BROWN (US) — Positive evidence presented in a weak way can make listeners suspicious of a predicted outcome, a finding that can have serious implications for professional persuaders like marketers and politicians.
“It’s not a conscious choice to behave this way,” says Steven Sloman, professor of cognitive and linguisticsciences at Brown University.
“When people are thinking forward in a causal direction, they just think about the cause they have in mind and the mechanism by which that would lead to the consequence they have in mind. They neglect alternative causes.”
Consider the following statement: “Widespread use of hybrid and electric cars could reduce worldwide carbon emissions. One bill that has passed the Senate provides a $250 tax credit for purchasing a hybrid or electric car. How likely is it that at least one-fifth of the U.S. car fleet will be hybrid or electric in 2025?”
The middle sentence is the weak evidence. People presented with the entire statement—or similar statements with the same three-sentence structure but on different topics—answered the final question lower than people who read the statement without the middle sentence.
They did so even though other people who saw the middle statement in isolation rated it as positive evidence for, in this case, higher adoption of hybrid and electric cars.
Give people a weak reason and they’ll focus too much on it. Give people no evidence and they’ll supply their own probably more convincing reason to believe that the outcome is likely. So, supportive but weak evidence seems to work against belief in a prediction.
“People take what you suggest and run with it,” says Philip Fernbach, postdoctoral associate at Brown and the paper’s first author.
Details of the study are published online in the journal Cognition.
The researchers say the “weak evidence effect” is strong enough to influence people when real money is in play.
In two of the five experiments participants were asked to bet on an outcome, such as whether Republicans would retake the house in 2010 or whether milk in the fridge would spoil by a certain date.
The participants could either opt to receive $10 no matter what happened, or could take the risk to receive $30 if the predicted outcome came to pass. Some were shown weak but positive evidence (for example, a GOP candidate in a close race received an endorsement; the power to the fridge was out for 30 minutes) and some were not.
In each experiment those who saw the mildly reinforcing evidence were less likely to take the risk that the prediction would come true.
The effect might help explain a peculiar pattern in consumer behavior: Sometimes adding a feature or promoting a product can make consumers less likely to buy it.
It might also explain why people have more trouble supporting sweeping policy proposals (such as the Affordable Care Act), even when they support individual initiatives within them.
But the effect is not an inevitable thought pattern. People in a variety of roles, including jurors, scientists, investors, and homebuyers often factor multiple pieces of evidence into their thinking.
“People have the potential to be good researchers if they have enough incentive to be,” Sloman says. “Although you’d be surprised.”
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