Their conclusion: Acetaminophen, the most common drug ingredient in the United States, can reduce a person's capacity to empathize with another person's pain, whether that pain is physical or emotional.

"We don't know why acetaminophen is having these effects, but it is concerning," senior author Baldwin Way, an Ohio State psychologist, said in a statement. "Empathy is important. If you are having an argument with your spouse and you just took acetaminophen, this research suggests you might be less understanding of what you did to hurt your spouse's feelings."

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In the first experiment, 80 participants were asked to drink a liquid. Half the subjects received something containing 1,000 mg of acetaminophen. The other half got something without the drug. After an hour, all subjects were asked to rate the pain experienced by characters in eight different fictional scenarios. In some of the stories, the character went through a physical trauma, in others an emotional trauma. In general, those subjects who had taken the acetaminophen rated the pain of the characters as less severe than those who had taken the placebo.

A second experiment exposed participants to brief blasts of white noise. They were then asked to rate the pain of another (anonymous) study participant who'd also been subjected to the unpleasant sounds. Again, those who had received the acetaminophen rated the other person's pain as being less severe compared to students who'd drunk the placebo liquid.

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As a further test, in which participants had to judge online skits involving social rejection, they split along the same lines as in the noise experiment.

"In this case, the participants had the chance to empathize with the suffering of someone who they thought was going through a socially painful experience," Way said. "Still, those who took acetaminophen showed a reduction in empathy. They weren't as concerned about the rejected person's hurt feelings."

The two experiments build on previous studies identifying a brain region that appears to be key to a person's empathetic response. The anterior insula, located deep in the folds between the front and side of the brain, is where mind and body are integrated. It also plays a key role in human awareness, including emotional awareness. The less pain a person feels, the less able he or she is to empathize with someone else's.

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"Because empathy regulates prosocial and antisocial behavior," the authors note, "these drug-induced reductions in empathy raise concerns about the broader social side effects of acetaminophen."

Acetaminophen is an ingredient in more than 600 different medicines, according to the Consumer Healthcare Products Association. About a quarter of all Americans take acetaminophen every week.