An excerpt from the NY Times:

If Carole Johnson, a retired school administrator who lives near Sacramento, Calif., happens to have a distressing thought while passing through a doorway, she needs to “clear” the thought by passing through the door twice more, doing it precisely three times.

My own challenge is fighting the urge to return to my parked car and check yet again that the parking brake is secure. If I don’t, how can I be sure my car won’t roll into something — or worse, someone?

To many of us with obsessive-compulsive disorder, those pleasures are invisible. We walk into a calm and civilized dining room and see things we won’t be able to control. This feeds directly into one of the unifying themes of the disorder: an often crushing inability to handle the unknown.

“The common thread, I think, has something to do with certainty,” said Dr. Michael Jenike, medical director of the Obsessive Compulsive Disorders Institute at McLean Hospital in Belmont, Mass., which is affiliated with Harvard Medical School. “If you have O.C.D., whatever form, there seems to be some problem with being certain about things — whether they’re safe or whether they’ve been done right.”

If lack of certainty is our common challenge, than warding off uncertainty is our common quest. For some of us battling obsessive-compulsive disorder, that means scrubbing our hands to make sure they’re clean, or checking and re-checking everything around us in the name of safety. For others, the need is to arrange various items in order, or repeat actions in ritualized sequences in vain attempts at removing doubt.

These quirks lead to some serious complications in our lives, especially when we find ourselves in a place that triggers obsessive-compulsive behavior, like a restaurant. Once Ms. Johnson gets past the door, she often needs to try out a few tables, looking for one that feels right, as a frustrated maître d’hôtel looks on.

Personally, I am fine with just about any table, although the wobbly ones can spell big trouble. I have harm obsessions, which means I am plagued by the fear that other people will be hurt by something I do, or don’t do. Seated at a less-than-sturdy table, I conjure images of fellow diners being crushed or otherwise injured should I fail to notify the restaurant’s management. This is called a reporting compulsion in the vernacular of the disorder, and before I learned to fight these urges, many a manager heard from me.

One of them was the woman running a coffee house I frequent. One day while sipping my latte at a fake-marble table I leaned forward, and the far end of the tabletop lifted. This barely moved my coffee cup, but it sent my nerves right through the roof. Before I realized it I was crouched over, my head upside down beneath the table. The only responsible thing to do, I decided, was to ask the woman behind the counter to come over for a look. Her lack of concern only exacerbated my problems.

Forget the tabletop, my friend Matt Solomon tells me; it’s what’s on top of the table, and precisely where, that really matters. Mr. Solomon is a 39-year-old lawyer in Fort Worth with order compulsions. To enjoy a meal he needs to separate the salt and pepper shakers, and, ideally, place a napkin holder or other divider midway between them.

Why? He can no more answer that than Ms. Johnson can tell you why she needs to chew her food in sets of three bites or drink her beverages three sips at a time. Three is her magic number. That is about as refined an explanation as any of us can give for our compulsions, rituals that we understand are entirely illogical.

Some of our other concerns may seem familiar. I imagine most diners, for example, have noticed and perhaps even struggled to remove white detergent spots that can sometimes be seen on silverware. But few, I suspect, have gone to the lengths Jared Kant has to get rid of them. Mr. Kant is a 24-year-old research assistant living outside of Boston who has obsessive fears of contamination. (He first came to my attention when I read a memoir he wrote about living with obsessive-compulsive disorder.) Last year he visited a Chinese restaurant with several friends, one of whom pointed out that their silverware was spotted and seemed dirty. Mr. Kant collected all the utensils at the table and attempted to sterilize them by holding them above a small flame at the center of a pu-pu platter, quickly attracting the attention of their waiter.

Help is available, in the form of a therapy called exposure response prevention. As the name suggests, the technique calls for exposing people with obsessive-compulsive disorder to situations that trigger obsessions, then preventing them from acting on them. The therapy addresses low-level anxieties, and works up from there.

With restaurant cleanliness, for example, a therapist might have an client rate his anxiety about challenges ranging from simply touching spotted silverware to eating from a spotted plate. Then the therapist would ask him to face those situations while fighting the compulsion to clean or replace spotted items.

The therapy attempts to alter behavior, but it appears to alter much more than that. Dr. Sanjaya Saxena, the director of a program for obsessive-compulsive disorders at the University of California at San Diego, said that exposure response prevention therapy “certainly is changing the brain at the molecular level — that is, at the level of particular proteins that are expressed and created and on the level of neurotransmitter function.” In that sense, he said, “behavioral therapy is biological therapy.”