Rare objects harder to find

Screeners searching for weapons like guns or bombs are more prone to error when the incidence of such threats is small, say US researchers.

When people look for something rare - like a gun or knife or an explosive device hidden in a suitcase - they often have trouble spotting it, researchers have found.

The reverse is also true. When something is very common, people tend to see it everywhere they look, even when if it is not there, says Professor Jeremy Wolfe of Harvard Medical School, whose study appears in the journal Current Biology.

"It is clear that if you don't find it often, you often don't find it," he says.

That means that if you look for 20 guns in a stack of 40 bags, you'll find more of them than if you look for the same 20 guns in a stack of 2000 bags.

"We really want to understand why that is happening," says Wolfe.

For the current study, Wolfe and his colleagues worked with the US Department of Homeland Security's Transportation Security Laboratory. The department sent Wolfe's team images of empty suitcases and images of items typically found in them.

"We have software that basically packs bags. It takes an empty bag and throws some clothes in it and maybe some odds and ends - and maybe it throws a gun or a knife in there," Wolfe said. "We either had those show up 50% of the time or 2% of the time," he says.

Then they recruited 13 volunteers to look for guns or knives in the bags.