A brain scan study pinpoints the changes associated with child abuse that may raise people's risk of depression, PTSD and addictions later in life.

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Child maltreatment has been called the tobacco industry of mental health. Much the way smoking directly causes or triggers predispositions for physical disease, early abuse may contribute to virtually all types of mental illness.

Now, in the largest study yet to use brain scans to show the effects of child abuse, researchers have found specific changes in key regions in and around the hippocampus in the brains of young adults who were maltreated or neglected in childhood. These changes may leave victims more vulnerable to depression, addiction and post-traumatic stress disorder (PTSD), the study suggests.

Harvard researchers led by Dr. Martin Teicher studied nearly 200 people aged 18 to 25, who were mainly middle class and well-educated. They were recruited through newspaper and transit ads for a study on “memories of childhood.” Because the authors wanted to look specifically at the results of abuse and neglect, people who had suffered other types of trauma like car accidents or gang violence were excluded.

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Child maltreatment often leads to conditions like depression and PTSD, so the researchers specifically included people with those diagnoses. However, the study excluded severely addicted people and people on psychiatric medications, because brain changes related to the drugs could obscure the findings.

Overall, about 25% of participants had suffered major depression at some point in their lives and 7% had been diagnosed with PTSD. But among the 16% of participants who had suffered three or more types of child maltreatment — for example, physical abuse, neglect and verbal abuse — the situation was much worse. Most of them — 53% — had suffered depression and 40% had had full or partial PTSD.

The aftermath of that trauma could be seen in their brain scans, whether or not the young adults had developed diagnosable disorders. Regardless of their mental health status, formerly maltreated youth showed reductions in volume of about 6% on average in two parts of the hippocampus, and 4% reductions in regions called the subiculum and presubiculum, compared with people who had not been abused.

That’s where this study begins to tie together loose ends seen in prior research. Previous data have suggested that the high levels of stress hormones associated with child maltreatment can damage the hippocampus, which may in turn affect people’s ability to cope with stress later in life. In other words, early stress makes the brain less resilient to the effects of later stress. “We suspect that [the reductions we saw are] a consequence of maltreatment and a risk factor for developing PTSD following exposure to further traumas,” the authors write.

Indeed, brain scans of adults with depression and PTSD often show reductions in size in the hippocampus. Although earlier research on abused children did not find the same changes, animal studies on early life stress have suggested that measurable differences in the hippocampus may not arise until after puberty. The new study suggests that the same is true for humans.

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The findings also help elucidate a possible pathway from maltreatment to PTSD, depression and addiction. The subiculum is uniquely positioned to affect all of these conditions. Receiving output from the hippocampus, it helps determine both behavioral and biochemical responses to stress.

If, for example, the best thing to do in a stressful situation is flee, the subiculum sends a signal shouting “run” to the appropriate brain regions. But the subiculum is also involved in regulating another brain system that, when overactive during chronic high stress such as abuse, produces toxic levels of neurotransmitters that kill brain cells — particularly in the hippocampus.

It can be a counterproductive feedback loop: high levels of stress hormones can lead to cell death in the very regions that are supposed to tell the system to stop production.

What this means is that chronic maltreatment can set the stress system permanently on high alert. That may be useful in some cases — for example, for soldiers who must react quickly during combat or for children trying to avoid their abusers — but over the long term, the dysregulation increases risk for psychological problems like depression and PTSD.

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The subiculum also regulates the stress response of a key dopamine network, which may have implications for addiction risk. “It is presumably through this pathway that stress exposure interacts with the dopaminergic reward system to produce stress-induced craving and stress-induced relapse,” the authors write.

In other words, dysregulation of the stress system might lead to intensified feelings of anxiety, fear or lack of pleasure, which may in turn prompt people to escape into alcohol or other drugs.

With nearly 4 million children evaluated for child abuse or neglect in the U.S. every year — a problem that costs the U.S. $124 billion in lost productivity and health, child welfare and criminal justice costs — child maltreatment isn’t something we can afford to ignore.

Even among the most resilient survivors, the aftereffects of abuse may linger. Not only are such children at later risk for mental illness, but because of the way trauma affects the stress system, they are also more vulnerable to developing chronic diseases like diabetes, high blood pressure, heart attack and stroke.

We can do better for our kids.

The study was published in the Proceedings of the National Academy of Sciences.

Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.