Recovering from a concussion typically takes female athletes more than twice as long as males, according to a new study that tracked hundreds of teenagers active in sports. The finding adds to a growing body of evidence that vulnerability to this injury—and aspects of the healing process—may vary by sex.

A handful of studies published since the mid-2000s have suggested that girls in high school and college may sustain a higher rate of these injuries on the playing field than boys do, and investigations over the last few years have indicated they may also take longer to recover. As a result, when sports medicine researchers and experts convened in Berlin last fall for the 5th International Consensus Conference on Concussion in Sport, their subsequent statement cited evidence girls were more likely to suffer concussions that required a more lengthy recovery period than their male counterparts did. “But there wasn’t enough data to [definitively] say that this was the case,” says John Neidecker, a sports medicine physician with the Orthopaedic Specialists of North Carolina. “We thought that we'd take a look back at the athletes that we saw over a three-year period and actually [provide] some objective data.”

Neidecker and his colleagues analyzed the medical records of 212 middle and high school athletes who visited a sports medicine practice in southern New Jersey—110 boys and 102 girls—who had experienced their first concussion while playing an organized sport such as football, field hockey or wrestling. (Only initial head injuries were considered to rule out the possible effect of prior incidents.) Their analysis revealed the median recovery time for girls was 28 days—more than double that of boys, which was 11 days. The results appeared in the Journal of the American Osteopathic Association.

After a concussion, some individuals experience migraines and mental health issues, such as depression, which can contribute to longer recovery times. Yet researchers have also found evidence suggesting longer bounce back times are associated with suffering from those conditions prior to a head injury—raising some questions for Neidecker’s group. Although previous studies have reported a longer recovery period for girls, “what nobody has brought up is that all these preexisting [conditions] that seem to affect concussion recovery are more prevalent in females,” Neidecker says. “So maybe it’s actually not the concussion that’s still giving them the symptoms but the preexisting problems that were exacerbated [by the injury].”

The research team’s analysis partially supports this hypothesis—he and his colleagues examined the students’ medical histories and found that the girls were more likely to have previously suffered migraines than the boys. They also suggest psychological factors, such as depression or anxiety disorder, may play a role as well. In this new work there was a slightly higher prevalence of mental illness in girls versus boys, they note, but this difference was not statistically significant. Neidecker says he suspects this effect might be more pronounced in a larger sample, however.

Michael Collins, a concussion scientist at the University of Pittsburgh Medical Center who was not involved in this work, points out that other factors could also contribute to the disparity in recovery times. His prior research has shown, for example, that women also tend to experience more eye movement and visual stability issues following a concussion than men, which can require longer recovery times.

Other researchers have proposed potential biological explanations for the gender difference such as women’s smaller necks, which give them less strength to absorb shock, and higher rates of glucose metabolism (a process that generates the body’s energy). A woman’s menstrual cycle may also directly impact recovery—one 2014 study of 144 women reported brain injuries during certain phases of the cycle might take longer to heal, which researchers think might be due to a sudden drop in levels of progesterone, a female sex hormone. And other research suggests menstrual patterns, which are often associated with headaches and other symptoms of discomfort, might also affect self-reports both before and after concussions.

Across the sexes, concussion is common in contact sports such as soccer and hockey, where heads bang and helmets clash. And, over the years, the public has become more aware of the serious health effects associated with repeated blows to the head. The National Football League has publicly acknowledged the link between its sport and degenerative brain disorders such as chronic traumatic encephalopathy. Such head injuries are also prevalent in amateur and recreational athletics. In a study recently published in the Journal of the American Medical Association, for example, researchers found that in a sample of 13,088 U.S. teens, around 20 percent reported at least one diagnosed concussion in 2016. Among those who had participated in a contact sport recreationally, such as football or wrestling, the prevalence was 31.5 percent.

Overall, this latest study adds to a growing body of literature that shows girls have a higher incidence of concussions than boys and might also experience more persistent symptoms, says Robert Cantu, a neurosurgeon and professor at Boston University who did not take part in the new research. “This is one of the most robust studies in terms of the numbers of people involved,” he says, confirming “girls take longer to recover.”

Yet some concussion experts caution this latest work has some limitations. Mayumi Prins, who studies traumatic brain injury in children  at the University of California, Los Angeles,  points out that whereas Neidecker’s findings are consistent with what others have reported, a key consideration is that the authors relied on the athletes’ self-reports to determine their conditions at baseline (before the injury happened)—measures physicians typically use to determine whether a concussion has resolved. “Self-reporting or parental reporting is often fraught with errors,” Prins says.

More generally, there is also evidence of gender differences in symptom reporting across concussion studies. For example, a study published earlier this year in the Journal of Athletic Training found that although high schoolers of both sexes were equally knowledgeable about concussion symptoms, girls were more likely to disclose sports-related injuries to authority figures such as a medical professional or coach.

Ultimately, Prins says, researchers need an objective test to determine whether an individual has had a concussion. Scientists are currently working on developing better neuroimaging measures and identifying biomarkers in blood and other bodily fluids. In the meantime, however, one thing does appear to be clear: Concussion risk factors—and how they may differ by sex—require further scrutiny. Understanding what preinjury conditions are associated with recovery has important implications for treatment, Collins says. “The bottom line here is the injury needs to be recognized, the patient needs to be taken out of play and the [concussed] kids need to go to the clinics where they can get the multidisciplinary care that they need.”