When people look for things that are rare, they often fail to find it and when looking for something common, they think they see it even when it isn't there, a new study has revealed.
Lead researcher Jeremy Wolfe of Harvard Medical School helps explain why this happens - something which could lead to some simple methods to help airport security personnel looking for weapons and radiologists looking for tumours get better at their jobs.
"We know that if you don't find it often, you often don't find it," said Wolfe.
Rare stuff often gets missed. That means that if we look for 20 guns in a stack of 40 bags, we'll find more of them than if we look for the same 20 guns in a stack of 2,000 bags.
The study also showed that people do send false alarms when looking for common items even if the object is not there.
"When nothing is there, they don't give up on the response," said Wolfe.
"It's all terribly adaptive behaviour for a beast in the world. 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 that adaptive inclination in nature can cause problems when people start looking for rare things, like guns in baggage or breast cancer.
Airport screeners know there probably isn't a gun in your bag, and radiologists know that a tumour probably isn't going to be there, but they really want to catch it if there is.
"We aren't well-built for that and make more errors than we'd like," Wolfe added.
He thinks that there may be ways to solve this problem, or at least to improve upon our searching skills.
He said that his team suspects error rates may be lowered by offering people in jobs like these some simple retraining at the start of every shift.
If they spend a couple of minutes doing a simulated search for common weapons or tumours, they might then do a better job at really finding rare ones for the next 30 minutes or so.
The study appears in journal Current Biology.