(dissenting, with whom Marshall, C.J., joins). If the Legislature had not provided its own definition of “a party’s exercise of its right of petition,” G. L. c. 231, § 59H, the various limitations on that term imposed by today’s decision might represent a plausible interpretation of the statute. We are not, however, called on to craft our own definition, or to interpret the statute based on our own understanding of what a party’s “exercise of its right of petition” ought to entail, because the Legislature has defined the term for us. The statute provides that “a party’s exercise of its right of petition” means, inter alla, “any written or oral statement made before or submitted to a legislative, executive, or judicial body, or any other governmental proceeding.” Id. Here, the claims against Gastfriend are expressly based on Gastfriend’s having submitted a “written . . . statement,” in the form of an affidavit, to an “executive . . . body,” the Board of Registration in Medicine (board), see G. L. c. 13, § 10, in connection with a “proceeding,” a disciplinary proceeding against Kobrin. The conduct for which Gastfriend has been sued thus comes within the literal *343meaning of the term “a party’s exercise of its right of petition,” as that term is defined in the statute.
The court today sidesteps this straightforward application of the statutory definition by emphasizing the words “its right” in the term “a party’s exercise of its right of petition,” and claims that the protections of § 59H must be limited in order not to render those words “superfluous.” Ante at 332-333. However, the statute provides us with the Legislature’s definition of the entire term “a party’s exercise of its right of petition.” We should look to that definition, not our own assessment of what the words “its right” might connote, if we did not have a definition from the Legislature. The definition itself unambiguously applies to the present case, and no component of the definition is rendered “superfluous” by that application. Apparently discomfited by the broad scope of the definition (for fear that it would protect “every statement ever made to a government body,” ante at 333), the court ignores the definition and reads its own limitation into the words being defined. The court then seeks to justify that limitation by suggesting, without citation to any authority, that one’s constitutional right of petition is limited to petitioning on one’s “own” behalf in pursuit of one’s “own interests,” and that the additional phrase “under the constitution” therefore connotes the Legislature’s desire to limit the protections of § 59H in the same fashion. See ante at 330, 332-334.
“[I]n interpreting a statute a court should always turn first to one, cardinal canon before all others. We have stated time and again that courts must presume that a legislature says in a statute what it means and means in a statute what it says there. . . . When the words of a statute are unambiguous, then, this first canon is also the last: ‘judicial inquiry is complete.’ ” (Citations omitted.) Connecticut Nat’l Bank v. Germain, 503 U.S. 249, 253-254 (1992), quoting Rubin v. United States, 449 U.S. 424, 430 (1981). “Courts are not free to read unwarranted meanings into an unambiguous statute even to support a supposedly desirable policy not effectuated by the act as written.” 2A N.J. Singer, Sutherland Statutory Construction § 46:1, at 129 (6th ed. 2000).
There are, of course, occasions when we depart from the literal wording of a statute, despite the unambiguous nature of *344that literal wording. However, such departures from the Legislature’s straightforward wording are rare, reserved for those instances where application of the literal meaning would result in “absurd or unreasonable” consequences, Champigny v. Commonwealth, 422 Mass. 249, 251 (1996), quoting Attorney Gen. v. School Comm. of Essex, 387 Mass. 326, 336 (1982), or would “defeat [the] purpose” of the legislation, Champigny v. Commonwealth, supra, quoting Lehan v. North Main St. Garage, 212 Mass. 547, 550 (1942). We should be especially leery of narrowing the literal meaning of this particular definition, as the breadth of the definition was repeatedly criticized by the Governor at the time the statute was being enacted, and the Legislature deliberately chose to reject that criticism and to maintain the definition in its extremely broad form.1
Today’s opinion acknowledges that history, but concludes that it stands for nothing more than the fact that “the Legislature rejected the Governor’s position.” Ante at 337 n.11. What this history signifies is that the Legislature’s attention was drawn to the fact that the statute’s definitions were very broad indeed, and that they went well beyond the prototypical SLAPP suit. The Legislature’s decision to keep the statute’s broad definitions in the face of the Governor’s repeated objections indicates at a minimum that the breadth of those definitions was not the product of inartful draftsmanship or legislative inadvertence. Given the attention that was paid to the definitions at the time of enactment, we should be even more inclined to interpret the definitions consistent with their literal wording — their breadth *345is not some drafting error that we need to correct to make the statute comport with the Legislature’s ostensible intent.
Today’s opinion makes no claim that according the protections of G. L. c. 231, § 59H, to this particular defendant would result in “absurd or unreasonable” consequences, or that doing so would “defeat the purpose” of § 59H. Champigny v. Commonwealth, supra. Rather, the court takes it upon itself to narrow the protections of § 59H to a smaller class of persons that the court finds to be more deserving than expert witnesses who are paid by the government, rather than accept the literal statutory mandate that such protections are to be accorded to all persons — paid or unpaid, expert or lay, private or officially retained — submitting “written or oral statements]” to governmental bodies in connection with pending proceedings. Before tampering with the Legislature’s definition of “a party’s exercise of its right of petition,” we should consider whether it would be “absurd or unreasonable” to protect all such persons.
There is nothing “absurd or unreasonable” about protecting all witnesses from lawsuits based on the statements they give during the course of agency proceedings. To the contrary, absolute immunity from suit has long been accorded to witnesses in judicial proceedings, even if their testimony is knowingly false. See Correllas v. Viveiros, 410 Mass. 314, 319-320 (1991); Aborn v. Lipson, 357 Mass. 71, 72-73 (1970); Mezullo v. Maletz, 331 Mass. 233, 236-237 (1954); Sheppard v. Bryant, 191 Mass. 591, 592 (1906); Hoar v. Wood, 3 Met. 193, 197 (1841). The privilege is grounded in the view that “it is more important that witnesses be free from fear of civil liability for what they say than that a person who has been defamed by their testimony have a remedy.” Aborn v. Lipson, supra at 72. Massachusetts law recognizing such absolute immunity accords with well-established law across the country. See Briscoe v. LaHue, 460 U.S. 325, 330-334 (1983), and cases cited; Blevins v. Ford, 572 F.2d 1336, 1338 (9th Cir. 1978); Brawer v. Horowitz, 535 F.2d 830, 836-837 (3d Cir. 1976); Sacks v. Stecker, 60 F.2d 73, 75 (2d Cir. 1932) (absolute immunity for witnesses “is the practically universal rule in this country”). This immunity is accorded not merely for statements made as part of a witness’s testimony at trial, but for statements made “in the context of a *346proposed judicial proceeding.” Correllas v. Viveiros, supra at 321. See Dolan v. Von Zweck, 19 Mass. App. Ct. 1032, 1033 (1985), quoting Sullivan v. Birmingham, 11 Mass. App. Ct. 359, 361 (1981) (“absolute privilege applies to defamatory statements made ‘in the institution or conduct of litigation or in conferences and other communications preliminary to litigation’ ”); Frazier v. Bailey, 957 F.2d 920, 932 (1st Cir. 1992); Leavitt v. Bickerton, 855 F. Supp. 455, 458 (D. Mass. 1994); Restatement (Second) of Torts § 588 (1977) (“A witness is absolutely privileged to publish defamatory matter concerning another in communications prehminary to a proposed judicial proceeding or as a part of a judicial proceeding in which he is testifying, if it has some relation to the proceeding”). This absolute privilege applies not only to oral testimony in court, but also to statements or testimony given in written form. See Mezullo v. Maletz, supra (absolute privilege protected physician who signed certificate in commitment proceeding certifying that plaintiff was insane); King v. Hildebrandt, 331 F.2d 476, 478 (2d Cir. 1964) (immunity accorded psychiatrist who signed affidavit that launched commitment proceeding, noting “well-established rule that statements in pleadings and affidavits made in the course of judicial proceedings are absolutely privileged so long as they are relevant to the issues involved in the proceeding”); Williams v. Williams, 169 F. Supp. 860, 862 (D.D.C. 1958) (statement in affidavit); Todd v. Cox, 20 Ariz. App. 347, 348 (1973) (affidavit); Overman v. Klein, 103 Idaho 795, 800 (1982) (immunity applicable to witness who filed affidavit in child custody proceeding, noting that witness immunity extended “as to virtually any statement in documents which have been filed in a judicial proceeding”); Resciniti v. Padilla, 72 A.D.2d 557, 558 (N.Y. 1979) (affidavit); Jarman v. Offutt, 239 N.C. 468, 472 (1954) (affidavit); Vieira v. Meredith, 84 R.I. 299, 301 (1956) (statement in pleadings).
Finally, of particular relevance to the distinction that today’s decision reads into § 59H, common-law immunity from suit extends to witnesses who are employed by or working for the government — it is not limited to private persons giving evidence in such proceedings. See Briscoe v. LaHue, supra at 335-336 & n.15 (common law provided absolute immunity for *347all witnesses, “governmental or otherwise,” and “drew no distinction between public officials and private citizens”; 42 U.S.C. § 1983 did not override witness immunity for police officers who allegedly testified falsely); Overman v. Klein, supra (witness immunity extended to social worker employed by government agency). See also LaLonde v. Eissner, 405 Mass. 207, 211-212 (1989) (absolute immunity for court-appointed experts). Indeed, “to the extent that traditional reasons for witness immunity are less applicable to governmental witnesses, other considerations of public policy support absolute immunity more emphatically for such persons than for ordinary witnesses.” Briscoe v. LaHue, supra at 342-343.
From this vast body of precedent supporting absolute immunity for statements made by any kind of witness in connection with judicial proceedings, the anti-SLAPP statute takes the modest step of extending a more limited degree of immunity to all persons submitting statements in connection with other types of governmental proceedings. There is nothing remarkable, let alone “absurd or unreasonable,” about protecting all persons who provide government agencies with information in the course of agency adjudications, including those who have been sought out and paid by the government. What is being protected is not merely the “rights” of the person submitting such information, but the interests of the government agency in acquiring information germane to the proceedings before it. See preamble to 1994 House Doc. No. 1520 (purposes of anti-SLAPP legislation include encouragement of “robust discussion of issues before legislative, judicial, and administrative bodies”). As with immunity from suit for a witness’s participation in judicial proceedings, immunity for witnesses in agency proceedings removes what might otherwise be a powerful disincentive against participation, a disincentive that would operate to rob the agency itself of the benefit of those witnesses’ information, views, and expertise.2 The agencies themselves are best served by having witnesses and participants *348of all types protected from lawsuits stemming from their testimony and participation. Indeed, that such agencies are the functional equivalent of courts, requiring comparable protection for those involved, has long been recognized. See Butz v. Economou, 438 U.S. 478, 512-517 (1978) (absolute immunity accorded to administrative law judges and to agency attorneys presenting case, as they are equivalent of court judges and prosecutors); Bettencourt v. Board of Registration in Med., 904 F.2d 772, 782-785 (1st Cir. 1990) (absolute immunity for board and staff members in connection with disciplinary proceedings against physician).
Whereas the common law made witnesses in judicial proceedings absolutely immune from suit, the anti-SLAPP statute, protecting persons making statements in connection with government proceedings, extends a form of protection that is slightly less than absolute immunity. Under G. L. c. 231, § 59H, the suit against the person who made the statement will be dismissed unless it can be shown that the person’s statement “was devoid of any reasonable factual support or any arguable basis in law,” the burden being on the plaintiff to demonstrate that the participant’s statement lacked such support or basis. The statute also establishes a procedural mechanism to enforce this immunity in a pragmatic way at the outset, before the defendant has been subjected to the expense and anxiety of protracted litigation. See Fabre v. Walton, 436 Mass. 517, 520-522 (2002), S.C., 441 Mass. 9 (2004) (allowing interlocutory appeal from denial of motion to dismiss under § 59H). Extending such protection to persons who submit information to government agencies, whether they do so on their own personal initiative or as witnesses retained by the government, is an extremely modest extension of the law of witness immunity.3 It is hardly “absurd or unreasonable” to include all such persons *349within the scope of this protection, and I therefore see no reason to depart from the literal language of § 59H and carve out an exception merely because the witness’s participation was not in pursuit of “a grievance of his own” or “on his own behalf.” Ante at 330.
Beyond the troubling ramifications today’s decision has for government agencies seeking to obtain opinions from private experts, the case introduces a new and somewhat amorphous argument with which to defeat special motions to dismiss under § 59H, namely, the argument that the defendant who made the statement to a government agency did not have his or her “own interests” at stake, was not acting on his or her “own behalf,” or lacked the requisite “interest in” the subject of the proceedings. See ante at 330, 332, 334, 337. Even where the suit in question is unabashedly based on a person’s “statement” to a government agency in connection with a pending matter, the motives and relationships underlying the person’s decision to give such a statement must apparently be examined to make sure they are sufficiently pure to come within the ambit of the narrower version of § 59H outlined in today’s decision.
While the precise contours of this evaluation of what caused the person to submit a statement to the government agency are unclear, it is apparent that affiliation with the government, or the receipt of filthy lucre, or perhaps a combination of the two, may suffice to taint the person making the statement, such that the person is no longer acting in his or her “status as citizen[]” *350or exercising a “constitutional” right and should therefore be deprived of the quasi immunity and procedural protections of § 59H. Ante at 332, 334-335, 338. The court today posits that there may be persons who are “solicited by the government” to participate in proceedings, but who nevertheless could be protected by § 59H, as long as that solicitation did not result in “performing services for the government” or “a mere contractual connection to the proceedings.” Ante at 332, 338.4 The court cites no authority for the proposition that either receipt of compensation for one’s time or agreement to provide the government with information or expert advice deprives someone of the “status” of “citizen” or otherwise curtails someone’s “constitutional” rights. Yet, according to today’s opinion, such persons are not deserving of protection and must therefore somehow be excluded from the unambiguous definition of § 59H. Recognizing the ambiguity inherent in its own (as opposed to the Legislature’s) definition, the court acknowledges that “some difficult factual situations will have to be assessed on a case-by-case basis.” Ante at 332 n.8. The court then muses whether § 59H would be available to “a government ‘whistle-blower,’ ” apparently tom between the fact that a whistleblower may have “personal knowledge and concern” about the matter being reported and the fact that such a whistleblower, working for the government, might just be pursuing the government’s interest. Ante at 340 n.16.
Today’s decision casts a pall of uncertainty over the status of many persons who make statements to the government. What if an expert is hired by a petitioner — will the petitioner’s “own interests” in the matter allow us to extend the protections of *351§ 59H to the petitioner’s disinterested expert, ante at 334, or does the fact that the individual expert has no “grievance of his own,” ante at 330, deprive the expert of those protections? What about lobbyists or lawyers? They are customarily making statements to government officials on behalf of their clients, not on their own behalf, and are compensated for doing so. Is their connection to the proceeding also a “mere contractual connection” that deprives them of protection? See ante at 338. What about persons who testify before agencies after being subpoenaed (by either the agency or by any of the parties) — such persons submit “statements]” in connection with the agency proceeding, but if they did not want to make statements of their own volition, are they pursuing their “own” grievances, or exercising their “right of petition under the constitution”? See ante at 330.
In my view, such questions are irrelevant, as the straightforward definition provided by the Legislature does not require us to consider who the person making the statement works for, whether the statement was a product of the declarant’s “own interests,” ante at 334, or what other constellation of factors may have influenced the person to submit a statement to the agency. The definition of “a party’s exercise of its right of petition” contains no reference to the motives or affiliations of the person making the “statement ... to a legislative, executive, or judicial body,” and special motions to dismiss under § 59H should not be bogged down by such considerations.
The elusive nature of the additional element that the court has inserted into § 59H is best illustrated by the difficulty the court has in conjuring some distinction between the present case and Baker v. Parsons, 434 Mass. 543 (2001). There, as here, a government agency solicited the views of an outside expert in order to assist the agency, and that expert was then sued for the statements she had made in rendering her opinion.5 There, as here, the plaintiff argued that the anti-SLAPP statute *352was not applicable. Rather than engage in any analysis of the expert’s “independent interest in the controversy” that predated the government’s solicitation of her input, ante at 339, the court in Baker reasoned that “the plain language of the statute . . . squarely encompasses the facts of this case.” Baker v. Parsons, supra at 549. The case came within that “plain language” because “Parsons, a biologist, responded to inquiries from State and Federal environmental officials, in connection with government agency reviews of Baker’s application to develop property by constructing a pier on an island that has historically been a home for many aquatic birds. As a result of her responses, Baker eventually sued Parsons and the nonprofit organization for which she works, thus, according to the defendants, ‘chilling’ any further participation by the defendants in assisting State and Federal agencies gathering information on the merits of Baker’s application.” Id. That same “plain language” analysis can be applied to the present case using the exact same reasoning: Gastfriend, a physician, responded to inquiries from the board, in connection with the board’s review of Kobrin’s fitness to practice medicine. As a result of his responses, Kobrin eventually sued Gastfriend, thus, according to the defendant, “chilling” any further participation by him in assisting the government in any proceeding against Kobrin.6
In attempting to craft some distinction between Gastfriend’s *353status in the present case and Parsons’s status in Baker v. Parsons, supra, the court claims that Gastfriend was “serv[ing] on behalf of the government to further its interests,” whereas Parsons was “seek[ing] redress for [her] grievances.” Ante at 339. That Parsons was pursuing her “own” grievance was demonstrated by the fact that she had “personally” conducted research on the island site at issue, and thereby had an “interest” in the preservation of the island that was formed “prior to the solicitation of [her] comments by the government.” Id. Gastfriend, by comparison, only articulated his views after he was “hired by the government.” Id.
This proffered distinction between Parsons and Gastfriend assumes that an expert who starts out as a neutral, disinterested expert solicited (and paid) by an agency cannot thereafter become a genuine petitioner who, having become familiar with the facts and circumstances, develops his “own” desire to see the government take a particular action. To the extent that it was Parsons’s sincere view about the undesirability of the proposed pier construction that made her a viable candidate for protection under § 59H, how can we tell that, by the time he submitted his affidavit, Gastfriend was not equally sincere in his view that Kobrin should not be allowed to practice medicine? May not an initially neutral physician, who starts out with no knowledge of the physician being disciplined, review the information made available to him and then become indignant at what he views as dangerously substandard medical practice, and thus intensely desirous that the offending physician’s license be revoked? While Gastfriend did not have such views at the time the board first contacted him, because he at that time knew nothing of Kobrin or the deaths of his patients (that ignorance, in the court’s view, being a feature that makes him distinguishable from Parsons, ante at 338), who is to say that he had not developed an “interest” in the outcome of the matter by the time he submitted his affidavit? Having learned about Kobrin’s practice, may not Gastfriend have developed his “own” concern for the image of his profession, or his “own” compassion for patients who were, in his opinion, at risk of dying from Kobrin’s improper practices, and thus have his “own” interest in seeing Kobrin’s license suspended? If he had *354developed the requisite desire that the government take action against Kobrin, why would his agreement to review the records and render an opinion diminish his “constitutional” right as a “citizen” to express his views to the governmental body that had the power to suspend Kobrin’s license? The court today cynically assumes that Gastfriend, having begun as a “hired gun,” would forever be a “hired gun” and would not actually care about the outcome of the disciplinary proceedings against Kobrin, whereas Parsons, because she had already studied the island habitat in question, subconsciously cared about the outcome of the pier permit proceedings before she even knew about them. There is nothing in this record to suggest that, at the time he submitted his affidavit, Gastfriend actually had no preference as to what the board did with Kobrin’s license. Indeed, it is insulting to suggest that, for money, Gastfriend was simply saying whatever “the board” or “the government” wanted him to say, and thus merely acting “on behalf of the board” or “the government.” See ante at 333, 339. If, as today’s decision states, § 59H will not apply if the person making the statement to the governmental body lacks some “interest” in the matter, there is no basis on which to conclude that Gastfriend ultimately lacked such an “interest” in the matter before the board.
I see nothing to be gained, and much to be lost, in requiring such parsing of motives as part of a special motion to dismiss under § 59H. In my view, nothing in § 59H suggests that its protections hinge on the manner in which the person making the statement came to know or care about the agency proceedings in question. Only by reading such irrelevant considerations into the statute, and then assuming that Gastfriend cannot satisfy those irrelevant considerations, does the court deny him the protections of § 59H. Gastfriend, and the many other expert witnesses who assist a wide array of government agencies, are entitled to those protections under the unambiguous wording of the statute’s definition, and it is in the best interests of both the witnesses and the agencies that such witnesses be accorded those protections in the prompt manner that § 59H envisions. I see no indication that the Legislature intended to deny such witnesses those protections, and therefore respectfully dissent.
Specifically, when the first enactment of the bill was sent to the Governor, he vetoed it, explaining that the bill as drafted “applies to a broad group of potential claims, sweeping in cases that are far beyond the types of lawsuits which the bill’s proponents wish to control,” and noting the stringent consequences for any claim “falling within its broad definition.” Letter from Governor William F. Weld to House of Representatives and Senate (Jan. 13, 1994). When the Legislature passed another bill using the same “broad definition” (see 1994 House J. 1118; 1994 Senate J. 1294), the Governor again articulated his concern about the ostensible overbreadth of the bill and recommended an amendment to narrow its scope. 1994 House Doc. 5570. When the bill passed again without amendment (see 1994 House J. 1247; 1994 Senate J. 1418), the Governor again expressed the view that it was too broad and, for that reason, vetoed it. See 1994 House Doc. No. 5604. The Legislature proceeded to override the Governor’s veto. 1994 House J. 1306; 1994 Senate J. 1491-1492.
The amicus brief of the Attorney General points out that the Board of Registration in Medicine (board), like many other government agencies, regularly needs the assistance of outside experts in order to perform its functions.
With specific reference to the board, it is also an extremely modest modification of the earlier statute that gave qualified immunity to persons filing complaints, reporting or providing information, “or assisting the board at its request in any manner in discharging its duties and functions.” G. L. c. 112, § 5. That statute precludes imposition of liability as long as the person complaining, reporting, or assisting did so “in good faith and without malice.” Id. The anti-SLAPP statute substitutes a more stringent qualification of the immunity that is granted, i.e., the immunity is lost only if the person’s state*349ment “was devoid of any reasonable factual support or any arguable basis in law.” G. L. c. 231, § 59H. And, to give teeth to that immunity, the anti-SLAPP statute makes available a procedural vehicle ensuring that the immunity issue will be addressed promptly. Given that the Legislature had already extended some degree of immunity to persons “assisting the board,” it cannot be “absurd or unreasonable” for the Legislature to tighten that immunity somewhat, create procedures to enforce it, and extend it to all persons who are “assisting” government agencies by giving “oral or written statement[s]” to those agencies.
Today’s decision correctly holds that, notwithstanding G. L. c. 112, § 5, persons exercising their “right of petition” before the board are entitled to the protections of G. L. c. 231, § 59H. Ante at 340-341. That this defendant would also be covered under G. L. c. 112, § 5, does not preclude him from claiming the more rigorous protections of the anti-SLAPP statute if the suit against him is “based on” his having made a “statement” to the board. G. L. c. 231, § 59H.
This suggests that retained expert witnesses, who are compensated for their time, will not be protected, unless perhaps they can show that they had some interest in or connection to the matter prior to entering that contractual arrangement. Ordinarily, when seeking an expert’s opinion, one purposely seeks out an expert who does not have some prior involvement or interest in the matter, as that deliberate detachment is viewed as important to the neutrality and validity of the resulting expert opinion. Under today’s ruling, biased experts who are compensated by agencies can perhaps claim the protection of § 59H when they are sued for rendering faulty opinions, because they can demonstrate that they had their “own” interest in the outcome of the matter before they were retained. However, neutral experts — the only kind of experts worth hiring — cannot claim that protection.
More specifically, the United States Army Corps of Engineers had itself solicited comment from other government agencies, including a unit within the Massachusetts division of fisheries and wildlife (division), with respect to a pending application to construct a pier on an island. The division, in preparation for its own response to the Federal agency, in turn sought input from Parsons, a scientist who had conducted research on the island where the *352proposed pier would be located. Parsons provided her written opinion, opining that the value of the island as a nesting habitat for aquatic birds made it an inappropriate site for the pier. That opinion allegedly caused others (not Parsons herself) to petition the Executive Office of Environmental Affairs for an environmental impact review of the proposed pier. Baker, the applicant seeking permission to construct the pier, then sued Parsons for the damages allegedly incurred as a result of the delay in issuance of the permit for the pier. Baker v. Parsons, 434 Mass. 543, 545-546 & n.6 (2001).
Inexplicably, today’s decision treats as irrelevant the fact that Gastfriend was a potential witness in a pending criminal case against Kobrin at the time Kobrin filed this lawsuit. Ante at 330 n.5. Gastfriend would not view the pendency of this lawsuit as irrelevant to his consideration whether he would be willing to assist the prosecutor, nor would it be viewed as irrelevant by the prosecutor, who would have to consider whether the lawsuit would make Gastfriend appear biased. Of course, whether the lawsuit was intended to “chill” Gastfriend’s participation in either the disciplinary proceedings or the criminal prosecution, the effect of today’s decision will unquestionably “chill” any neutral expert’s willingness to provide an opinion to any State or Federal agency.