Peter Sunde, a co-founder of the file-sharing website The Pirate Bay, recently spent time in prison for aiding and abetting copyright crimes. Last November, he described prison life to The Guardian.

What is most difficult to cope with is the boredom, Sunde says. The days in prison merge into a grey mass, indistinguishable from each other. Sunde has trouble sleeping at night. “You become brain-dead in here,” he says. “A guy who has been here a long time said it best: what I miss most are new memories.”

Sunde's depressing assessment may not be far off. According to a new review published in the journal Frontiers in Psychology, prison does the brain little good.

Reviewers centered out of VU University Amsterdam in the Netherlands poured through the scientific literature for any studies that compared the cognitive functions of inmates to those of the general population. Their search turned up only seven studies, but those seven studies told a convincing tale.

"Distinct executive function deficits were found in attention, set-shifting, working memory, problem-solving and inhibition," the researchers report.

Considering that a previous analysis found a strong link between executive function deficits and criminality, the researchers argue that prisons should focus on mending inmates' brains with a more "enriched" environment, instead of simply keeping them locked up. Doing so could very well lower recidivism rates, which currently range between 35 and 67 percent.

"Prison... is... a clear example of an impoverished and sedentary environment... characterized by a lack of demand on self-regulating functions," the researchers say. "For example, prisoners sit or lie on their beds for a striking 9.36 hours per day on average, besides the hours spend sleeping. As we know from animal studies, an impoverished environment has a negative effect on the prefrontal cortex, a brain region crucial for executive functions."

The researchers admit that they can't be sure whether the prison environment is actually deteriorating executive functions. After all, antisocial individuals who commit crimes likely already suffer from impaired mental faculties. It would be easy to find out, however.

"We advise recruiting new detainees for a baseline measurement, and reassessing these new detainees within a certain timeframe, e.g. three or six months," they suggest.

Source: Meijers J, Harte JM, Jonker FA and Meynen G (2015). Prison brain? Executive dysfunction in prisoners. Front. Psychol. 6:43. doi: 10.3389/fpsyg.2015.00043