TWENTY years ago, kids in preschool, kindergarten and even first and second grade spent much of their time playing: building with blocks, drawing or creating imaginary worlds, in their own heads or with classmates. But increasingly, these activities are being abandoned for the teacher-led, didactic instruction typically used in higher grades. In many schools, formal education now starts at age 4 or 5. Without this early start, the thinking goes, kids risk falling behind in crucial subjects such as reading and math, and may never catch up.

The idea seems obvious: Starting sooner means learning more; the early bird catches the worm.

But a growing group of scientists, education researchers and educators say there is little evidence that this approach improves long-term achievement; in fact, it may have the opposite effect, potentially slowing emotional and cognitive development, causing unnecessary stress and perhaps even souring kids’ desire to learn.

One expert I talked to recently, Nancy Carlsson-Paige, a professor emerita of education at Lesley University in Cambridge, Mass., describes this trend as a “profound misunderstanding of how children learn.” She regularly tours schools, and sees younger students floundering to comprehend instruction: “I’ve seen it many, many times in many, many classrooms — kids being told to sit at a table and just copy letters. They don’t know what they’re doing. It’s heartbreaking.”

The stakes in this debate are considerable. As the skeptics of teacher-led early learning see it, that kind of education will fail to produce people who can discover and innovate, and will merely produce people who are likely to be passive consumers of information, followers rather than inventors. Which kind of citizen do we want for the 21st century?