Why We Touch Our Mouths So Much: Forewarned is Forearmed

When I taught Introductory Psychology, I came across a study in which researchers put people in a room with food and watched them. They were looking for cycles in eating and drinking. They noticed that their subjects spent a lot of time touching the face near their mouth — what they called “the snout area”. After I read that, I noticed the same thing countless times. Right now I am at an airport waiting for a flight. Looking around, I see three of about 50 people touching their mouth or nearby.

Why do we do this? I propose an evolutionary explanation:  To expose our immune system to all the germs near us in small amounts. Mouth-touching is part of a larger sampling process:  1. We touch many things constantly. In particular, we shake hands, hug, and otherwise touch people near us. Germs that have managed to live in or on other people are the most dangerous. 2. We lick our lips often, moving germs on our lips inside our mouths. 3.  When you eat, food transfers bacteria from the inside of your mouth to your tonsils, which circle your throat. Tonsils are full of lymphocytes, the immune-system cells that detect germs. Once we have developed antibodies to a microbe, of course, we are much less vulnerable to it. The whole sampling process is a kind of self-vaccination.

We need conventional vaccination when self-vaccination fails. Polio vaccination was the first big vaccination program, and it worked: polio was nearly wiped out.  Before around 1900, polio was not a big problem. It became a big problem at roughly the same time that public health measures and the replacement of horses by cars caused cities to become much cleaner places. Others have theorized that this is why polio became a big problem. As recently as 1951, thousands of children died from polio.

This is related to but different than my ideas about our need for fermented food. (I believe we need to eat plenty of fermented food, day after day, to be healthy.) When we eat fermented food, we ingest large amounts of bacteria that  are familiar and safe. The amount is large because the food has been fermented. The bacteria are familiar because we eat the same food repeatedly. They are safe because the insides of our bodies are dramatically different than  what we eat (e.g., different temperature).  The sampling system I am proposing here exposes us to small amounts of unfamiliar dangerous bacteria. However, this sampling system and the factors that push us to eat fermented food (our liking for complex, sour, and umami flavors) both act to produce the best environment for our immune system. Fermented food resembles exercise and practice; the mouth-touching system resembles information.

A similar sampling system is our love of gossip. We love to hear it, we love to spread it. Gossip spreads information about the dangers around us. Again, forewarned is forearmed.

I am in Tokyo (for a few more minutes), an admirably clean city. Public rest rooms, for example, are convenient, clean, and free. (Unlike New York, Los Angeles, San Francisco, Beijing . . . ) The practical point of this idea isn’t that there is something wrong with public health measures, it is that they can go too far.











12 Replies to “Why We Touch Our Mouths So Much: Forewarned is Forearmed”

  1. Years ago, I recall a doctor made the rounds in the media claiming that nose picking is essentially the same thing.

  2. I have seen studies relating the increase in nut and other allergies to our modern obsession with cleanliness. When I was a kid in grade school, there was that kid in some other class or maybe in another grade that you had heard about having a nut allergy that would kill him if he came within 20 feet of a peanut but they were almost a myth. Now it seems every third kid has some kind of allergy issue. Maybe it is perception on my part.

  3. @Scott — part of the problem is that food allergies seem to be “trendy” now. Many people who think they’re allergic to foods are really not allergic to them:

    J Psychosom Res. 1999 Dec;47(6):545-54.

    Psychological characteristics of people with perceived food intolerance in a community sample.

    Knibb RC, Armstrong A, Booth DA, Platts RG, Booth IW, MacDonald A.
    School of Psychology, University of Birmingham, UK. rebecca.knibb@nottingham.ac.uk

    In most adults who believe themselves to be food intolerant there is no objective supporting evidence. It has therefore been proposed that the misperception of intolerance to food is linked to psychiatric illness or personality disorder. This hypothesis was tested in a community-derived sample of individuals who attributed an adverse symptom to a type of food. A random mailing recruited 955 participants aged > or =18 years, of whom 232 perceived themselves to be food intolerant (PFI). All recruits were sent two questionnaires, the General Health Questionnaire-28 (GHQ-28) and the shortened version of the Eysenck Personality Questionnaire (EPQ-R). A total of 535 GHQ-28 and 518 EPQ-R forms were returned that were correctly completed, an overall response rate of 55%. For the subscales of the EPQ-R, neuroticism was greater in those with a PFI than those without. Women with a PFI were more extroverted than control women. For the GHQ-28 subscales, women with a PFI had significantly higher scores than control women on somatic symptoms, anxiety, insomnia, and severe depression. There was a greater percentage of psychiatric caseness among women with a PFI than among men with a PFI or control women. Nevertheless, this percentage was no greater than that reported among a reference sample derived from NHS and university staff. It is concluded that perceived food intolerance is associated with psychological distress in women with a PFI, and neurotic symptoms in both men and women with a PFI, but there is no greater prevalence of psychiatric disorder among women or men with a PFI than there is in some professional groups.
    PMID: 10661602 [PubMed – indexed for MEDLINE]

  4. Much more plausible explanations:

    1. the mouth region, for obvious reasons, is one of the most sensitive and nerve-dense regions; face-touching is about dealing with itches and transient discomfort, in the same way people knead their fingers or scratch their hands or what not. (Look around, how many people are doing fiddling with their fingers in some way? I thought so.)
    2. Body language; the face in general is a major source of cues about one’s state. One’s hands can hide the face. Such obscuration is very useful in public.
    3. The head is fairly heavy and modern people do not get much exercise. It’s simply about resting the neck muscles.

    Germs… I would be quite surprised if that turned out to be the explanation. Well, at least it’s testable (manipulate immune systems with immunosuppressants, etc).

  5. I’d always enjoyed mixed nuts but at about age 60 they started upsetting my stomach. I stumbled across the explanation in Jim Watson’s book on DNA – he alluded to an allergy to the protein in Brazil nuts. Now if I stick to almonds, hazelnuts and walnuts, all is well. One Brazil seems to do little or no harm – a handful is bad news. To my surprise, when I checked the wikipedia page on Brazils no mention was made of the problem.

  6. Seth, you said:
    “I propose an evolutionary explanation: To expose our immune system to all the germs near us in small amounts.”

    So exposure to lots of germs is good for our immune system?

    In recent years, each time that I have been in a hospital it takes them about 2 days to give me a staph infection, 2 more days to give me MRSA, and another 2 days to give me VRE.

    If being exposed to germs is good, then I must be getting very healthy from all these hospital-acquired infections.

    But maybe I could get the immunity-conferring benefits without the inconvenience of a hospital stay: Maybe I could just sit in a hospital ER waiting room for an hour a day, and get acquainted with lots of germs.

    1. So exposure to lots of germs is good for our immune system?

      Lots of germs in small amounts is good. You want your first exposure to Germ X to be a small amount, not a large amount. If it is a small amount, you will survive later exposures to large amounts of the same germ. If your first exposure is to a large amount of Germ X, well, good luck.

  7. One aspect to keep in mind is that we may touch our face as an act of self soothing. Dacher Keltner had his students go to restaurants and observe people as they entered — more than 80% of the time, they touched their face. My impression is that when entering a room with a crowd of strangers, it can feel rather overwhelming; touching your face can ground and soothe you, or at least it can me

    Notice the woman in the bottom right who is holding her cheeks:

    I think I touch my face to make myself feel more secure — this does not contradict other motivations/effects necessarily of course.

  8. It’s interesting that usually the first response to eating a novel food is NOT having an allergic reaction to it.

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