Recall bias, or how I learned to stop worrying about measles and love my vaccination beliefs

The multi-state outbreak of measles in the US, which originated at Disneyland theme parks in California, has received much attention in the press recently. Sadly, the event isn't terribly surprising: many people, including children, lack immunity. The reasons for this are varied, and include the MMR vaccine being recommended for children over 12 months of age; persons not completing the recommended vaccination sequence; waning immunity in older persons; and lack of vaccination.

This latter issue is related to the anti-vaccination movement and this dimension is receiving attention in the media. Some of the discourse is revealing. One example appeared in the New York Times recently, which quoted a Santa Monica pediatrician who has cautioned against the way vaccines are used in the past (but does administer the vaccine in his practice):

“I think whatever risk there is -- and I can’t prove a risk -- is, I think, caused by the timing,” he said, referring to when the shot is administered. “It’s given at a time when kids are more susceptible to environmental impact. Don’t get me wrong; I have no proof that this vaccine causes harm. I just have anecdotal reports from parents who are convinced that their children were harmed by the vaccine.”

Perhaps such comments could be rephrased more bluntly: I believe that the vaccine could be dangerous to kids because people who know less about medicine and epidemiology than I do tell me, with great conviction, that they think it's dangerous. Even blunter still might be: The idea that vaccines can be dangerous has a high truthiness, so count me in. Or even, After talking to some patients I've decided to stop worrying about measles and love my unfounded beliefs about vaccination.

I've written before about the importance of understanding those who subscribe to anti-vaccination notions. Statements like this from a physician illustrate how much work has to be done. Several years ago Delgado-Rodríguez and Llorca wrote a very nice continuing professional education paper on bias in epidemiology, which physicians with similar beliefs should read. The following passage is especially important:

Recall bias: if the presence of disease influences the perception of its causes (rumination bias) or the search for exposure to the putative cause (exposure suspicion bias), or in a trial if the patient knows what they receive may influence their answers (participant expectation bias). This bias is more common in case-control studies, in which participants know their diseases, although it can occur in cohort studies (for example, workers who known their exposure to hazardous substances may show a trend to report more the effects related to them), and trials without participants’ blinding.

If someone wants to find a cause for a medical event, they will. The plural of "anecdote" is not "data", no matter how convincing the anecdotes may seem.

(image source: Wikipedia)

On truthiness, celebrities, and math

Katy Waldman recently published an article discussing how people's minds tend to grasp at the "low-hanging cognitive fruit" in daily life. She describes how sometimes we accept ideas as facts according their "truthiness" (a term coined by Steven Colbert in 2005). It's an interesting article and I recommend reading it. It makes me wonder about the role of truthiness in health-related behavior.

Colbert has described the notion of truthiness:

It used to be, everyone was entitled to their own opinion, but not their own facts. But that's not the case anymore. Facts matter not at all. Perception is everything. It's certainty. . . . Truthiness is "What I say is right, and [nothing] anyone else says could possibly be true." It's not only that I feel it to be true, but that I feel it to be true. There's not only an emotional quality, but there's a selfish quality.

Waldman surveys some of the evidence for truthiness: how people, instead of analyzing data critically to draw conclusions, sometimes accept ideas based on seemingly unrelated criteria, like the aesthetic presentation of a written message or the familiarity of a message bearer's name.

Within the realm of health behavior, truthiness can be devastating. In epidemiology, for example, it is often said that people who suffer from a condition tend to look for and find a cause, whether one exists or not. Truthiness reminds me of Jenny McCarthy's views on vaccination and autism, which she described during an interview on The Oprah Winfrey Show:

Winfrey: So what do you think triggered the autism [in your son]? I know you have a theory.
McCarthy: I do have a theory.
Winfrey: Mom instinct.
McCarthy: Mommy instinct. You know, everyone knows the stats, which being one in one hundred and fifty children have autism.
Winfrey: It used to be one in ten thousand.
McCarthy: And, you know, what I have to say is this: What number does it have to be? What number will it take for people just to start listening to what the mothers of children who have autism have been saying for years? Which is that we vaccinated our baby and something happened. . . .
McCarthy: Right before his MMR shot, I said to the doctor, I have a very bad feeling about this shot. This is the autism shot, isn’t it? And he said, “No, that is ridiculous. It is a mother’s desperate attempt to blame something on autism.” And he swore at me. . . . And not soon thereafter, I noticed that change in the pictures: Boom! Soul, gone from his eyes.

The post hoc, ergo propter hoc fallacy of the passage has a high truthiness to many, even though scientific bodies have debunked the notion that vaccination causes autism.

How do "truthy" fringe ideas persist and grow in the general population? Deffuant and coworkers in 2002 published a study applying agent-based modeling to analyze the propagation of extremist views. Nigel Gilbert's book describes the study succinctly:

In Duffuant et al's model, agents [individuals] have an opinion . . . and a degree of doubt about their opinion, called uncertainty . . . An agent's opinion segment is defined as the band centered on the agent's opinion, spreading to the right and left by the agent's value for uncertainty. Agents interact randomly. When they meet, one agent may influence the other if their opinion segments overlap. If the opinion segments do not overlap, the agents are assumed to be so different in the opinions that they have no chance of influencing each other. If an agent does influence another, the opinion of one agent (j) is affected by the opinion of another agent (i) by an amount proportional to the difference between their opinions, multiplied by the amount of overlap divided by agent i's uncertainty minus one. The effect of this formula is that very uncertain agents influence other agents less than those that are certain.

 . . . The model shows that a few extremists with opinions that are not open to influence from other agents can have a dramatic effect on the opinions of the majority . . .

In the case of vaccination and autism, the high truthiness (to some) of the idea that MMR vaccine causes autism produces a group of people very certain in their beliefs. Duffuant et al, working within the context of political ideas, show that such a group can impact the opinions of others, thereby propagating their ideas.

Perhaps similar dynamics apply to the case of vaccination, and potentially other health-related memes such as the raw milk movement. I also wonder about the truthiness of the local myths surrounding Ebola and how those might spread more widely, potentially affecting public health in outbreak areas adversely.

(image source: Wikipedia)