If you witnessed a car crash, how would you decide who’s to blame? Chances are, you’d imagine an alternative scenario in which one car’s path was different and the collision was avoided.

Cognitive scientists have known for a while that people consciously do this when trying to determine who or what is responsible for a given outcome. The process is called “counterfactual simulation.” For the first time, researchers now have direct evidence that people also unconsciously use counterfactual simulation when witnessing physical events.

“Based on previous work, we had good reasons to believe that participants engage in counterfactual simulation when making causal judgments about dynamic physical events,” Tobias Gerstenberg, a postdoctoral student at MIT and lead author of the study, which was published in Psychological Science, told Seeker. “In this experiment, we aimed to get more direct evidence for what we assumed was going on in people’s minds.”

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Study participants were asked to watch 18 videos that showed different outcomes of two billiard balls colliding with each other. As they watched the outcomes, such as whether ball B went through a gate or not, their eye movements were tracked. The researchers told some people to agree or disagree with statements about which ball influenced the other — for example, “Ball A caused ball B to go through the gate.” Others were just asked to describe the outcome of the collision in their own words.

The researchers observed that people who had been asked to assign blame to ball A traced their eyes along the imagined trajectory of ball B to determine what would’ve happened if ball A had not interfered. Infrared light technology was used to show where the subject’s eyes were looking, revealing that people imagined this trajectory even when they were unaware that they were doing it. They were unconsciously using counterfactual simulation.

“We found that when judging whether ball A caused ball B to go through the gate, participants spontaneously simulated where ball B would have gone if ball A hadn't been present in the scene,” Gerstenberg said.

The participants tended to imagine ball B’s trajectory more often in cases where there was a lot of uncertainty as to whether ball A had an effect on ball B.

“The results show that participants engaged more in counterfactual simulation when it was more difficult to anticipate what the counterfactual outcome would have been,” Gerstenberg said.

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In other words, it appears that we often employ counterfactual simulation in “close call” situations. The study’s results suggest that people may differ in assigning blame to one side over the other when they don’t agree on what would have happened in a different situation.

“However, judgments of blame go beyond judgments of causation in several ways,” Gerstenberg noted. “When we decide how much a person is to blame, we don't just look at whether their action caused the outcome, we also try to infer their underlying mental states [and ask,] ‘Does the person foresee and intend for the outcome to happen?’ We consider, more broadly, what the action reveals about the person.”

In real-world scenarios of causality, Gerstenberg and his colleagues think that their work could be most helpful in certain legal circumstances. One way the law uses counterfactual simulation is with the “but for” test, where it’s determined that an outcome may not have taken place without a specific interference — for example, the plaintiff would not have been injured “but for” the defendant’s carelessness.

“Our study shows that there is a close correspondence between the way in which people make causal judgments, and the tests that the law employs,” Gerstenberg said. “People consider not only what actually happened, but also what would have happened otherwise.”

Ultimately, Gerstenberg’s goal is to better understand how people think about causation.

“Our work helps to make different causal concepts more precise,” he said. “It gives legal scientists a better understanding of how the ordinary person reaches causal judgments.”

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