The Unknown Unknowns – Differential Etiology And Idiosyncrasy

Donald Rumsfeld once justified attacking Iraq without firm evidence that his target possessed the ultimately apocryphal "weapons of mass destruction" by citing fear of the unknown:

Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns − the ones we don't know we don't know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.

That worked really well in Iraq. But it works even worse with differential etiology, often referred to, not particularly correctly, as "differential diagnosis." That's because the "differential" part of this testimonial technique is really just a form of circumstantial evidence - a "process of elimination." "Differential diagnosis is accomplished by determining the possible causes for the patient's symptoms and then eliminating each of these potential causes until reaching one that cannot be ruled out or determining which of those that cannot be excluded is the most likely." Guinn v. AstraZeneca Pharmaceuticals LP, 602 F.3d 1245, 1253 (11th Cir. 2010). It is "a process of identifying external causes by a process of elimination." McClain v. Metabolife International, Inc., 401 F.3d 1233, 1252 (11th Cir. 2005).

A differential etiology is a process-of-elimination approach to determining a subject's cause of injury. Under this method, an expert considers all relevant potential causes of the symptoms and then eliminates alternative causes. Although the parties and the district court below refer to this method as a "differential diagnosis," that term is really a misnomer. A "diagnosis" is concerned only about naming the condition or ailment, not establishing its cause.

C.W. v. Textron, Inc., 807 F.3d 827, 832 n.4 (7th Cir. 2015) (citation omitted).

"Unknowns" are a recurring problem with this "differential" process. If one doesn't know what the possible causes are, then it will be impossible to eliminate them from the equation. A significant level of idiosyncratic causation is fatal to the valid use of differential etiology.

Although differential etiologies are a sound methodology in principle, this approach is only valid if general causation exists and a substantial proportion of competing causes are known. Thus, for diseases for which the causes are largely unknown, such as most birth defects, a differential etiology is of little benefit.

Michael D. Green, et al., "Reference Guide on Epidemiology," in Federal Judicial Center, Reference Manual on Scientific Evidence 549, 618 (3d ed. 2011).

This technique is more accurately described as a "differential etiology." It is most useful when the causes of a substantial proportion of the disease are known. . . . When the causes of a disease are largely unknown . . ., differential etiology is of little assistance.

Restatement (Third) of Torts: Physical & Emotional Harm § 28, comment c(4) (2010).

The validity of a differential diagnosis depends on a substantial proportion of the independent causes of the disease being known. . . . A differential diagnosis is of limited utility when a substantial portion of the incidence of a disease is of unknown etiology.

Id. at Reporter's Notes to comment c(4).

The Third Restatement relies on commentary that has long understood that "unknowns" preclude the proper use of differential etiology:

[It] becomes particularly important when . . . one cannot assume that most causes of any given ailment are known. Otherwise, . . . the elimination of other risk factors would...