The kind of DNA evidence that can conclusively prove innocence or guilt ...Oh dear, oh dear, oh dear!
For an explanation, please see here and here (comments provoked by the Central Park Jogger case).
And, dear Reader, just in case you are too lazy to click one of those two links, allow me to quote myself:
Even if we assume that DNA technology and laboratory procedures have improved so much that when a DNA test shows a "match" or "exclusion" we are effectively compelled to conclude, respectively, (i) that two samples containing DNA have a common source or (ii) that two such samples do not have a common source -- even if we assume that DNA evidence and laboratory procedures have gotten this good, the probative force of DNA evidence on a question such as criminal guilt or innocence always depends on a swarm of surrounding assumptions, evidence, and facts. Bottom line: DNA by itself never establishes a proposition such as "guilty" or "innocent."
I think that the Innocence Projects on the whole do God's work. (No one except a saint always does God's work.) My sense is that almost everyone thinks that the Innocence Projects are wonderful, divine, even saintly. Is it the desire to convey this sentiment -- to pass out congratulations and encouragement and to be counted among the good guys and gals -- that leads normally-thoughtful observers to exaggerate the epistemic (inferential, evidentiary) authority of DNA?
Think for a moment. Just for a minute. Please! (Try it. You might like it.) For example, consider questions such as these:
1. Does DNA [or a fingerprint] indicate -- conclusively indicate, that is -- the mental state of the person who is the source of the DNA [or fingerprint]?
2. Does the existence of one person's DNA [or fingerprint] in a particular place rule out the possibility of some other material in that same place that has another person's DNA [or fingerprint]? If not, will both samples -- with DNA or fingerprints -- always be found? If not, what follows?
3. Can a sample with a particular person's DNA [or fingerprint] be "planted"?
4. Does a DNA sample [or a fingerprint] conclusively establish when or how the DNA [or the fingerprint] came to be where it was found?Suppose a sample with Person A's semen is found on a window sill near the victim's body. What if anything does this discovery conclusively prove? Does it conclusively prove how the DNA or fingerprint sample got there? When it got there?5. Is the process of DNA identification [or fingerprint] fully automated? (Answer: No.) If it were, would the possibility of error be ruled out? Do machines ever malfunction? Do they ever wear out? Are they ever manipulated? (Is it hard or impossible to make a process so "automated" that it cannot be manipulated by a human being?)
6. Can a sample bearing a person's DNA be (literally) blown by the wind? Can a sample bearing a person's DNA be carried by a dog? (Can human hairs be carried by the wind? Do dogs move about and can they carry human hairs?) Can person A's DNA by transferred to person B by a handshake between A and B? (I don't know the answer to this question. But my guess is that the answer is "yes.") Can person A's DNA find its way onto person B if person B wears unwashed clothing that A has recently worn?
7. Does the presence of man A's semen in a woman X's vagina "conclusively" prove that man B did not have intercourse or attempt to have intercourse with woman X?
8. Do I need to keep asking such questions about such appalling situations to prove my point? (Alas, apparently I do.)
What is it that leads intelligent people to exaggerate the power of DNA evidence beyond all reason and common sense? Is it an anxiousness to affirm the great service that has been done by the Innocence Projects? Perhaps. But is such noble-minded exaggeration dangerous? I think it's possible -- I think it's probable -- that the answer is "yes."More is at stake here than a persnickety concern about the precise use of language.
If one believes that something is certain (whether it's guilt or innocence), one naturally has a tendency to think that such a certainty is not worth investigating. Why bother?
And what happens if a certainty is overthrown -- if, for example, a person said to be certainly innocent is shown to be probably guilty, or vice-versa? What kinds of feelings might one then have if "one" is, for example, a trial judge, a juror, a crime victim, a legislator, or just a member of the public who has a recollection of the past?
Incidentally, are prosecutors and expert witnesses for the prosecution always the only sinners? Should we trust defense counsel and expert witnesses for the defense for the same reason that we are asked to trust Google -- because we know or believe that they're the good guys and gals and wouldn't do anything wrong or even think of doing anything wrong? (And we're sure, aren't we, that the good guys and gals who run Google now will always run Google -- or that if the good guys and gals at Google prove to be mortal, their replacements will be just as good as the originals? Hasn't every President of the United States been as good as old George [Washington, I mean]? Q.E.D.)
Well, now I had better read the rest of Taylor's article. He is, after all, one of the best journalists around. He's ordinarily first-rate. But all of us get snookered now and then. Including me. It's entirely possible I've made a mistake in this very post. (That's a bit of a paradox, I suppose. But there you have it. And I still think -- for the moment -- that what I've said here -- or most of it -- is correct. But I could be wrong in thinking that.)