Got too many stamps? Baseball cards? Vintage cars? Blame your brain.

About 70 animal species, including rats and crows, hoard things—mostly food but occasionally useless objects such as beads. Primitive brain regions, including the hippocampus and amygdala, are involved, but in humans higher brain structures are at work as well. Steven W. Anderson, a neurologist at the University of Iowa, recently studied 86 people who had lesions in various well-defined areas; of the total, 13 were “abnormal collectors,” filling their homes with everything from junk mail to spoiled food or broken appliances. Although the subjects had average intelligence and reasoning ability, they would not stop hoarding nor allow anything to be discarded. This kind of defiant behavior can sometimes cause serious personal and even legal problems, such as eviction.

Using high-resolution magnetic resonance imaging, Anderson found that all 13 had suffered damage to the right mesial frontal region. When this particular area is injured, “the very primitive collecting urge loses its guidance,” Anderson says. He hopes to extend his work to defining the origins of normal collecting behavior.