people estimate the probability of an event or the frequency of a kind of thing by the ease with which instances come to mind.11 In many walks of life this is a serviceable rule of thumb. Frequent events leave stronger memory traces, so stronger memories generally indicate more-frequent events: you really are on solid ground in guessing that pigeons are more common in cities than orioles, even though you’re drawing on your memory of encountering them rather than on a bird census.
