Are "Exclusions" Deductive and "Identifications" Merely Probabilistic?

Lately I have heard people say that “source exclusions” are the product of deductively valid reasoning, whereas “source identifications” are less certain. But the difference between such conclusions does not arise from the fact that one is deductive and the other is not. On reflection, "exclusions" are no less probabilistic in nature than "identifications." 

Presumably, the reason people may think that exclusions are deductions is that “exclusion” can be part of a deductive argument. For example,

(1) No human being with Type O blood will leave Type A blood at a crime scene.
(2) Defendant has Type O blood.
(3) The crime-scene bloodstain is Type A.
Therefore,
(4) Defendant is excluded as the source of the stain.

This argument is a formally valid deduction. If the premises (1)–(3) are true, the conclusion (4) must be true. There is, however, no guarantee that any or all the premises are true. Perhaps something very strange (but not logically impossible) happened to convert a Type O stain into a Type A one. Or perhaps the defendant or the stain was mistyped. These are not very likely events, but deductive logic does not make them true. So although (4) is certain to be true conditional on (1)–(3), we cannot be absolutely certain that (4) is in fact true. In the symbolism of probability statements, the fact that Pr[(4) | (1)&(2)&(3)] =1 does not ensure that Pr(4) = 1 unless Pr(1) = Pr(2) = Pr(3) = 1.

One might think that I have misstated the argument. Indeed, the argument ending in exclusion might be reframed as follows:

(1) Everything we know tells us that no human being with Type O blood will leave Type A blood at a crime scene.
(2) A blood test shows that defendant has Type O blood.
(3) A blood test shows that the crime-scene bloodstain is Type A.
Therefore,
(4) The blood test excludes defendant as the source of the blood stain.

This too is a deductively valid argument. The premises entail the conclusion, and the premises are even harder to dispute than the ones in the previous example. But if this is all that a criminalist means by an exclusion, then an exclusion is not actually a statement that the defendant is not the source of the trace. The conclusion in our second argument only asserts that the test has excluded the defendant. Unless the test never errs, it does not follow (deductively) that the defendant was not the source of the bloodstain. To appreciate the force of the "deduction," we need to study how often criminalists report exclusions when examining items from different sources as opposed to items from the same source.

The situation is the same for “identification.” This conclusion also can come at the end of a deductive argument. For example,

(1) Fingerprints from the same finger always match.
(2) Fingerprints from different fingers never match.
(3) The questioned and known fingerprints being compared match.
Therefore,
(4) The fingerprints being compared are from the same finger — an “identification.”

As with an exclusion, the argument from (1)–(3) to (4) is logically impeccable. If (1)–(3) are true, then so is (4). And, once more, because propositions (1)–(3) might not all be true, the truth of the “identification” is not absolutely certain.

Again, we can rephrase the argument in a (vain) effort to make it appear that the desired conclusion is purely deductive:

(1) Fingerprints from the same finger always match.
(2) Fingerprints from different fingers never match.
(3) The questioned and known fingerprints being compared match.
Therefore,
(4) I have identified the questioned print as coming from the finger that left the known print.

But again, the deductive argument does not get us to the conclusion that the known finger is the source of the questioned print. To ascertain the probative value of a positive source classification (an “identification”), we need to study the performance of criminalists making these source attributions. We need to study how often criminalists report “identification” when examining items from the same source as opposed to items from different sources. In the end, if there is a difference in the certainty we can attach to an exclusion as opposed to an identification, it does not emanate from the difference between inductive and deductive forms of argument. It results from the fact that the premises of some inductive arguments are more probably true than the premises of other inductive arguments.

Reference

1. Brian Skyrms, Choice and Chance: An Introduction to Inductive Logic (4th ed. 2000).