"The average person misplaces up to nine items a day" and spends "an average of 15 minutes" looking for them, reports Sumathi Reddy in The Wall Street Journal (4/15/14). That’s based on a 2012 "online survey of 3,000 people," which maybe you remember (we couldn’t find the link, sorry). The most commonly lost items are "cellphones, keys and paperwork." The good news is that this type of "forgetfulness isn’t a sign of a serious medical condition" like dementia or necessarily a sign of growing older — "it is the norm for all ages."
It is possible that forgetfulness is a sign of "depression and attention-deficit hyperactivity disorders," but Daniel L. Schacter of Harvard offers a relatively more benign explanation: "It’s the breakdown at the interface of attention and memory," he says. "That breakdown can occur in two spots: when we fail to activate our memory and encode what we’re doing … or when we try to retrieve the memory." It helps if "your state of mind at retrieval" is the same as it was "during encoding."
For example, if you were hungry when you put down your keys, but full when you go looking for them, "the memory may be harder to access." Meanwhile, "a recent study … found that the majority of people surveyed about forgetfulness and distraction had a variation in the so-called dopamine D2 receptor gene (DRD2) leading to a higher incidence of forgetfulness. According to the study, 75% of people carry a variation that makes them more prone to forgetfulness … About half of the total variation of forgetfulness can be explained by genetic defects."