In Brief

When physicians gauge whether a patient has an illness such as pneumonia, they presumably consider the possibility of other illnesses as well.

While that process may seem beneficial, a study in the May Journal of Experimental Psychology: General (Vol. 135, No. 2) suggests that weighing such alternative explanations or diagnoses may actually end up clouding their judgments, particularly when they fail to regard irrelevant alternatives as extraneous and, instead, include them in their probability judgments.

"People like to think that they think about making judgments in a clean, logical way," says lead researcher Michael Dougherty, PhD, a University of Maryland cognitive psychology professor. "But in reality, we're often influenced by information that is completely irrelevant to the judgment in question."

In a set of studies, Dougherty and graduate student Amber Sprenger asked 189 college students to imagine they were making several trips to a grocery store. On the first three trips, participants imagined purchasing various amounts of produce, such as eight oranges or a dozen ears of corn. Each specific fruit or vegetable was purchased only one time. After each trip, participants judged the proportion of each kind of item in their shopping bag. As the researchers presented each additional grocery list, they aimed to increase the chance that participants would recall an irrelevant previous list item when making their judgments.

In the study's second phase, researchers instructed participants to imagine purchasing various amounts of beverages, such as milk, water and cola.

In the first task, participants consistently failed to discriminate between grocery lists. Moreover, across the first three lists, participants' probability judgments became more accurate in an absolute sense while becoming less accurate in a relative sense.

In the second task, when the items in the list were somewhat unrelated, participants were more accurate at discriminating between their current list and the prior three lists, which, in turn, led judgment accuracy to return to a similar level as after the first list.

The researchers propose that the participants conflated words across the first task because they couldn't discriminate between relevant and irrelevant lists, says Dougherty.

The participants' confusion as to which list an item belonged to is similar to physicians' failure to disregard implausible or irrelevant diagnoses when identifying an ailment, he says. And there may not be anything that people can do to mend the mistake, he adds.

"The problem is that people don't realize that they're being influenced by irrelevant alternatives, so there's no clear method for remediation," says Dougherty.

--Z. Stambor