Why rare objects are harder to find

Scientists have uncovered the scientific reason why hunting for a needle in a haystack is so hard - and often fruitless.

If you're a little bit scatterbrained when it comes to organising things and often spend an evening rifling through bits and pieces which, of course, were filed away in a "safe place",you can probably identify with the premise of this study.

It's said that when you're looking for unusual objects you're not as good at finding them. So, if you're on the security staff at an airport and you have 600 bags to scan, of which 15 contain some sort of weapon, you're much less likely to find all 15. Proportionate to the number of things, searchers will make more errors if the object they're looking for is rare. According to lead researcher Jeremy Wolfe, "If you don't find it often, you often don't find it."

The reverse is also true if you're looking for something that occurs more often, such as sun lotion, in which case you might think you can see it when it isn't there.

InCurrent Biologythis week, the researchers from Harvard have been looking at why this happens. They used two groups of between 12 and 13 volunteers to search through some computer-generated bag contents.

Common sense might tell you that, because something's rare you need to look through more boxes or bags and your brain just gets used to saying 'it's not there' all the time. And so you start to miss the thing you're looking for.

But the new study shows that it's actually the "Ooh, it's there!" response, which happens more slowly with rare objects.

This is an adaptive behaviour so that, in terms of our ancestors, if you were foraging for food you'd be more likely to stay in areas where it was plentiful and less likely to stray into areas where it was rare.

The findings of the study could be useful, say the researchers, in training security staff at airports or even for helping radiologists to spot more tumours on scans. According to the team, if people doing these sorts of jobs 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.