When a child sees a parent die, experiences severe poverty, or witnesses neighborhood violence, it can leave a permanent mark on her brain. This type of unmitigated, long-term "toxic stress" can affect a person's cardiovascular health, immune system, and mental health into adulthood.

“If you have a whole bunch of bad experiences growing up, you set up your brain in such a way that it’s your expectation that that’s what life is about,” James Perrin, president of the American Academy of Pediatrics, told me recently.

A new study in the journal Health Affairs finds that nearly half of all children in the U.S. have experienced one such social or family-related trauma.

Here's how the report authors found that number, according to the release:

For the study, [Johns Hopkins University family-health professor Christina] Bethell and her colleagues analyzed data from the 2011-12 National Survey of Children's Health, a survey of parents of 95,677 children under 17 from throughout the United States. The survey included questions about nine adverse childhood experiences as reported by parents: extreme economic hardship, parental divorce/separation, lived with someone with a drug or alcohol problem, witness or victim of neighborhood violence, lived with someone who was mentally ill or suicidal, witnessed domestic violence, parent served time in jail, treated or judged unfairly due to race/ethnicity, and the death of a parent. The survey includes myriad data on family and neighborhood environments and parental well-being in addition to children's schooling and medical care, and contains some data about child resilience.

The study found that 48 percent of children have experienced one of these childhood traumas, and 23 percent experienced two or more. But kids in some states fared worse than others. New Jersey had the lowest percentage of children with two or more traumas, at 16 percent, while Oklahoma had the highest, at 33 percent. Here's a map showing the general ranking of the states: