Why do some people hoard tons of junk while others can keep their lives clutter-free? Why are some people avid collectors of anything from stamps to beermats? American scientists think they have located the region of the rain that is responsible for hoarding behaviour- it's called the mesial prefrontal cortex, a tiny region found around half-way back in the brain. When they studied this region in pathological collectors, people who collect often value-less junk obsessively, they found the patients had damage to this area. Most people can discriminate between things that are worth collecting, and things that have no value. But when this part of the brain gets damaged or plays up, this discrimination is lost. Perhaps natural variations in this region could account for stamp collecting habits or even trainspotters!