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In Spain, prisoners’ brains are being electrically stimulated in the name of science

A new era of neurointerventions is upon us. Will it improve the criminal justice system or create an ethical nightmare?

A visualization of a brain and a person’s reflection in a mirror.
The use of neurointerventions in the criminal justice system is ethically fraught.
Dan Kitwood/Getty Images

A team of scientists in Spain is getting ready to experiment on prisoners. If the scientists get the necessary approvals, they plan to start a study this month that involves placing electrodes on inmates’ foreheads and sending a current into their brains. The electricity will target the prefrontal cortex, a brain region that plays a role in decision-making and social behavior. The idea is that stimulating more activity in that region may make the prisoners less aggressive.

This technique — transcranial direct current stimulation, or tDCS — is a form of neurointervention, meaning it acts directly on the brain. Using neurointerventions in the criminal justice system is highly controversial. In recent years, scientists and philosophers have been debating under what conditions (if any) it might be ethical.

The Spanish team is the first to use tDCS on prisoners. They’ve already done it in a pilot study, publishing their findings in Neuroscience in January, and they were all set to implement a follow-up study involving at least 12 convicted murderers and other inmates this month. On Wednesday, New Scientist broke news of the upcoming experiment, noting that it had approval from the Spanish government, prison officials, and a university ethics committee. The next day, the Interior Ministry changed course and put the study on hold.

Andrés Molero-Chamizo, a psychologist at the University of Huelva and the lead researcher behind the study, told me he’s trying to find out what led to the government’s unexpected decision. He said it makes sense to run such an experiment on inmates because “prisoners have a high level of aggressiveness.”

In the long term, he hopes tDCS — which he says is not painful — can make life better for inmates, both by making prison a less violent environment for those in it and by serving as a method of offender rehabilitation that’ll eventually allow inmates to get out.

In harboring these wishes, he’s not alone. Many people who float the idea of using neurointerventions on prisoners are motivated by the belief that mass incarceration is immoral and often counterproductive. They say we need a more humane way to treat people who’ve committed crimes — an approach that aims not to punish but to rehabilitate. That’s a noble goal.

But testing out techniques like tDCS on incarcerated people raises serious ethical concerns, which philosophers have been hashing out in a slew of journal articles and books. In the past, biomedical interventions in the criminal justice system have mostly been limited to chemical castration for sex offenders, with libido-reducing drugs being used in Europe and the US. Over the past few years, as neuroscientific advances raised the possibility of treating a wider array of offenders, ethicists have begun to probe its moral limits.

All the inmates participating in Molero-Chamizo’s study are volunteers who’ve signed consent forms, yet they’re also in an inherently coercive situation. Under those conditions, the ethicists ask, is it even possible to meaningfully consent to an experiment like this?

How the study will work if it’s approved

Assuming the Interior Ministry reissues permission for the experiment, here’s what it will do. A psychologist will apply an electrical current to the brain of each participating inmate for 15 minutes a day, three days in a row. It’s a gentle jolt, Molero-Chamizo said, adding, “it doesn’t have any adverse effects — maybe just some tingling in the skin.”

The subjects will also include psychology students. Like the prisoners, they will receive an explanation of the procedure and have to sign a consent form. Molero-Chamizo said these will be unpaid volunteers, some culled from his own classes, who are interested in seeing firsthand how the tDCS process works.

Before any current is applied, the participants will fill out a questionnaire asking them whether they agree with statements like, “I sometimes feel like a powder keg ready to explode.” At the end of the study, the participants will again fill out the same questionnaire. The researchers will analyze the difference in responses to gauge the effect of the tDCS.

In other words, the team won’t be able to tell whether the neurointervention decreases the actual violent impulses in inmates; instead, it’ll rely on self-reported aggression as the best available proxy. That’s not an ideal measure, though it’s fairly common given the ethical and practical difficulties of tracking violence.

Molero-Chamizo also hopes to collect saliva samples from the participants to establish their cortisol levels, indicators of stress that might hint at aggressive intentions. But he doesn’t have ethics approval for that component; it was outstanding even before the Interior Ministry pressed pause on the whole study Thursday.

The proposed experiment is similar to Molero-Chamizo’s previous study, which found that electrical stimulation of the prefrontal cortex — also for 15 minutes per day, over three consecutive days — reduced self-perceived aggression. The subjects were 41 male prisoners, including 15 convicted for murder. Students weren’t involved. The control group was composed of inmates who had electrodes strapped to their foreheads, but no current applied. No saliva samples were collected.

Molero-Chamizo designed the new study to address what he acknowledges were limitations of the first study. As his Neuroscience paper puts it:

The study was single-blinded, since only one researcher was allowed to enter the prison and a device for coded blinding was not available. … The inclusion of a non-imprisoned population as an added tDCS control group might have further strengthened the results and helped to answer the question if this intervention is only efficient for reduction of pathologically enhanced aggression or has a more general impact. Finally, long-term effects of stimulation were not obtained, thus future studies should include measures at different time-points to explore the duration of respective effects.

Also, the first study only looked at what happens when activity in the prefrontal cortex is increased through electrical stimulation. The new experiment aims to record what happens when activity is purposely decreased.

This line of research is fraught with ethical issues

Apart from the scientific issues with the studies, there are serious ethical issues to consider.

The biggest one has to do with consent. Incarcerated people are held against their will — a situation that is coercive by definition — and that makes establishing meaningful consent very difficult.

Roland Nadler, a neuroethicist at the University of Ottawa, told me that when inmates sign a consent form, they may be doing it under a kind of psychological duress. “Every criminal defendant will have an incentive to show progress toward rehabilitation — they’ll have the motivation of securing a more lenient sentence,” he said. “The exchange need not be a formal quid pro quo in order to effectively happen.”

Another concern with the neurointerventionist approach is that it places the burden for improving a deeply flawed prison system squarely on the prisoner. Inmates are expected to change their brains while the system is absolved of the need to change.

“To me this is a classic and genuinely difficult ethical dilemma,” Nadler said, “whereby the availability of a technological solution that promises immediate reduction in avoidable suffering also poses a risk of draining the moral urgency out of correcting a more systemic problem.”

Some philosophers, however, have suggested that neurointervention could be a radical tool for abolishing mass incarceration. If vast numbers of prisoners can be quickly treated for aggressive impulses, the argument goes, then there’ll clearly be no need to keep so many people behind bars for so many years. Molero-Chamizo emphasized to me that the minimally invasive neuromodulation his team administers is “easy, cheap, and portable.”

More broadly, Molero-Chamizo said, we’re at a point where research like his — assuming it continues — could have applications in the general population, too. If tDCS is proven effective at curbing antisocial behavior and if it gets regulatory approval, he told me, “you can do it in the prison or the hospital or even in your own home.”

The question of whether that would be a desirable outcome is part of a bigger debate about moral enhancement. Some, like the philosophers Julian Savulescu and Ingmar Persson, say we have a duty to develop new tools — drugs, surgery, or other treatments — to make ourselves into a more moral species. Otherwise, we may trigger our own annihilation through climate change and other global catastrophic risks.

Others object that these biomedical interventions could diminish our free will. If tools for morally enhancing ourselves were to become widely available, some would argue we’re morally required to use them, and that could cause people to feel socially coerced into submitting to medical treatments. Plus, the bioethicist John Harris notes, the treatments themselves could erode our free will by eroding the brain’s ability to make less-than-moral choices — what he calls “our precious freedom to fall.”

“This is a problem well explored in works like A Clockwork Orange,” Nadler pointed out. “We’d rightly worry about having a system that inexorably changes behavior to fit a standard prescribed by society.” But, he added, “there are no cost-free answers.”

This issue is incredibly fraught. One thing, however, is clear. The era of neurointerventions is upon us — and it’s coming for prisoners first.


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