Researchers at Harvard University have discovered that our experience of pain depends on whether we think someone caused the pain intentionally. In their study, participants who believed they were getting an electrical shock from another person on purpose, rather than accidentally, rated the very same shock as more painful. Participants seemed to get used to shocks that were delivered unintentionally, but those given on purpose had a fresh sting every time.

The research, published in the current issue of Psychological Science, was led by Kurt Gray, a graduate student in psychology, along with Daniel Wegner, professor of psychology.

It has long been known that our own mental states can alter the experience of pain, but these findings suggest that our perceptions of the mental states of others can also influence how we feel pain.

"This study shows that even if two harmful events are physically identical, the one delivered with the intention to hurt actually hurts more," says Gray. "Compare a slap from a friend as she tries to save us from a mosquito versus the same slap from a jilted lover. The first we shrug off instantly, while the second stings our cheek for the rest of the night."

The study's authors suggest that intended and unintended harm cause different amounts of pain because they differ in meaning.

"From decoding language to understanding gestures, the mind distills meaning from our social environment," says Gray. "An intended harm has a very different meaning than an accidental harm."

The study included 48 participants who were paired up with a partner who could administer to them either an audible tone or an electric shock. In the intentional condition, participants were shocked when their partner chose the shock option. In the unintentional condition, participants were shocked when their partner chose the tone option. Thus, in this condition, they only received a shock when their partner did not intend them to receive one. The computer display ensured that participants both knew their partner's choice and that a shock would be coming, to ensure the shock was not more surprising in the unintentional condition.

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Despite identical shock voltage between conditions, those in the intentional condition rated the shocks as significantly more painful. Furthermore, those in the unintentional condition habituated to the pain, rating them as decreasingly painful, while those in the intentional condition continued to feel the full sting of pain.

Gray suggests that it may be evolutionarily adaptive for this difference in meaning to be represented as different amounts of pain.

"The more something hurts, the more likely we are to take notice and stop whatever is hurting us," he says. "If it's an accidental harm, chances are it's a one-time thing, and there's no need to do anything about it. If it's an intentional harm, however, it may be the first of many, so it's good to take notice and do something about it. It makes sense that our bodies and brains might amplify our experience of pain when we know that the pain could signal threats to our survival."

These findings speak to how people experience pain and negative life events. If negative events are seen as intended, they may hurt more. This helps to explain why torture is so excruciating – not only are torture techniques themselves exceptionally painful, but it's the thought that counts—and makes torture hurt more than mere pain.

On the other hand, if negative events are seen as unintended, they may hurt less. This may explain, in part, why people in abusive relationships sometimes continue to stay in them. By rationalizing that an abusive partner did not intend harm, some victims may reduce their experience of pain, which could make them less likely to leave the relationship and escape the abuse.

The research was supported by the National Institute of Mental Health, the Canadian Social Sciences and Humanities Research Council and the Institute for Humane Studies.