Weak supporting evidence can undermine belief

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Article: Weak supporting evidence can undermine belief in an outcome

Defying logic, people given weak evidence can regard predictions supported by that evidence as less likely than if they aren’t given the evidence at all.

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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?”

That 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.

Paper: When good evidence goes bad: The weak evidence effect in judgment and decision-making

Abstract:

An indispensable principle of rational thought is that positive evidence should increase
belief. In this paper, we demonstrate that people routinely violate this principle when pre-
dicting an outcome from a weak cause. In Experiment 1 participants given weak positive
evidence judged outcomes of public policy initiatives to be less likely than participants
given no evidence, even though the evidence was separately judged to be supportive.
Experiment 2 ruled out a pragmatic explanation of the result, that the weak evidence
implies the absence of stronger evidence. In Experiment 3, weak positive evidence made
people less likely to gamble on the outcome of the 2010 United States mid-term Congres-
sional election. Experiments 4 and 5 replicated these findings with everyday causal
scenarios. We argue that this ‘‘weak evidence effect’’ arises because people focus dispro-
portionately on the mentioned weak cause and fail to think about alternative causes.

Article: Weak supporting evidence can undermine belief in an outcome

Defying logic, people given weak evidence can regard predictions supported by that evidence as less likely than if they aren’t given the evidence at all.

...

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?”

That 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.

Paper: When good evidence goes bad: The weak evidence effect in judgment and decision-making

Abstract:

An indispensable principle of rational thought is that positive evidence should increase
belief. In this paper, we demonstrate that people routinely violate this principle when pre-
dicting an outcome from a weak cause. In Experiment 1 participants given weak positive
evidence judged outcomes of public policy initiatives to be less likely than participants
given no evidence, even though the evidence was separately judged to be supportive.
Experiment 2 ruled out a pragmatic explanation of the result, that the weak evidence
implies the absence of stronger evidence. In Experiment 3, weak positive evidence made
people less likely to gamble on the outcome of the 2010 United States mid-term Congres-
sional election. Experiments 4 and 5 replicated these findings with everyday causal
scenarios. We argue that this ‘‘weak evidence effect’’ arises because people focus dispro-
portionately on the mentioned weak cause and fail to think about alternative causes.

Generally, if you're given evidence for something, the evidence-giver is trying to convince you of that something. If you're given only weak evidence, that itself is evidence that there is no strong evidence (if there is strong evidence, why didn't they tell you that instead?), and so in some circumstances it could be rational to downgrade your probability estimate.

When I read the title I had expected that this was the point of the post. Perhaps because I've been intending to write a post to that effect for the last three years or so.

So I agree completely.

5jhuffman15y

Also in this case, we have evidence that there is only token support in congress for public measures to improve adoption. I'm kind of surprised the control found this evidence to be net positive really. And I wonder if the evidence gets evaluated a little different when people have to use it rather than just evaluate it.

9shokwave15y

Sure, this makes perfect sense in a political environment - or in the ancestral environment, where I'm sure this kind of thing was very important to breeding (I could even take a shot at an evolutionary argument for this kind of instinct!). But that instinct is a net positive only in political situations; our current environment is significantly more factual-uncertainty based than political-uncertainty based. This may make the instinct a net negative.

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