POSNER, Circuit Judge.

The defendants were convicted by a jury of federal drug offenses and sentenced to 300 months in prison (Ingram) and 360 months (Dawson). Among the grounds for reversal that they press on us with misplaced zeal in more than 130 pages of briefs is that in every trial of a federal drug offender the prosecution must prove that the defendant's offense had an actual impact on interstate or foreign commerce. Otherwise, they argue, the prosecution may exceed the regulatory power conferred on Congress by the commerce clause of the Constitution. The statutes under which the defendants were prosecuted, 21 U.S.C. §§ 841, 846, do not, it is true, make effect on commerce an element of the crime, and so no proof of such an effect was presented at the trial. But it is common knowledge that the traffic in most illegal drugs — certainly including cocaine, the drug involved in this case, very little of which is manufactured in the United States because no coca is grown here — is national or (in the case of cocaine) international rather than local. Of course there are local sales of the drugs, at the end of the commerce chain; but these, taken in the aggregate, certainly affect the interstate and international traffic in these drugs, and that is the only handle that Congress requires to be empowered by the commerce clause to legislate with respect to the local sales. Gonzales v. Raich, ___ U.S. ___, ___-___, 125 S.Ct. 2195, 2205-06, 162 L.Ed.2d 1 (2005). The Lopez decision, on which the defendants principally rely, reaffirmed Wickard v. Filburn, 317 U.S. 111, 63 S.Ct. 82, 87 L.Ed. 122 (1942), which had held that wheat grown by a farmer for his own consumption was within the commerce power because the wheat is traded in a national market and home consumption affects the amount available for that trade. United States v. Lopez, 514 U.S. 549, 559-60, 115 S.Ct. 1624, 131 L.Ed.2d 626 (1995). Similarly, local sales of drugs affect the demand for their importation. United States v. Thomas, 159 F.3d 296, 297-98 (7th Cir. 1998).

The government's principal evidence of the defendants' guilt consisted of testimony and recordings by Oscar Diaz, who after years as a major drug dealer following his illegal entry into the United States had agreed to cooperate with the DEA in exchange for a fee of 20 percent of any proceeds of drug sales that the government recovered as a result of Diaz's efforts; he received additional compensation in an unknown amount. The DEA also, though apparently not pursuant to an explicit agreement with Diaz, forbore to inform the INS that he had, as the DEA knew, lied on his application for U.S. citizenship when he said he hadn't committed any crimes. (By the time of trial, Diaz was a U.S. citizen.) He was also assured that as long as he continued cooperating and told the truth, he would not be prosecuted for his past drug dealing. Before and after becoming a government informant, Diaz sold cocaine to the defendants.

Diaz recorded (or transmitted to government agents who recorded) a number of his conversations, both telephonic and in person, with the two defendants. He later reviewed the tapes and testified at trial that they accurately recorded the conversations, although not completely; some words were unintelligible and Diaz did not have an independent recollection of what they were. There were also gaps in the tapes — sometimes the signal from the transmitter was interrupted, and sometimes Diaz had failed to start his recorder at the beginning of the conversation or had ended it before the conversation ended. There may also have been erasures, though these may have been of conversations that did not involve the defendants. No "chain of custody" was established; that is, the government could not account for every person who had had access to the recordings between when they were made and the trial.

Because of these irregularities, the defendants argue that the tapes were not "authenticated" and should therefore have been excluded from evidence. Fed.R.Evid. 901. Evidence that is not oral testimony must be shown to be what it purports to be rather than a forgery or other fabrication or an innocent misidentification. But there are no rigid rules, such as chain of custody, for authentication; all that is required is adequate evidence of genuineness. United States v. Brown, 136 F.3d 1176, 1181 (7th Cir. 1998); United States v. Tropeano, 252 F.3d 653, 661 (2d Cir. 2001). (There are such rules for electronic surveillance governed by Title III, but Title III is inapplicable to conversations that, as here, are recorded with the consent of one of the participants. 18 U.S.C. § 2511(2)(c); United States v. Eschweiler, 745 F.2d 435, 437 (7th Cir. 1984); Meredith v. Gavin, 446 F.2d 794, 798 (8th Cir. 1971).) The defendants do not deny that it is their voices on the tapes. Nor is there any indication of splicing or other alterations that might have changed the meaning of what they had actually said. The only complaint is that the gaps or erasures might contain exculpatory material, although there is no evidence that they do. Even if they did, this would not "de-authenticate," and hence make inadmissible, the tapes of the conversations that were recorded. United States v. Robinson, 956 F.2d 1388, 1395 (7th Cir. 1992); United States v. Vega, 860 F.2d 779, 790-91 (7th Cir. 1988); United States v. Calderin-Rodriguez, 244 F.3d 977, 987 (8th Cir. 2001). And it would not raise a Brady issue even if (though there is no indication of this) the government was aware of what was said in the conversations but not recorded, because the defendants, being parties to the conversation, were equally aware. Brady requires disclosure only of exculpatory material known to the government but not to the defendant. E.g., Gauger v. Hendle, 349 F.3d 354, 360 (7th Cir. 2003); Buie v. McAdory, 341 F.3d 623, 625-26 (7th Cir. 2003); Fullwood v. Lee, 290 F.3d 663, 685-86 (4th Cir. 2002).

The defendants argue that Diaz should not have been allowed to testify at all (in which event he could not have authenticated the tapes, and so the government's case would have collapsed) because of the benefits he received in exchange for his assistance to the prosecution, in particular the 20 percent bounty that he received. The defendants imprecisely describe this as a contingent fee for his testimony. A bounty and a witness fee are different. A bounty is a reward for rendering a service that the offeror wants done, whether it's shooting wolves that prey on sheep or catching criminals who prey on humans. E.g., 19 U.S.C. § 1619; Crabill v. Trans Union, L.L.C., 259 F.3d 662, 665 (7th Cir. 2001); United States v. Jennings, 160 F.3d 1006, 1015 n. 3 (4th Cir. 1998); Doe v. United States, 100 F.3d 1576, 1581-82 (Fed. Cir. 1996); cf. United States v. Cervantes-Pacheco, 826 F.2d 310, 311-12 (5th Cir. 1987) (en banc). Here the bounty was for helping the authorities nail drug offenders. Although rather than being a flat fee it was a percentage of the money that the government recovered from the offenders, United States v. Estrada, 256 F.3d 466, 468 (7th Cir. 2001), this form of compensation is what economists call "incentive compatible" ("motivational" would be an alternative term for it): it gives the bounty hunter an incentive to concentrate on the biggest prey.

The bounty was not a contingent fee for testimony because it was paid whether or not Diaz testified. Since most cases, including forfeiture cases (and the forfeiture components of criminal cases), are settled rather than tried, probably Diaz usually earned the bounty without having to testify. Of course when he did testify, as in this case, his bounty may have been riding on the outcome (though this is unclear), giving him a monetary incentive to testify favorably to the government. From the standpoint of incentive to lie, a bounty based on an adjudicative outcome is much more suspect than one obtained simply by procuring a seizure of forfeitable goods. And the defendants are right to point out that paying witnesses (other than experts) for their testimony (beyond the tiny fees permitted, in the case of federal trials, by the Judicial Code, 28 U.S.C. § 1821) is forbidden. 18 U.S.C. § 201(c)(2). Even an expert witness, and a fortiori an occurrence witness, may not be paid more if the party for whom he is testifying wins the case. Tagatz v. Marquette University, 861 F.2d 1040, 1042 (7th Cir. 1988); United States v. Cervantes-Pacheco, supra, 826 F.2d at 313; United States v. Gray, 626 F.2d 494, 499 (5th Cir. 1980); United States v. Palow, 777 F.2d 52, 54 (1st Cir. 1985).

Yet whether violation even of that rule requires exclusion of the testimony from being used against a defendant is a separate question. Tagatz v. Marquette University, supra, 861 F.2d at 1042; United States v. Valona, 834 F.2d 1334, 1343-44 (7th Cir. 1987); United States v. Cervantes-Pacheco, supra, 826 F.2d at 316 (concurring opinion). Exclusion confers windfalls on the guilty and therefore, at least as a device for enforcing non-constitutional rules, is disfavored. United States v. Caceres, 440 U.S. 741, 755-56, 99 S.Ct. 1465, 59 L.Ed.2d 733 (1979) ("a rigid application of an exclusionary rule to every regulatory violation could have a serious deterrent impact on the formulation of additional standards to govern prosecutorial and police procedures"); Buntrock v. SEC, 347 F.3d 995, 999 (7th Cir. 2003); United States v. Newell, 239 F.3d 917, 921 (7th Cir. 2001) ("exclusionary rules are disfavored as remedies for non-constitutional violations of law"); United States v. Gilbert, 942 F.2d 1537, 1541-42 (11th Cir. 1991). An exclusionary rule would be not only costly but also gratuitous in the present setting, because a jury should be competent to discount appropriately testimony given under a powerful inducement to lie. An exclusionary rule for that class of evidence was rejected in United States v. Condon, 170 F.3d 687, 688-89 (7th Cir. 1999), and cases cited in that opinion. It should not be resurrected, whether in the form of the discredited "outrageous government conduct" doctrine here futilely pressed on us by the defendants, United States v. Sherman, 268 F.3d 539, 550 (7th Cir. 2001); United States v. Boyd, 55 F.3d 239, 241-42 (7th Cir. 1995); United States v. Nolan-Cooper, 155 F.3d 221, 229-30 (3d Cir. 1998), or by raising the bar of Rule 403 and presuming that evidence given under strong inducements — such as the bounty at issue here, not to mention permitting Diaz to become a citizen and escape criminal punishment for the drug dealing that he engaged in before he agreed to become a government informant — is so unreliable that its probative value is clearly outweighed by a tendency to confuse the jury.

Any general policy (whether enforced by exclusion or by some other means) against giving a witness inducements to testify, rather than relying entirely on his love of truth or the terrors of the oath, would require reinstating the old rule (illustrated by the trial of the eponymous hero of The Pickwick Papers for breach of promise) that the party to a case may not testify at all because he has an interest, pecuniary or otherwise, in the outcome and is therefore not a disinterested witness. Williams S. Holdsworth, Charles Dickens as a Legal Historian 132-35 (1928). Most of the key witnesses in cases of drug dealing and other "victimless" crimes (that is, crimes consisting of voluntary transactions, so that there is no complaining victim) are criminals testifying in the hope of obtaining leniency or other benefits from the government. Such testimony is frequently indispensable, and it is not worthless; Diaz's testimony, for example, was amply corroborated, and not only by the recordings.

Judges are in no position to evaluate the government's need to offer monetary or other inducements to the criminals whom it hopes to enlist in the "war against drugs." Should we tell the Justice Department that 20 percent was too generous a bounty to pay Diaz? That 5 percent would have been sufficient? That bounties that are too generous might actually induce people to become drug dealers, in the thought that if the business becomes too hot they can always enlist as bounty hunters for the DEA? Our job, so far as it bears on whether Diaz should have been excluded from testifying at all, is to make sure that grossly unreliable evidence is not used to convict a defendant. We do this by requiring (in effect) that the inducements be disclosed to the jury, which can use its common sense to screen out evidence that it finds to be wholly unreliable because of the inducements that the witness received. Wisehart v. Davis, 408 F.3d 321, 323-24 (7th Cir. 2005). The more extravagant the blandishments, the more likely the jury is to discredit the witnesses who received them.

Another issue presented by the appeals concerns the judge's response to a note that she received from the jury during its deliberations. The note asked whether "given the wording [in the indictment] `Pierre Dawson and Alfonso Ingram did conspire with each other . . .,' can one defendant be separate from the other? It's difficult to establish a separate verdict given the wording." The judge responded: "In answer to your question, a conspiracy involves an agreement between a defendant and one or more other persons to accomplish an illegal purpose. Therefore, it is possible to make an individual determination as to each defendant." The defendants argue that the judge should have said " must make an individual determination" rather than " it is possible to make an individual determination," since her wording may have been understood to mean that the jury didn't have to consider the guilt or innocence of each defendant separately.

But it is better for a trial judge to answer the question that the jury asks than another question, unless answering just the jury's question would entrench some misunderstanding revealed by their question. The jury asked "can one defendant be separate from the other" and the judge answered, "yes, one defendant can [`it is possible'] be separated from the other." Had the judge answered "not only can, but must," this might have set the jurors to scratching their heads, wondering what they had missed.

In her original instructions the judge had told the jury that "even though the defendants are being tried together, you must give each of them separate consideration. . . . You must separate consideration both to each count and to each defendant." The jury was not questioning that instruction. It was asking whether it was possible, given that the defendants were accused of conspiring with each other, to convict (or acquit) one defendant but not the other. It was possible, as the judge instructed, but only if there were additional conspirators.

If the only conspirators were Dawson and Ingram (Diaz, who was working undercover for the government when the transactions for which the defendants were prosecuted were made, could not count as a conspirator, United States v. Duff, 76 F.3d 122, 127 (7th Cir. 1996)), then either both were guilty of conspiracy or neither. In that case, to tell the jury that they "must" separate the defendants might well have misled them, in part by giving undue emphasis to the "separate consideration" instruction. There was, however, evidence of other, though uncharged, conspirators with Dawson and Ingram in their very large drug business. And so the jury, if for example it thought the evidence of Ingram's participation in the conspiracy weak (the evidence of Dawson's participation was very strong), could "separate" the defendants and find only Dawson guilty of conspiracy. The supplemental instruction permitted, but rightly did not compel, such separation, since, to repeat, if the jury found that Dawson and Ingram were the sole conspirators, their verdict on the conspiracy count would have to be the same for both of them. It would, however, have been better had the instruction told the jury explicitly that the conspiracy could have involved other persons besides just the defendants and the informant.

In suppression hearings in two previous cases, involving other defendants, the trial judges had disbelieved testimony by three government agents who also testified for the prosecution in our case. Defense counsel in our case wanted to use those judges' rulings to impeach the three witnesses' testimony. The judge refused on the basis of Rule 608(b) of the Federal Rules of Evidence, which provides that "specific instances of the conduct of a witness, for the purpose of attacking or supporting the witness' character for truthfulness, other than conviction of crime as provided in rule 609, may not be proved by extrinsic evidence." Our defendants were not proposing to use extrinsic evidence, however, but merely to ask each witness whether a judge had disbelieved him or her in a previous case. But the error was harmless. In plea bargains that resolved the previous cases, the defendants had acknowledged that the witnesses whom the judges had disbelieved had been telling the truth. The prosecutor would have trotted out the plea bargains on redirect examination in our case, thereby neutering the cross-examination.

Ingram complains that he should not have been denied a sentencing discount for having been a minor participant in the conspiracy. U.S.S.G. § 3B1.2(b). The government argues that though Diaz's dealings were mainly with Dawson there was evidence that Ingram was actually the kingpin. The government points out that the higher you are in an illegal business, the less likely you are to be present at individual transactions, since that would increase your chance of being arrested. Leaders rationally as well as selfishly seek to insulate themselves from the risks to which their underlings are exposed, because a leader is a scarcer commodity than a follower. Steven D. Levitt Sudhir Alladi Venkatesh, "An Economic Analysis of a Drug Selling Gang's Finances," 115 Q.J. Econ. 755, 761-62 (2000). The larger the transaction, however, the likelier the leader is to be present. At a sale of 38 kilograms of cocaine, Ingram carried the $269,000 in cash for the purchase price from Memphis, the headquarters of the gang, and assured Diaz that the deal would go through and that Diaz would get all the money. That was the talk not of a courier but of a manager, Dawson's equal or superior. And another witness testified that it was Ingram who always took possession of the drugs that the gang held at its Memphis headquarters.

So we find no error, but since the sentencing determinations were made under the pre- Booker regime, we have to order the limited Paladino remand. United States v. Paladino, 401 F.3d 471, 483-84 (7th Cir. 2005).