Military service members deployed to conflict zones may be at greater risk of developing post-traumatic stress disorder if they were physically, emotionally, or sexually abused in childhood, according to a new study.

This, along with other findings from the study, help clarify how adverse experiences early in life can make people more vulnerable to trauma later on.

“To understand why some service members develop PTSD symptoms while others don’t, researchers have often looked at how much combat or trauma a service member has experienced while deployed. Obviously, that matters,” says Marcus Credé, an industrial psychologist and associate professor at Iowa State University and coauthor of the study published in the Journal of Traumatic Stress.

“But people respond differently, and it seems like some, by virtue of what they went through in childhood, are simply more susceptible.”

The US Centers for Disease Control and Prevention define a traumatic event as one “marked by a sense of horror, helplessness, serious injury, or the threat of serious injury or death.” Previous studies have linked frequent and high levels of traumatic stressors, including adverse childhood experiences, to PTSD.

But researchers have disagreed about the strength of the connection and whether the effect is “additive” or “multiplicative.”

An additive effect is like putting weights on a scale, Credé says. An adverse childhood experience is one weight. Trauma during deployment is another. PTSD or elevated symptoms develop if the cumulative weight is too heavy.

With the multiplicative concept, it’s more like a chemical reaction. Someone who was abused as a child may respond more strongly to trauma as an adult because their system for coping has been affected.

Read the full article about PTSD and childhood abuse by Rachel Cramer at Futurity.