A hypochondriac is someone who, given certain physical symptoms, is inclined to imagine the worst possible disease. If a doctor were to do that when examining a patient, he would be wrong most of the time. For that reason, there is a saying in medicine: “When you hear the sound of hooves, think of horses and not zebras.” In other words, think of the common causes for diseases, not those that are rare.

Some of my patients with health anxiety are always thinking of zebras. “Wouldn’t it be awful if I had pancreatic cancer, or cancer of the gallbladder?” they think, even when acid indigestion is much more common. The stakes in that situation are very high—the possibility of death—but the odds of having contracted those particular diseases are very low.

The tendency to focus on the stakes rather than the odds is a common human failing. It is the reason people buy lottery tickets. “Wouldn’t it be great if I won 50 million bucks?” Yes, but what are the odds? Chances are you could buy a lottery ticket every hour for the next hundred years without winning. With those odds, the stakes are not worth considering.

Our entire lives are governed by decisions that have a statistical quality, although we make these choices for the most part without consciously weighing the odds or even being aware that such a decision is being made. Some of these decisions are more obvious than others. Some of them are precautionary. When we put on seat-belts, choose to give up smoking, drive at the speed limit, visit the dentist, put on sunscreen, carry an umbrella, or duck when we walk through a low doorway, we are going to some trouble to avoid risk. The decision to undertake a medical treatment such as an operation is always understood explicitly in terms of risk-benefit. Sometimes because the risk is not obvious, or because it is so low, we don’t think of it consciously at all.

When we cross an intersection with the light—or against it—we have made a decision based on an estimate of risk. The choice of whether to live in a city or in suburbia, whether to buy a big car, or whether to talk back to a boss, are all made partially on the basis of an unconscious analysis of risk. Estimating risk is a matter of knowing the odds. Most of the time—with a few striking exceptions—we know the odds well enough to get through our daily routine without much difficulty. We have learned from the mistakes of others that it is not a good idea to go drag-racing or to eat mushrooms that we pick ourselves.

But we do not always act sensibly in such a way. Smoking cigarettes is probably the single most dangerous activity anyone can engage in day after day. The habit of smoking kills enough people to fill up a half dozen jumbo jets every few weeks. Why does a significant percentage of the population still smoke? Because most of the days that people smoke, they do not die. That has been their experience for years. They argue from their personal, past experience. What they read in reports of scientific studies seems remote from that day-to-day experience. Besides, cigarette smoking seems to them more or less under their control and is, therefore, less frightening—as opposed to panic attacks, for example, which are, in fact, harmless, but which appear dangerous because they seem out of their control. Also, cigarettes kill people out of sight one at a time without the dramatic impact of a jumbo jet crashing at major airport.

In some situations—airplane crashes are one—people tend naturally to focus on the stakes rather than the odds. “l know that mile for mile I’m safer in an airplane than traveling by car," someone might think, "but if the car crashes, I’ve still got a chance. If the plane explodes in the air, I’ve had it.” That person is focusing on the stakes—certain, sudden death—rather than on the odds, as he should. We live and die by the odds. If the odds are a million to one that the plane will not crash, what difference does it make how awful a plane crash would be? Yet the stakes do matter to people. We try to avoid very uncommon mistakes that are likely to be catastrophic.

Consider again the lottery. Millions of people buy lottery tickets every day dreaming about the stakes—a golden life with enough money to buy happiness, or at least a dozen expensive cars—and so they ignore the odds. They see the winners on television. But nobody interviews all the losers. The odds of winning a lottery are so small that buying tickets is equivalent to throwing away money. But it is a pleasant daydream, and no one wants to think about reality.

What is harder to understand is why some people dwell so obsessively on unpleasant daydreams—illness and death—when the odds indicate these outcomes are very unlikely. Health worriers are concerned endlessly with the stakes—cancer, heart attack, AIDS, a lonely and painful death in a hospital—and forget the odds. Or they do not know the odds. They argue from their daily experience—a neighbor who just had a heart attack—or today’s obituaries; and they think these events are more common than they really are. Nobody comments on all the people who did not die yesterday or did not get sick today. They worry for the same reason smokers do not worry: their personal experience has led them astray.

In addition, health worriers are superstitious. They may think they have been singled out for bad luck. If the chance of their having AIDS is one in 1,000, they think they will get it.  Also, they think one in 100,000 is not much different than one in 1,000.  A number of years ago, I had an odd experience that brought home to me just how difficult it is to interpret odds. In 1981 my youngest son received two blood transfusions during his first attack of ulcerative colitis. A few months later the first reports of AIDS appeared in the medical literature. I was naturally concerned that he might have contracted AIDS as a result of receiving the blood. I called the local blood bank which assured me the odds were “one in a million,” but I was not comforted since at that time no one really knew what the odds were. In the next few years some statistics were reported, and I was able to figure out that the odds were actually about one in 20,000. I found these really very low odds not very reassuring somehow; and on those occasions when my son got sick with anything at all, I found myself worrying about AIDS. Additional, less optimistic, statistics were reported, and I figured the odds again at, perhaps, one in 1,000. Finally, taking into consideration every unfavorable circumstance I could imagine, construing every medical report in the worst way, I figured that the odds rose to one in 200. I discovered, then, unexpectedly, that the level of my concern dropped considerably. It took me a while to understand why. The answer, I realized, was that I knew from my own experience how unlikely one in 200 was. All the rest were simply numbers to me.

I did not ask my son to submit to an AIDS test when it became available since I knew the odds of his having the disease were exceedingly low and I did not want to put him—and me—through the psychological difficulties attendant upon unnecessary medical testing, especially when no effective treatment was available. Now, many years later, the odds of his having contracted AIDS so long ago are about one in 4,000,000.

The trick, then, is first of all, to start thinking about the odds. If you have a swollen lymph node under your chin, what is the chance of it being due to a lymphoma compared to the chance of it being due to a sore throat? If you have a breast lump, what are the chances of it being cancer rather than a cyst? If it is a solid tumor, what are the chances of it being cancer rather than a benign fibroma? If it is cancer, what are the chances that it has already spread, rather than being still localized? If it has spread, what are the chances of it being lethal quickly rather than the kind that may kill only 20 years later? Often the incidence of a particular disease is reported in a medical textbook. Sometimes a doctor can give you a rough guess. Remember, the odds change for each condition depending on age and sex. Most illnesses are benign. Most of us get only one fatal disease per lifetime.

Second, you must recognize that the statistical laws that govern everyone else’s life also rule yours. If you have been sick many times in the past, it does not mean you are more likely to get sick in the future, unless you have one of those immunological illnesses that predispose to getting sick. Third, since, like me, you may have trouble visualizing the odds. I suggest the following exercise: make single marks on a piece of paper until you have reached 1,000 marks. It takes 10 minutes. Look at that piece of paper. You will get a feel for how unlikely it is that you will be one in 1,000. One in 10,000 is 10 times less likely. You can mark that off in less than an hour and a half.

If your chance of getting a particular disease is even less, you can post similar pieces of paper, unmarked, on the wall next to each other in order to get a feeling for just how unlikely these conditions are.

(c) Fredric Neuman  Follow Dr. Neuman's blog at fredricneumanmd.com/blog/ or ask advice at fredricneumanmd.com/blog/ask-dr-neuman-advice-column/

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