In reply, I would emphasize that you err in all of your specific legal examples. In actuality, the practices you think impossible are followed routinely.
First, courts hold that the mere personal stake of experts is a relevant ground for doubting the testimony of the expert. This is reflected systematically throughout the law. Under the rules of procedure, judges order expert witnesses to disclose the amount of compensation that they receive. Under the law of evidence, lawyers are free to cross-examine experts on the amount of money that they receive for their service in the current and other cases since it is considered relevant as a ground for doubting their credibility. Before deliberation, juries are routinely instructed to consider the compensation received by experts as affecting their credibility. Most critically, expert witnesses whose compensation is dependent on the outcome of the case are frequently prohibited from testifying entirely. A lawyer who knowingly offers evidence of an expert witness who will be paid money if a litigant is successful will be disciplined. Nothing more is required than the fact of financial interest; no direct evidence of influence is required whatsoever.
Second, with respect to criminal defendants, courts have always recognized that the interest of a criminal defendant in acquittal is grounds to doubt his credibility. Indeed, this was precisely the basis for the common law rule prohibiting them from testifying. While criminal defendants are allowed to testify today in order to afford them every possible means of self-vindication, the U.S. Supreme Court has throughout its whole history upheld the propriety of instructing the jury about the doubt that the defendant’s interest casts on his testimony. See, most recently, Portuondo v. Agard, 529 U.S. 61, 68 (2000)(noting the propriety of ordering the jury to consider “[t]he deep personal interest which [the defendant] may have in the result of the suit” in determining whether to give his testimony any weight. This was true, the Court said despite the fact that the instruction was occasioned only by the sheer personal stake and “did not rely on any specific evidence of actual fabrication for its application.” The Court has defended this rule by noting that when a criminal defendant chooses to testify, then he should be subject to the ordinary rules of credibility that apply to all testifying litigants.
Your final example is not legal, but in determining the credibility of complaints, the usual process would admit both the evidence supporting allegations of racism (the complaints) and evidence that undermines such allegations (inter alia, evidence of interest).
First, it needs to be noted that not one of my examples is specifically legal, or, indeed, legal at all. The first is about experts; the second about a newspaper columnist dismissing someone's claim of innocence because they have a personal stake in the matter; the third about someone hearing about an accusation of police racism. None of these are legal matters. And this is quite as it should be, since the case at hand was not a legal matter, but a claim that we should be suspicious of someone's casual testimony in an interview. Moreover, the claim Pensans had made in the comment, the one to which I was responding, was that as a civilization and in our society, we systematically doubt the testimony of interested persons. And this certainly cannot be established by legal examples alone, particularly given that the instance to which it was applied here was not a legal example.
But let's take the legal cases. With regard to the response to the first example, the original example did not say anything about compensation; the personal stake involved was reputation and career in the larger professional community. So the response really doesn't tell us anything about the matter. In any case, being paid for testimony is not a "mere personal stake"; it's potential evidence for a distorting interest. Thus it is incorrect to say that this shows that the mere personal stake of experts is a relevant ground for doubting their credibility; rather, what is happening here is that the law is recognizing the fact that compensation for testimony can be evidence for distorting interest -- at least that it's one of the things that needs to be taken into account if you are concerned about distorting interest (which the law has to be).
The response to the second example is more interesting, since I think it can much more plausibly be argued that this is a case in which the law supports the questioning of a person's credibility simply in virtue of their personal stake. Of course, I'm inclined to say that this shows one area of law that needs to be reformed in order to be made more just, since as it stands it allows someone to cast aspersions on someone else's testimony the basis of no evidence at all for what may be, for all that can be assumed at that point in the process, nothing but the sheer bad luck of being in the wrong place at the wrong time. But I'm willing to concede that it's a point at which our society treats mere personal stake as a reason to doubt someone's testimony that isn't itself directly motivated by the two motivations I had suggested, namely, short-circuiting investigation or casting aspersions. To that extent I concede the point. But it's a very good example, and worth thinking about, and that's one reason why I wrote this as another post rather than as a comment.
