People with brain injuries are more likely to be placed in federal prison, new data suggest.

Researchers looked into the records of more than 1.4 million people who were between 18 and 28 years old in 1997 in Ontario, Canada. They found that those with traumatic brain injury were more than twice as likely to be federally incarcerated in that country more than a decade later by 2011, according to a study in the Canadian Medical Association Journal.

That particular age group was chosen “because of its high risk of TBI and involvement in the criminal justice system,” a statement from St. Michael’s Hospital says. While those without a history of traumatic brain injury had an incarceration rate of about 0.2 percent, the rate was about 0.5 percent among those with such a history.

Read: Mindfulness Can Make Prisons Better

Traumatic brain injury, in which a sudden trauma damages the brain, can happen when someone hits their head hard, like in a car crash, “or when an object pierces the skull and enters brain tissue,” the National Institute of Neurological Disorders and Stroke explains. “Every TBI is different, and symptoms of a TBI can be mild, moderate, or severe, depending on the extent of the damage to the brain.” For instance, some patients lost consciousness while others do not.

Symptoms associated with the injury could include a headache, confusion, memory and attention problems, dizziness, blurred vision, ringing in the ears, fatigue, sleep problems, vomiting or seizures. The institute also notes that TBI could affect behavior, mood and thinking or reasoning, depending on how severe the case — behavior or mental health side effects could include “depression, anxiety, personality changes, aggression, acting out, and social inappropriateness.”

Although the findings published in CMAJ Open are not comprehensive, as only federal prison sentences were included, as opposed to provincial incarcerations, they “contribute to emerging research suggesting traumatic brain injury is an important risk factor for involvement with the criminal justice system,” lead author Dr. Flora Matheson, of the Centre for Urban Health Solutions at St. Michael’s Hospital, said in the hospital statement. “This may be just the tip of the iceberg as our study focused only on people with more serious TBI.”

Read: How to Spot a Psychopath

It is also unclear how the association would translate to other nations and prison systems. The United States, Canada’s neighbor to the south, has a comparable society in many ways but has a drastically different population makeup. Compared to Canada’s current population of about 36 million, the U.S. has more than 325 million people. And just a couple of years ago, according to statistics from the U.K.-based Institute for Criminal Policy Research, Canada’s prison population was about 40,000, compared to 2.2 million total in the U.S. — which means people in the states are several times more likely to be incarcerated than Canadians.

In Canada, “the authors stressed that the overall risk of someone who has experienced TBI being incarcerated was still low,” according to St. Michael’s Hospital. “However, knowing that people who are incarcerated are much more likely to have experienced brain injury than the general population will help health care providers better identify people at risk for incarceration and provide better care for people within the prison system.”

See also: Microbes Are A New Way To Identify Criminal Suspects

See also: 3 Weird Ways People Beat Drug Tests