Correlation, causation, vaccination

By Megan McArdle

You would have to be harder hearted than I to ignore the anguished search of parents of autistic children for the cause of their child's condition. "I saw it," they say; "one shot, and then the child who had talked and laughed started retreating into himself." It's hard to argue with someone's pain.

Nonetheless, I'm going to. Our brains are designed to learn by associating events that happen at the same time, or in close sequence. When you were a little kid, that's how you learned that if you touched the stove, you got burned, long before you understood how combustion worked. We have other ways of learning as well, but that temporal link is the strongest, most primal association. We go through our whole lives looking for those connections.

This is not a bad heuristic, but of course, it often leads one astray, which is why there are so many ridiculous "cures" for hiccups. The fact that your child regressed after having a shot doesn't mean that the shot caused the regression. It suggests a theory . . . but that theory has been tested, and found wanting.

The anti-vaccination websites sustain their belief by systematically excluding anyone offering counterevidence from the domain of acceptable sources. Pharma studies can't be trusted because they have a profit motive. The CDC is in hock to big business. The "medical establishment" wants to make money giving your children unnecessary shots. In fact, the only person you can trust is the guy writing the website.

This is the sure sign of a crank. It is possible that all these people are wrong--science has had much more spectacular failures in the face of clear evidence. But there is no such thing as a multi-million person conspiracy.

Those sites pick apart the studies that show no link between vaccinations and autism, while ignoring much more glaring problems with the studies supporting that link. The state of medical research is indeed pretty horrifying to people who do other kinds of research, thanks to a combination of human subject problems and generally abysmal statistical practice. But the problems are even more likely to be found in the poorly funded, inadequately staffed, and usually ineptly designed studies purporting to show a link between vaccination and autism.

A doctor who has been working with autistic children for decades offers a very interesting paper on the topic, which I found via Blissful Knowledge. He reports that well before the vaccine theory, parents always identified something to causally link with their child's late onset autism:

On the Children’s Unit at WMHI we had both forms of Autistic Disorder. In some children the clinical signs and symptoms of autism had been present from birth. In others, the child was quite normal (neurotypical) at birth and reached developmental milestones, including language acquisition at the usual times and in the usual manner. But then at age two, three or four, a conspicuous regressive process began robbing the child of all of that natural progress. Interestingly, in these late onset cases the parents all had some sentinel event that, in their mind, accounted for the cause of this dreadful regressive pattern: “ever since he fell off the pier and nearly drowned”; “the time he got trapped in the silo”; or “ever since he went into the hospital to have his tonsils removed”.

The point is that there is a natural tendency on the part of parents to seek out and blame some event or procedure for the onset of such startling regression in a child who has otherwise been developing normally. Dr. Down called that regression the “loss of wonted brightness”. Dr. Down attributed this regression to the “second dentition”. Obviously, in seeking causes, one has to separate out temporal relationship to causal relationship.
. . .
I saw cases of late onset autism well before there was an acceleration of vaccine schedules to their present levels. And in each of those cases the parents pointed toward some special event that in their mind was responsible for the onset of the regression. A full century earlier Dr. Down described cases of late onset autism well before there were any vaccination schedules at all. As I pointed out above, he ascribed that regression, temporally at least, to the ‘second dentition’.



Looking for those links is entirely natural. But fingering vaccines has real and terrible consequences. Millions of children die worldwide every year from childhood diseases that we've eliminated here through vaccination. Now, because these websites are frightening people about vaccination, we're seeing a resurgence of those diseases. People are dying from them again, and others are being left with permanent health impairment. Leaving children unvaccinated means going back to

  • Leg braces and iron lungs for people with polio (57,628 cases in 1952)

  • Encephalitis and sterility for people with mumps (200,000 cases a year in the 1960s)

  • Congenital rubella syndrome for children whose mothers contracted the illness during pregnancy.

  • Blindness, pneumonia, encephalitis, and death--one per thousand--for people with measles (nearly 1 million cases a year in the US before vaccines).

  • Encephalitis and pulmonary hypertension for people with whooping cough--thanks to people who don't vaccinate their kids, in 2001, 17 people, mostly infants, died of pertussis (200,000 cases in 1940).

  • Cardiac arrest and paralysis for people with diptheria (207,000 cases and about 15,000 deaths in 1920).



The vaccines scare us because the diseases don't. And they don't because of the vaccines.

This article available online at:

http://www.theatlantic.com/business/archive/2008/03/correlation-causation-vaccination/3087/