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Edward J. Imwinkelried is a professor of law at the University of California at Davis and co-author of Scientific Evidence (3d ed. 1999). A “battle of the experts” is a common occurrence these days. The plaintiff or prosecutor fires an opening salvo with one expert, and the defense responds in kind. Increasingly, defense counsel are no longer content to cross-examine the opposing expert; counsel goes farther and puts his or her own rebuttal witness on the stand. I was recently corresponding with my good friend, Professor David Crump of the University of Houston, who was kind enough to share with me a draft of an article he is working on. As I was reading his draft, one sentence jumped off the page at me. After noting that the subsequent decisions have treated Daubert v. Merrell Dow Pharmaceuticals, 509 U.S. 579 (1993), as announcing a rigorous standard for introducing expert testimony, Crump observed: “The Supreme Court has not recognized a difference in the standard for rebuttal witnesses.” Recalling Blackmun’s passage in ‘Daubert’ That sentence caught my eye. On the one hand, I realized that Crump was right. The Daubert opinion does not expressly state that there is a different, lower admissibility threshold for rebuttal witnesses. For that matter, I could not think of any lower court opinion that even suggested that such a difference exists. On the other hand, I recalled the passage in Daubert in which Justice Harry Blackmun explained that several features of the adversary system minimized the risk of allowing the introduction of “shaky but admissible evidence” under Daubert. Id. at 596. In addition to mentioning “[v]igorous cross-examination . . . and careful instruction on the burden of proof,” the justice asserted that the “presentation of contrary evidence” is a “traditional and appropriate means of attacking” the weight of the proponent’s evidence. Id. Blackmun’s opinion certainly contemplates the possibility that the opponent will proffer rebuttal testimony. However, in light of Crump’s observation, does the opponent always have to lay a Daubert foundation for rebuttal evidence? In some cases, the answer is yes. Suppose that in a toxic tort case, the plaintiff alleges that her illness was caused by exposure at home to the defendant’s pesticide. The plaintiff calls an expert toxicologist on the issue of general causation. The expert relies on an epidemiological study supposedly establishing a statistically significant relationship between exposure to the pesticide and the incidence of the illness. In Daubert, the plaintiffs proffered epidemiological evidence. Thus, it is clear that the trial judge should insist that the plaintiff lay a validation foundation for the causation opinion by the plaintiff’s expert. Assume further that the defense’s position is that the plaintiff’s illness was caused by her exposure to a different chemical at work. The defendant also calls an expert toxicologist. Like the plaintiff’s expert, the defense expert intends to rely on an epidemiological study, this one investigating the connection between exposure to that industrial chemical and the incidence of the illness. This is a classic “battle of the experts.” Both experts are prepared to opine on general causation, and both intend to rely on the same type of data, namely, epidemiological research. If the plaintiff’s expert must run the Daubert gauntlet, by parity of reasoning, the defense expert’s rebuttal testimony should also be subject to Daubert scrutiny. It is submitted, though, that in many other cases, the opponent proffering rebuttal testimony need not lay a Daubert foundation. To understand why that is the case, we need to appreciate the typical components of the direct testimony by the proponent’s expert. In an ordinary case, that testimony will include the following elements: Initially, the proponent elicits the witness’s testimony demonstrating the witness’s qualification as an expert; then the witness describes his or her major premise-the general theory or technique the expert proposes relying on; next, the witness specifies the minor premise-the case-specific information such as a patient’s symptoms that the witness applies the major premise to; the witness then applies the major premise to the minor premise; and, lastly, the witness draws the final conclusion, yielded when the major premise is applied to the minor. “The ‘Bases’ of Expert Testimony: The Syllogistic Structure of Scientific Testimony,” 67 N.C. L. Rev. (1988). Those elements would be the essential components of the foundation for the final conclusion. However, so long as the opponent’s expert stopped short of drawing a final opinion on the merits, the opponent could use a rebuttal expert to attack any of these elements without the necessity of a Daubert foundation. Suppose that during his direct examination, the proponent’s expert testified that he is a member of a certain professional organization-one with an impressive title that might prompt the jury to attach more weight to the expert’s opinion. It certainly would be permissible for the opponent to call an expert to attack the weight of the first expert’s credentials. For instance, the opponent’s expert might point out that the only requirement for joining the organization is paying annual dues. Alternatively, the opponent’s expert could note that although applicants must pass a test to join the organization, on the last examination the passage rate was 99.5%. The opponent’s expert is testifying to factual information rather than attempting to opine. For that reason, the opponent’s rebuttal should be admissible without a Daubert foundation. Likewise, occasionally the opponent will be entitled to attack the expert’s general theory or technique without laying a Daubert predicate. By way of example, suppose that on direct examination, the proponent’s expert testifies that a particular study conducted at Johns Hopkins University validates the theory or technique the expert relies on. Even without laying a Daubert foundation, the opponent’s expert ought to be permitted to provide the jury with factual information about the study-the relatively small number of subjects in the study, facts suggesting that the composition of the database was unrepresentative, and data about the error rate. Also, again if the opponent’s expert stops short of proffering an ultimate opinion contradicting that of the proponent’s witness, the opponent’s expert arguably should be allowed to describe a University of California at Los Angeles Medical School study yielding contrary findings. The jury must decide how much weight to give to the proponent’s expert’s opinion, and it is certainly relevant for the jury to appreciate that there are studies calling into question the validity of the plaintiff’s underlying theory. By the same token, the opponent’s expert can question the proponent’s expert’s minor premise without directly challenging the opinion stated by the proponent’s expert. The quality or trustworthiness of the information comprising the minor premise might be suspect. Suppose, for example, that in a criminal case the accused pleads insanity. The accused calls a psychiatrist to testify that the accused suffers from a well-recognized psychosis. Assume that the diagnostic criteria for the psychosis are well recognized and set out in detail in the American Psychiatric Association’s DSM-IV, the fourth edition of the Diagnostic and Statistical Manual. On those assumptions, the defense expert’s major premise may be practically unassailable. However, assume further that the psychiatrist gained all his information about the accused’s case history from the accused in an interview conducted after the accused had entered an insanity plea. The prosecution should be permitted to rebut defense testimony by calling another psychiatrist to challenge the defense expert’s minor premise. For instance, the prosecution psychiatrist might testify that in psychiatric circles, it is unacceptable to reach a final diagnosis on the basis of uncorroborated information from biased sources when verification from independent sources would be feasible. If the accused mentioned an incident involving a third party, the psychiatrist should arguably make an effort to contact the third party. J. DeMay, “The Plaintiff’s Personal Injury Case: Its Preparation, Trial and Settlement” (1977); Tigar, “Handling the Expert Like an Expert: Back to Basics,” NLJ, March 1, 1982, at 33. The prosecution should certainly be permitted to challenge the defense expert’s minor premise in this fashion without assuming the burden of laying a complete Daubert foundation. Even when the proponent’s expert’s major premise is sound and the case-specific information is trustworthy, without establishing a Daubert foundation the opponent is entitled to call an opposing expert to establish that it is improper to apply that major premise to the minor premise. General Electric Co. v. Joiner, 522 U.S. 136 (1997), is illustrative. There, the plaintiff’s experts relied on a number of research projects, including animal studies, as support for their conclusion that the plaintiff’s industrial exposure to PCBs had contributed to his cancer. The Supreme Court upheld the trial judge’s ruling excluding the opinions. Rehnquist finds too great an ‘analytical gap’ Writing for the majority, Chief Justice William Rehnquist marshaled all the differences between the studies and the facts of the instant case: The studies involved infant mice, but the plaintiff was an adult human being; the mice received “massive doses” of PCBs while, relative to his body size, the plaintiff’s exposure was “far less”; the PCB doses were injected directly into the stomachs of the mice, but the plaintiff received only dermal application; and while the plaintiff developed small-cell carcinomas, the mice suffered from alveologenic adenomas. Id. at 144. The chief justice believed that there was “simply too great an analytical gap” to permit that “extrapolat[ion]” from the animal studies. Id. at 146. Assuming that the trial judge had ruled the plaintiff’s opinion evidence admissible, defense experts presumably would be allowed to provide rebuttal testimony detailing all factual differences between the pending case and the parameters of the studies. Again, that defense testimony would be relevant even if the defense expert stopped short of proffering a contrary opinion, which would necessitate a Daubert foundation.

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