given skill rather than because of the text’s content. Rarely do the topics connect: Students might read a book about bridges one day, zebras the next, and clouds the day after that.

Cognitive scientists have known for decades that simply mastering comprehension skills doesn’t ensure that a young student will be able to apply them to whatever texts they’re confronted with on standardized tests and in their studies later in life.