But as a growing body of research suggests that youth tackle football is harmful to children’s brains, not everyone is cheering.
“Why bring girls into it? We should be taking the boys out of it,” said Dr. Robert Stern, director of clinical research for Boston University’s C.T.E. Center, which studies chronic traumatic encephalopathy, the degenerative brain disease caused by repeated blows to the head. “It doesn’t make sense to expose our children to repetitive head impacts during periods of incredible maturation of the most important organ in our body, the brain.”
The number of girls playing tackle football is still low compared to boys — of the 225,000 athletes in Pop Warner youth football programs, for example, just 1,100 are girls. According to the Sports and Fitness Industry Association, of the 5.5 million Americans who report playing tackle football, 596,000 — or 10.9 percent — are female.
It is notable that more girls want to play even as annual survey by the National Association of State High School Federations reported that participation in high school football went down 3.5 percent over the past five years.
Valerie Palmer-Mehta, a professor of communication at Oakland University whose work focuses on women and rhetoric, said the change is evidence of larger cultural shifts.
“We can thank a constellation of cultural forces for women’s involvement in football today, from Title IX to the women’s movement, to strong female athletes who have persisted in pursuing their athletic dreams despite a lack of broader cultural support,” Dr. Palmer-Mehta said.
Team sports like football provide well established social, physical and psychological benefits. But a Boston University study released last year found that kids who played tackle football before age 12 may be at higher risk for emotional and behavioral problems later in life. Another study took MRIs of the brains of kids before and after a single season of tackle football, removing from the study anyone who had a diagnosable concussion. Those researchers found that there was a change in the brain’s white matter after just one season of play. And a study published in January in the journal Brain found the kind of changes typical of C.T.E. in the brains of four teenage athletes who had died after impact injuries.
Dr. Stern said that it is important to understand that the real danger for C.T.E. is not necessarily concussions, but subconcussive impacts. That is, repeated hitting is damaging — even if it doesn’t cause a concussion.
Crystal Sacco, league president and co-founder of Utah Girls Tackle Football, which started in 2015 and is expecting about 400 girls this season, said she doesn’t hear many concerns from parents about their daughters playing tackle football.
“I think they feel safe because they’re playing against other girls,” she said. And Dr. Stern noted that it is not possible to say whether the research — which looked only at boys — can be generalized to include girls because in an all-girls league, the force of the hits could be different.
But several studies have found that in sports with comparable rules between girls and boys, the rates of concussion are actually higher in women. Not only that, a 2012 statement from the American Medical Society for Sports Medicine highlighted research showing that female athletes sustain more concussions than their male counterparts, report more severe symptoms and have a longer duration of recovery than men.
“When it comes to female athletes’ participation — regardless of age — we’re lacking in truly understanding their experience around head injury,” said Donna Duffy, co-director of the Female Brain Project, a research team at the University of North Carolina Greensboro studying head injuries in female athletes. “We’re on the cusp of this; there’s a growing body of literature suggesting that biological sex hormones may be impacted or disrupted when a head injury is sustained.”
But while Dr. Duffy cautions more research is needed, she agrees with other researchers that prepubescent kids should avoid playing tackle football.
Parents whose daughters want to play football may feel they have a difficult decision to make. Several programs suggest flag football as a healthier option for both boys and girls, because they learn the strategy of the game and develop agility skills without risking the injuries of tackling.
Last month, the Concussion Legacy Foundation, composed of doctors and former N.F.L. players, recommended that no children play tackle football before age 14. They’ve created the Flag Football Under 14 program to encourage kids who want to play football to start with flag football until they’re older, and Chris Nowinski, the foundation’s co-founder and chief executive, says those guidelines apply to both boys and girls.
The N.F.L. has also been promoting its flag football program, which partners with U.S.A. Football to allow kids who play flag football to sport the uniform and logo of an N.F.L. team. The N.F.L. Flag program is open to both girls and boys, and according to Mr. Mumtaz, participation increased 45 percent in the past five years to more than 409,000 in 2017.
Girls varsity flag football has been sanctioned as a high school sport in five states. Jen Welter, the first female coach in the N.F.L., founded Grrridiron Girls, a flag football camp program designed to allow girls of all experience levels to participate. Her most recent camp in Boston had 90 participants.
Dr. Welter suggested that if a girl is interested in playing football, rather than shutting down the conversation entirely, parents could discuss what drives her desire to play and consider ways to play safely.
“Flag is one of those ways,” she said, to allow them “to learn about the game and to develop their skills.”
An earlier version of this article gave the wrong date for the N.F.L. Women’s Summit. It was Friday, not Sunday.