Head injury as a cause of ADHD

I don't often read the British Medical Journal (this is a first), but there is an interesting article in the newest issue about the cause of attention-deficit hyperactivity disorder. It was previously believed that head injury in two-year olds was a factor, based on evidence that there is a correlation between incidents of head injury and ADHD later in life. The diagram below show the hypothesized causal relationship, in which head injury causes ADHD:

That makes a lot of sense. ADHD is a disorder of the brain, and hitting the head hard we know can result in all sorts of maladies of the brain. All that was left was to find the damaged part of the brain, and causation would have been established. But that was not to be.

New evidence also finds a correlation between burn injuries in two-year olds and ADHD later in life. The same line of thinking as with head injury might make one suggest that somehow burn injury causes ADHD, but most would probably agree that such an inference would be absurd. Another factor could be in play, such as "poor impulse control and increased risk taking behaviour" (this is a British journal, not an American jornal):

This makes a lot more sense in terms of head and burn injury. Unruly kids are of course more likely than average to be injured. However, who is to say that poor impulse control etc. is a direct cause of ADHD? At least as plausible is the explanation that some other factor, e.g. a developmental abnormality, causes both poor control and ADHD:

The authors conclude that while head injury is not a causative factor of ADHD, it may be a marker for subsequent diagnosis of ADHD.

This paper is a great example of the importance of getting causal relationships right. Correlation does not imply causation. It takes more than that, such as a plausible theory to explain the hypothesis. But even then we can't feel too certain that we're not fooling ourselves, as we were in this example until burn injuries was included in the study.

Via Politiken.

Reference:
H. T Keenan, G. C Hall, S. W Marshall (2008). Early head injury and attention deficit hyperactivity disorder: retrospective cohort study BMJ, 337 (nov06 2) DOI: 10.1136/bmj.a1984