Checking behavior is a symptom of several diseases, including Alzheimer's and Obsessive Compulsive Disorder. A watered-down version of this debilitating symptom can be found in "normal", everyday people. I am a checker.
The very attributes that make me a decent scientist, cook, and planner are also responsible for keeping me up at night. When your mind races even at rest and when you are programmed to constantly look for connections between seemingly unrelated concepts, you too may become a checker.
For instance, every night I ask my husband if he locked the doors, even if I actually witnessed him locking them. I then ask, after he says yes, if he is sure that he locked the doors. Then, I roll over and squint my eyes (I really need to check out Lasik eye surgery) to see if the alarm has been set, even if I had just set it myself moments before. I then lie in bed and go through my day. Did I remember to do this, did I turn off that, did I email so-and-so, did I lock the lab door.....
I may be a checker because, on more than one occasion, I have found the coffee pot in the refridgerator, the ice-cream in the pantry, and my toothbrush on my vanity. If you are smiling, you know where I'm coming from. At least 50-times-a-day I check my pockets to make sure I still have my keys. Did I mention that I have yet to turn 30? Did I mention that I successfully multitask multiple projects every day?
It's a chicken or the egg question. Am I a checker because of the high demand placed on my brain circuitry, or do I have an efficient mind because I'm a checker.
By the way, I'm not OCD. So next time you are worried because you think you, your child, your spouse, your crazy neighbor, or your evil grandmother are ADHD, Bipolar, or whatever, take a deep breath and calm down. There's a good chance you are instead witnessing the concept of inherent personality. Ask anyone who really has one of these diseases for their story, and you'll understand the difference. I'm saying this because, as much as we want to believe that seemingly odd or counter-intuitive behaviors are actual diseases, sometimes they're not. Sometimes it's easier for us to attribute behaviors to disease than to accept the truth about ourselves. Hitler was not schizophrenic -- he was a reflection of what power and greed and hate can do to a person given the right (or, actually, wrong) personality. Be careful who you vote for and who you befriend. Stick with the checkers.