These are averages, and researchers emphasize that there are many boys with strong skills and girls with lagging skills. Other factors also contribute -- the gaps in kindergarten readiness by family income and by race are larger than they are for gender.
But over the last two decades, as those gaps have narrowed, the gender gaps have become more consequential. Kindergarten has become significantly more academic because of a national law passed in 2001, with children expected to spend more time sitting still and learning math and reading -- and many boys do not enter with the skills to meet those expectations.
Adding to that, childhood has changed in recent years in ways that could have set back boys further. The isolation of the pandemic delayed young children's development, parents are increasingly stressed, and children are spending more time on screens. These factors affect all children, but they may have been particularly hard on boys, who scientists have shown are more vulnerable to hardship.
Taken together, these changes set boys on a disadvantaged path throughout school. Jayanti Owens, who studies inequality in schools at the Yale School of Management, has found that boys' behavior at ages 4 and 5 predicted the amount of schooling they finished by their mid-20s.
Skills build on themselves, so children who don't master kindergarten phonics or counting could remain behind in future grades. And children who struggle with academics or behavior risk developing negative perceptions of themselves as learners.
"That instigates a cycle where a boy doesn't think, 'I'm smart,' doesn't think, 'I'm good at school,' and if you're told enough times that you're not good at what the teacher is expecting of you, you start to manifest that," Owens said.
Kindergarten readiness data show that many children enter unprepared. But consistently, fewer boys are ready than girls, by about 10 percentage points.
The newest national data come from the federal government's National Survey of Children's Health, which since 2016 has included a survey of parents of children ages 3 to 5. It asks such questions as how many letters children can identify, how long they can focus on a task and how often they lose their temper. In 2022 and 2023 combined, 58 percent of boys and 71 percent of girls were considered on track.
Some states test children at the beginning of kindergarten and generally find that fewer than half of students are ready for kindergarten, and often only about a third of boys.
Test scores from Ohio show how big a role race and family income also play in kindergarten readiness. White girls were most likely to be ready, and Hispanic boys least likely. Just over half of children from economically stable families were prepared, but only a quarter from low-income families.
Perhaps the most comprehensive study, the Early Childhood Longitudinal Study, gathers information from children, parents, and teachers, and follows children over time. It has found that when children start school, cognitive gaps are small -- slightly favoring girls in reading and boys in math -- but that gaps in skills like studiousness, persistence, and self-control are bigger. (Data about the latest cohort of children in the study, which is conducted by the Education Department, have not been released, and a contract for the study was canceled by the Trump administration.)
Researchers at the University of Virginia compared kindergarten in 2010 and 1998. They found that in just over a decade, teachers had allocated much more time to academic subjects and desk work, and less time to art, music, and activities like blocks or dramatic play. The share who said students should learn to read in kindergarten increased to 80 percent from 31 percent.
Amanda Nehring, a kindergarten teacher in Crystal Lake, Ill., said the expectations for kindergartners had become more like what had been asked of first or second graders.
She's had to give up playtime for math and literacy because that's what students are tested on. But some students, often boys, now struggle so much that support staffers pull them out of class for "movement breaks."
In report cards in May, she had to record whether students could write the alphabet with a pencil. Nearly all the girls could, but just a quarter of the boys. Yet they knew their letters, she said, and could build them with Play-Doh or write them with a crayon.
"It's not that they don't get it," Nehring said. "Boys are just as capable, but we don't provide them with the means to show this."
Faced with these pressures, some teachers seem to have less tolerance for boys' behaviors, researchers said. They rate boys below girls -- even when they perform similarly on tests or exhibit the same behaviors, and especially if they are Black or Hispanic.
And since the pandemic, children are entering kindergarten with fewer skills than before. Young boys' development seems to have been particularly affected.
A big change is increased time spent on screens. While it affects all young children, kindergarten teachers said that boys are having more trouble than ever paying attention.
Nehring has always rewarded her class with pajama and movie days -- but in recent years, she said, kindergartners had lost the ability to sit through a movie. "They only want quick little YouTube shorts or TikTok," she said. "You get 10 minutes max. We had to show 'Encanto' in three parts. It's boys and girls, but much more often my boys are the first to go."
Researchers say there are ways to support young boys. Starting them in kindergarten a year later could help close gender gaps in maturity. Male kindergarten teachers could be role models who know what it's like to be a boy in school.
A powerful way to help boys -- and girls too -- is to bring back more play into the early years of school, because it's how young children learn best, researchers and teachers said. Movement, music, time outside, games with peers and activities like puzzles all help children build skills like self-regulation and executive function. Play-based preschool has been shown to shrink gender gaps.
Pat Shaw, the director of a bilingual preschool in Davidson, North Carolina, said she gets pressure from local kindergartens to teach academic topics. Instead, her students make butter when they learn about cows, cover the floor with an art project when they learn about the solar system, and dig for bones in the sandbox when they learn about dinosaurs.