The findings on the effect of weak evidence may seem counterintuitive, because it would seem that any positive evidence -- even weak facts -- should boost our confidence in what we hope will be the eventual outcome. But it doesn't, according to the research.
Of course, we all like to think we are more deliberative than that. But Fernbach argues that many of our judgments are intuitive, based to some degree on what we've just learned.
"This is potentially quite pervasive and important," he said, "judging the potential state of the world given that I know some piece of information. That is just so prevalent in our lives. Am I going to take an umbrella today, it's cloudy? Will my stocks go up? Will Libya have stability if we do a no-fly zone?"
Some of that probably calls for "deliberative thinking," as he put it, but the decision is frequently based on shaky ground. After all, who wants to lug around an umbrella just because it's cloudy? Clouds are weak evidence, so leave the umbrella behind.
And then, the deluge.
There is an upbeat part of this story, however. The scores of participants who took part in the research apparently had no trouble figuring out that the evidence was weak. That's the good news. The bad news is that they usually made the wrong decision.