Football players for Wise and Oxon Hill High Schools in Maryland. (Photo by Jonathan Newton/The Washington Post)

A lot has changed in the past 10 years when it comes to sports concussions. As evidence has grown of the devastating effects of traumatic brain injuries, athletes and parents have become increasingly alarmed and demanded preventive steps. Some of the most tangible results of that concern are new laws passed in all 50 states and the District of Columbia that try in some form to address head injuries among young athletes.

A new study, published Thursday in the American Journal of Public Health, explores how effective those laws have been in reducing recurring concussions. The study found that the new laws have led to a noticeable nationwide decline in repeated concussions among teenagers.

The laws, which vary in strength, were passed between 2009 and 2014. The researchers burrowed into a set of national data tracking the number of concussions and analyzed numbers for different states before and after a law went into effect.

They found that immediately after each law passed, the number of concussions increased, which they attributed to improved reporting of concussions because of greater awareness and concern generated by the laws. But about 2.6 years after each law was passed, the numbers of repeated concussions began to decline, a promising result that seems to indicate the laws are having an effect.

“We were very happy to see that,” said Ginger Jingzhen Yang, a sports injury researcher at Nationwide Children’s Hospital and Ohio State University. Yang and her co-authors used a nationally representative sample of high schools that included 100 schools each academic year from 2005 to 2015.

The study found that high school athletes reported an estimated 2.7 million concussion injuries from fall 2005 through spring 2016. Of these reported concussions, 89 percent were new injuries and 11 percent were recurrent.

The youth sports laws that have been passed mostly require educating coaches, trainers, athletes and parents on the effects of concussions. Most of them also require athletes to be removed from play after a concussion and prevent them from returning until they have been cleared by medical staff, so that they do not experience a recurrence, which can greatly worsen such injuries.

Some of the results in the research published Thursday echo the findings of other recent studies. The rate of concussions among high school football players, for example, was massively higher than any in other sports in Yang's study. Teenage boys who play football had an average annual concussion rate that was more than double that of boys playing soccer. The researchers also found that the rate of concussion during a competitive game was five times higher than in practice.

When football was taken out of the equation, however, in matched sports played by both boys and girls, the female athletes experienced much greater rates of concussion than the males — a puzzling result that other researchers have also encountered.

“We don’t know if this is because girls, for some reason, carry a higher risk for this kind of head injury or if they are more likely to report it,” Yang said. “This is something we all have to study further.”

A study in March by the American Academy of Orthopaedic Surgeons similarly found that female athletes, especially soccer players, suffer greater rates of concussions than male equivalents. The authors of that study theorized that the disparity may be caused by lack of protective gear for female athletes and more competitive, harder play in girls' soccer compared to boys' soccer.

One possible explanation may be that “the neck muscles of girls just aren’t as developed as boys' are,” said Wellington Hsu, an orthopedic surgeon and one of the authors of the earlier study released in March. “So if girls experience an impact, it makes sense they might be affected by it more than boys if they don’t have the muscles to cushion that impact.”

The new research on concussion comes at a time when school officials and parents are grappling with how to respond to the growing evidence of how severely brain injuries can affect athletes. Many parents have begun to hold their kids back from participating in football in particular.

One especially alarming study that came out this summer detailed how researchers found signs of neurodegenerative disease in 99 percent of the brains donated by families of former NFL players. Out of 202 brains that belonged to men who played football at all levels and were later donated for research, the researchers found chronic traumatic encephalopathy (CTE) in 177 of them — 87 percent. The results provoked reactions from sports officials and lawmakers in Washington, who have begun discussing federal regulations that would better protect players.

Despite the rise of state laws in recent years, enforcement and widespread data collection continue to be sparse. Yang and other experts also point out that the state laws do not prevent the concussions from occurring in the first place.

“These laws focus on recurrence and education, which is important,” she said. “But they don’t have prevention as part of the law, and that needs to be included. We need more strategies for minimizing body and head contact.”

Read more:

Repetitive hits, concussions, and a mother’s long goodbye to youth football

New research on football and brain trauma sounds an alarm

New concussion treatments go beyond just resting in a dark room

Trump knocks ‘softer’ NFL rules: ‘Concussions — ‘Uh oh, got a little ding on the head?’