Hunting for a misplaced set of keys or a dead cell phone can be a nuisance. But for people who search for concealed weapons or malignant tumors, finding a target—and one they're not sure is even there—could be a matter of life or death. Unfortunately, research has shown that the rarer an item has proved to be, the less likely people are to find it when it is there.

"We know that if you don't find it often, you often don't find it," Jeremy Wolfe, a professor of ophthalmology at Harvard Medical School, said in a prepared statement. Likewise, searches for common objects tend to turn up way more false positives. So are people as hasty to judge that an inspected bag doesn't contain a weapon (a rare item), as they are to assume it has a more common item?

Wolfe and his colleague, Michael Van Wert of the Brigham and Women's Hospital in Cambridge, were determined to hunt down an answer to how reaction times changed based on what people were used to finding. Their results were published online January 14 in Current Biology

Their experiment, in which two dozen participants looked for weapons in simulated baggage, showed that people do not adapt their searching time equally to different odds of finding items. Individuals were given a consistent likelihood of finding an object, say 98 percent, and were judged both on the time it took them to declare whether a bag contained a weapon—and whether they were correct. Those in experiments who are faced with consistently 50-50 odds of finding a target might be expected to take the longest to arrive at an answer, and those with very rare or very frequent incidences of finding weapons should be the fastest. That is, with similarly slim odds of finding or not finding a target, people might be expected to make decisions rapidly about whether they see it or not.

But that's not what Wolfe and Van Wert's data revealed. Those who looked for items that were frequently present were quick to ID them (often creating false-positives when the items were not there), but those who had infrequent targets were much slower to dismiss a baggage sample as not containing a target. "When nothing is there, they don't give up on the response," Wolfe said. If people are bad at finding rare objects, why do we spend so long looking for them anyway?

"It's all terribly adaptive behavior for a beast in the world," Wolfe said. "If you know berries are there, you keep looking until you find them. If they are never there, you don't spend your time hunting."

But these ancient adaptations aren't always so beneficial today, especially for those who are charged with hunting for these rare—and sometimes dangerous—objects. "We aren't well-built for that and make more errors than we'd like," Wolfe said.

And looking for an unknown number of targets presents an even bigger dilemma. Searchers have to continue looking until they are satisfied with their hunt. This "'satisfaction of search' is a known problem in search for an unknown number of targets," the authors wrote. "The probability of detecting one target is lower if another target has been detected first."

All of this knowledge, however, could help those who search for objects, such as airport security personnel or medical technicians, to do a better job. Wolfe noted that pre-shift training in which a person was exposed to high frequency examples before they went on duty might help them find that rare hidden bomb or malignancy. Though it might not help you track down that pen you just misplaced.

Image courtesy of iStockphoto/Yakobchuk