People who score highly on cognitive tests may be less vulnerable to post-traumatic stress disorder (PTSD) than people with only average scores, according to a study in the August issue of the Journal of Abnormal Psychology (Vol. 115, No. 3, pages 484–495).

The study, which examined Vietnam War veterans and their twin brothers, found that such traits as high general IQ and advanced verbal memory skills may offer some protection.

Psychologists have long known that, on average, Vietnam veterans who develop PTSD don’t do as well on some neurocognitive tests as veterans without PTSD, according to Harvard Medical School and Manchester VA Medical Center psychologist Mark Gilbertson, PhD, the lead author of the study. But researchers didn’t know whether those cognitive deficits were a result of PTSD or a risk factor that existed before the trauma. The new study’s twin-sample design allowed the researchers to investigate the causality question.

“[The twin study] gave us a unique sample to look at the question of vulnerability versus toxicity,” Gilbertson says.

He says he hopes the results may lead researchers to develop methods to teach people at risk for trauma exposure—such as police officers, firefighters and soldiers—new ways to protect themselves against PTSD.

Same genes, different experiences

In the study, Gilbertson and his colleagues tested 43 Vietnam War veterans and those veterans’ twin brothers, none of whom had been exposed to combat. Nineteen of the veterans developed PTSD, while none of the noncombat twin brothers did.

The researchers gave all 86 participants a battery of cognitive tests, including a general IQ test and tests of verbal memory, visual memory, attention, visuospatial ability and executive function.

They found, as expected, that the veterans with PTSD had significantly lower IQ, verbal memory, attention and executive function scores than veterans without PTSD. They also found, however, that the noncombat twins mirrored their brothers’ scores: Those whose brothers had developed PTSD generally had significantly lower scores than those whose brothers had not. And in both cases, the twins’ scores didn’t differ significantly from the scores of their combat-exposed brothers.

The results, Gilbertson says, suggest that the cognitive differences in the literature represent pre-existing, familial genetic risk factors for PTSD, rather than trauma-induced cognitive problems.

“It’s hard to get at these chicken-or-the-egg, cause-or-effect issues,” says Tulane University psychologist Jennifer Vasterling, PhD, who also studies cognitive function and PTSD. “Using this twin paradigm was a very innovative way of beginning to address it.”

Also important, Gilbertson says, is that almost all of the participants—Vietnam vets and brothers; those with PTSD and those without—scored well within the range of normal.

“Most of the vets and their brothers performed in the average range on all the tasks,” he says. “The folks who didn’t develop PTSD were above average.”

In other words, it’s not that people with low cognitive skills are more likely to develop PTSD; it’s that people with above-average skills are less likely to develop it.

“This is not a deficit factor, but a protective factor,” Gilbertson says.

Of course, cognitive ability is not the most important factor that determines whether a person will develop PTSD, according to Vasterling—that is still likely to be the severity and potency of the trauma. And having an extra-good memory, she says, doesn’t guarantee you’ll never develop PTSD.

“The nastier and more persistent the trauma, the more likely you are to develop PTSD—those tend to be the most potent predictors,” she says. “I think it’s important to highlight that because the last thing people need is to feel like, ‘Oh, there’s something innately wrong with me because I developed PTSD.’”

Remembering in words or images

But why would cognitive skills help protect a person from PTSD?

“That’s a good question, and one we’re not able to definitively answer,” Gilbertson says. It’s possible, he suggests, that people with better verbal skills more naturally put their memories into words. Verbally encoded memories, he says, may be less likely to produce flashbacks and other hallmarks of PTSD than memories encoded in images.

It’s also possible that high IQ scores simply reflect better problem-solving abilities, says Richard McNally, PhD, a Harvard University psychologist and PTSD researcher not involved with the study. “Recovering from the effects of intense emotional experience is certainly a problem to be solved,” he says.

Finally, McNally suggests, high IQ and other cognitive skills may make it easier for people to do the sorts of things that re-engage them in postwar life: going to school, getting a job and recruiting social support from friends and colleagues.

The next research step, Gilbertson says, is to try to figure out more precisely which cognitive skills protect people from PTSD.

“Once we figure that out, the question then becomes: Are those skills that we can teach or enhance in folks that don’t have them as intuitively?” he says. “I think that since there do appear to be cognitive resilience factors, it would be worth it to invest money in finding out what those factors are, so we can use them to build resilience in everyone.”