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There's no getting around the fact that bad things happen to people— things that are often outside of their control. And the psychological effects of wars, natural disasters, and other types of violence and accidents don't just go away when the war ends.

They can leave people traumatized. Many people who are exposed to traumas like war get diagnosed with post-traumatic stress disorder, or PTSD, where thoughts or anxieties about the events can plague them in other parts of their lives, and seemingly benign things can trigger a fear response. But hopefully, once a war goes down in history and few generations pass, we'll be free of the trauma, right?

Unfortunately, it's not so simple. Trauma can affect future generations, too. For example, in a study of 100 Holocaust survivors, researchers found that their children were more likely to get a PTSD diagnosis, even though they didn't report any more traumatic experience in their lives than a similar control group did.

What they did get was stories from their parents— really distressing stories. And hearing about threats to your family, even in the past, can be traumatic on its own. But that might not be the only explanation.

Trauma could also be inherited biologically, sort of like what gets passed on through our genes. And since a big part of understanding people is knowing about the different factors that can affect their thought processes and behaviors, psychologists want to figure out how this works. So far, we do have some evidence that trauma can be passed biologically from one generation to the next, but it's very limited.

A lot of it comes from a study published in 2015, where researchers did some genetic tests on a sample of 32 Holocaust survivors and 22 of their children, as well as a demographically similar control group. They were looking at what's known as the methylation of the genes, where a DNA molecule gets a sort of hop-on, called a methyl group. If a gene has a lot of methyl groups attached to it, they can create problems with copying that gene, or can keep the gene from doing what it's supposed to.

Studying methylation is part of the field of epigenetics, the study of what can be inherited or passed on from parent to child biologically, but outside of the actual letters of your DNA. Since methylation levels can change based on your environment, it's a way things that happen in your life can affect what you pass down. And the researchers found that compared to the control groups, both the Holocaust survivors and their children had higher methylation levels in a gene related to stress response.

Previous research showed that gene was related to an increased risk of developing a psychological disorder like PTSD in people who experienced some kind of trauma in their childhood. So the team thought they might have found a biological way for trauma to be inherited: through increased methylation that gets passed on. But there are some problems with this study.

Among other things, the sample size was pretty small. And we'd have to look back a few more generations to see if the methylation changed when the trauma happened, or if it was always higher in that group. So, the evidence so far is kinda tenuous, and we'll need a lot more research before we know for sure.

That said, the idea that trauma can be inherited isn't too far-fetched. We know that other chronic stresses can have an effect on children, sometimes through a biological mechanism. But even if we never find a biological way to explain how parents who've experienced trauma can pass it on to their kids, there's a much simpler explanation that has a lot more research to back it up.

Usually, parents shape a lot of the environment their kids grow up in, which can make a big difference when it comes to their mental health. We know this happens with some clinical conditions, like depression. Depression can run in families, and we know about some genes that might be risk factors.

If one person gets diagnosed with depression, it's not only more likely that a child will get a diagnosis, but also that a grandchild will. But even in cases in which there isn't this long family pattern, having a depressed parent has effects on a child's functioning, like how well they handle daily life stresses. You can see similar patterns with alcoholism.

Even though there's a known genetic element that makes children of alcoholics more likely to be alcoholic themselves, there are other ways people can inherit that stress from their parents. Having an alcoholic parent makes people more likely to show higher neuroticism, psychiatric stress, and aggressive behavior in adolescence— but the thing is, it makes a difference whether their parent is a recovered alcoholic or not. Meaning, it's not just their shared genetics.

The environment the child is living in can put them at increased risk. And if a parent has experienced trauma, that can have a big effect on their kids' environment. For children of Holocaust survivors, for example, even hearing stories from their parents could change their view of the world.

So we know there's a risk that's carried to children, even if the epigenetics idea turns out to be a fluke. But it's also important to remember that a risk factor isn't an equal sign. Having a parent that experienced trauma or had a clinical disorder doesn't mean you will too.

Research shows that things like promoting positive parent-child interactions and teaching coping skills to children of depressed parents can reduce some of these risks. So even though kids are at risk of inheriting some negative consequences if their parents have experienced trauma, it is certainly not inevitable. Thanks for watching this episode of SciShow Psych, and if you're interested in learning more about the science of epigenetics, you can check out our video about it on the main SciShow channel. [OUTRO ♪].