Full opinion text
SELYA, Circuit Judge. This is a landmark case; for the first time in its history, this court must review a sentence of death imposed by a federal judge. To that extent, we are writing on a pristine page. We are guided in this path-breaking endeavor, however, by a variety of reliable sources, including Supreme Court precedent, decisions of other courts of appeals in capital cases, and legal principles of general application. With this brief preface, we turn to the particulars of the case at hand. Defendant-appellant Gary Lee Sampson entered a guilty plea to two counts of carjacking resulting in death. See 18 U.S.C. § 2119(3). On January 29, 2004, the district court sentenced Sampson to death on the recommendation of a jury of his peers. Sampson’s appeal from his sentence raises a host of claims. The first six include five claims that contest the constitutionality of the Federal Death Penalty Act, 18 U.S.C. §§ 3591-3598 (FDPA), pursuant to which the district court pronounced sentence, and one that contests the constitutionality of the death penalty in general. There follows a litany of claims concerning alleged errors specific to Sampson’s penalty-phase trial. The district court’s rulings on many of these issues are embodied in a series of published opinions. See United States v. Sampson, 335 F.Supp.2d 166 (D.Mass.2004) (Sampson IV); United States v. Sampson, 332 F.Supp.2d 325 (D.Mass.2004) (Sampson III); United States v. Sampson, 275 F.Supp.2d 49 (D.Mass.2003) (Sampson II); United States v. Sampson, 245 F.Supp.2d 327 (D.Mass.2003) (Sampson I). We begin this opinion by sketching the background of the case. We then discuss Sampson’s arguments about the constitutionality of the FDPA and the death penalty itself. Finally, we address the myriad claims of trial-related error. In the end, we reject Sampson’s asseverational array in its entirety and affirm his capital sentence. I. BACKGROUND We briefly recount the facts underlying Sampson’s claims. Many of these facts are rehearsed in Sampson IV, 335 F.Supp.2d at 174-75, and McCloskey v. Mueller, 446 F.3d 262, 264-65 (1st Cir. 2006), and we assume the reader’s familiarity with those opinions. Sampson committed a series of bank robberies in North Carolina in May, June, and July of 2001. He then fled to Massachusetts. On July 23, he called the FBI’s Boston office and offered to self-surrender. The call was disconnected and, although he waited for the police to arrive, Sampson was not apprehended. The next day, Phillip McCloskey, a 69-year-old retiree, was driving his car in Weymouth, Massachusetts. He picked up Sampson, who was hitchhiking. When McCloskey later tried to drop Sampson off, Sampson pulled out a knife and told McCloskey to keep driving. Once they reached Marshfield, Sampson forced McCloskey out of the car and attempted to restrain him with a belt. When McClos-key resisted, Sampson stabbed him multiple times and then slit his throat, nearly decapitating him. Sampson proceeded to steal McCloskey’s money and tried to steal his car, which would not start. Three days later, Jonathan Rizzo, a 19-year-old college student, picked up Sampson (who was posing as a stranded traveler) along a road in Plymouth. Sampson forced Rizzo at knifepoint to drive to Ab-ington, where Sampson maintained a makeshift campsite. Sampson tied Rizzo to a tree, gagged him with a sock and a bandana, stabbed him repeatedly in the neck and chest, and slit his throat. After Rizzo was dead, Sampson stole his car and drove to New Hampshire. On July 29, Sampson broke into a home on Lake Winnipesaukee. The next day, the caretaker (Robert Whitney) arrived. Sampson tied him to a chair, gagged him with a washcloth, and strangled him to death with a rope. Sampson then appropriated Whitney’s car and drove to Vermont. On July 31, William Gregory picked up Sampson, who was hitchhiking, near West Bridgewater, Vermont. Sampson attempted to force Gregory at knifepoint onto a dirt road so that he could tie him to a tree and steal his car. Gregory, however, pulled into a rest area and escaped on foot. Sampson made off with Gregory’s car. Later that day, he broke into a home near the Killington ski area. He then called 911 and offered to turn himself in for carjacking Gregory and for the earlier bank robberies. Vermont state troopers arrested Sampson at that locus. Following his detention, Sampson waived his Miranda rights, see Miranda v. Arizona, 384 U.S. 436, 444-45, 86 S.Ct. 1602, 16 L.Ed.2d 694 (1966), and made several detailed confessions to the authorities. On October 24, 2001, a federal grand jury charged Sampson with two counts of carjacking resulting in death (namely, the murders of McCloskey and Rizzo). Sampson offered to plead guilty in exchange for a sentence of life imprisonment without parole but that overture was rejected. In short order, the government filed a superseding indictment to comply with Ring v. Arizona, 536 U.S. 584, 609, 122 S.Ct. 2428, 153 L.Ed.2d 556 (2002), and then served a notice of intent to seek the death penalty, see 18 U.S.C. § 3593(a). After filing numerous pretrial motions that unsuccessfully challenged the constitutionality of the FDPA, Sampson eventually .entered a guilty plea to both counts of the superseding indictment. The district court empaneled a death-qualified jury to determine what punishment should be imposed. See id. § 3593(b)(2)(A); see also United States v. Green, 407 F.3d 434, 436-37 (1st Cir.2005) (discussing “death-qualified” jury requirement). On December 23, 2003, after a six-week penalty-phase trial conducted in accordance with the FDPA, the jury unanimously recommended that Sampson be sentenced to death on both counts of the superseding indictment. The district court sentenced Sampson to death on both counts. United States v. Sampson, 300 F.Supp.2d 275, 276 (D.Mass.2004). The district court denied Sampson’s ensuing motions for judgment as a matter of law, a new penalty-phase trial, and other relief. Sampson III, 332 F.Supp.2d at 341. This appeal followed. II. THE CONSTITUTIONAL CLAIMS Sampson raises six types of constitutional claims. Most of them are attacks on the FDPA. First, Sampson argues that the FDPA, which authorizes prosecutors to decide whether to seek the death penalty, is unconstitutional because it does not require the aggravating factors needed for a sentence of death to be presented to a grand jury as mandated by Ring. Relatedly, he argues that the presentation of aggravating factors to the grand jury in this case was tantamount to executive and judicial redrafting of the statute in derogation of the principles of separation of powers and legislative authority. Second, Sampson argues that because the federal death penalty is so rarely sought or imposed, the FDPA operates in a fundamentally arbitrary and capricious manner (and, thus, is invalid as violative of the Eighth Amendment). Third, he argues that the absence of a principled basis for distinguishing between cases in which the federal death penalty is imposed and those in which it is not renders the FDPA unconstitutional. Fourth, he argues that the federal death penalty is sought on the invidious basis of race and the irrational basis of geography (and, thus, is unconstitutional). Fifth, he argues that continued enforcement of the federal death penalty will lead to execution of a significant number of innocent persons and that, therefore, the FDPA and the death penalty itself are unconstitutional. Finally, he argues that the death penalty is per se unconstitutional. Sampson supports his arguments with Brandeis-brief type information from studies about the operation in fact of the FDPA. The district court rejected each of these claims after careful analysis. See Sampson II, 275 F.Supp.2d at 62-66, 71-94; Sampson II, 245 F.Supp.2d at 330-38. Concluding, as we do, that nearly all of the claims are foreclosed by Supreme Court precedent, we echo this holding. Before addressing the issues, we confirm some basic principles applicable to judicial review. A district court’s rulings on questions of law, including constitutional questions, engender de novo review. See United States v. Bender, 221 F.3d 265, 268 (1st Cir.2000); see also United States v. Marenghi, 109 F.3d 28, 31 (1st Cir. 1997). Statutes duly enacted by Congress are presumed to be constitutional. INS v. Chadha, 462 U.S. 919, 944, 103 S.Ct. 2764, 77 L.Ed.2d 317 (1983). Thus, the burden of proving that the FDPA is unconstitutional is on the challenger (here, Sampson). Lujan v. G & G Fire Sprinklers, Inc., 532 U.S. 189, 198, 121 S.Ct. 1446, 149 L.Ed.2d 391 (2001). Last — but surely not least — when the Supreme Court has directly decided an issue, we must “follow the case [that] directly controls, leaving to [the Supreme] Court the prerogative of overruling its own decisions.” Rodriguez de Quijas v. Shearson/Am. Express, Inc., 490 U.S. 477, 484, 109 S.Ct. 1917, 104 L.Ed.2d 526 (1989). With these principles in mind, we turn to Sampson’s constitutional claims. A. Ring-Related Claims. We begin with Sampson’s multifaceted claim that the FDPA is unconstitutional under Ring. Sampson argues that the FDPA “suffers from a fatal flaw,” Appellant’s Br. at 199, in that it provides for the prosecutor, not a grand jury, to set out by allegation the aggravating factors necessary for a sentence of death whereas the Constitution, as interpreted by the Ring Court, requires that these factors be presented to a grand jury and charged in an indictment. He further argues that allowing the prosecutor to select the aggravating factors and present them to the grand jury in his case constituted “improper executive and judicial redrafting of the statute.” Id. We do not agree. The FDPA defines the circumstances under which defendants who commit certain federal crimes may be eligible for the death penalty. A defendant who commits a qualifying offense is death-penalty eligible only if a jury finds beyond a reasonable doubt that the defendant acted with the statutorily required intent, see 18 U.S.C. §§ 3591(a)(2), 3593(b), and that at least one statutorily defined aggravating factor exists, see id. §§ 3592, 3593(c)-(e). It is only after these threshold findings are made that a jury, considering both mitigating and aggravating factors, may determine that the death penalty is appropriate. See id. § 3593(e)-(e). Part of the statute, section 3593(a), authorizes the Department of Justice (DOJ) to determine whether to seek the death penalty in a particular case. If the government elects to seek the death penalty, the statute requires it to give the defendant notice of its election and of the aggravating factors that it plans to prove. Id. § 3593(a). The FDPA makes no mention of the grand jury. This omission is understandable. Congress enacted the FDPA in 1994 against the backdrop of Walton v. Arizona, 497 U.S. 639, 110 S.Ct. 3047, 111 L.Ed.2d 511 (1990), in which the Supreme Court held that “the Sixth Amendment does not require that the specific findings authorizing the imposition of the sentence of death be made by the jury.” Id. at 648, 110 S.Ct. 3047 (quoting Hildwin v. Florida, 490 U.S. 638, 640-41, 109 S.Ct. 2055, 104 L.Ed.2d 728 (1989) (per curiam)) (internal quotation marks omitted). Essentially, the Walton Court held that the facts necessary to render a defendant eligible for the death penalty were not elements of the offense itself, making inapplicable the Fifth Amendment requirement that the elements of an offense be charged by a grand jury in an indictment. See id. at 649, 110 S.Ct. 3047. The Supreme Court first cast doubt on Walton in Jones v. United States, 526 U.S. 227, 119 S.Ct. 1215, 143 L.Ed.2d 311 (1999). There, the Court considered 18 U.S.C. § 2119, the carjacking statute involved in this case, and held that the statute created three separate offenses. See Jones, 526 U.S. at 251-52, 119 S.Ct. 1215. It further held that the fact that a defendant caused serious bodily injury to another during the commission of a crime was an element of a greater offense that had to be both charged in an indictment and found by a jury at trial beyond a reasonable doubt. See id. at 232-39, 252, 119 S.Ct. 1215. In reaching these conclusions, the Jones Court distinguished the sentencing factors upheld in Walton. See id. at 251, 119 S.Ct. 1215. The next Term, the Court held that any fact that increases the maximum author--ized statutory sentence “is the functional equivalent of an element of a greater offense,” which must be charged in an indictment and proved beyond a reasonable doubt. Apprendi v. New Jersey, 530 U.S. 466, 490, 494 n. 19, 120 S.Ct. 2348, 147 L.Ed.2d 435 (2000). Finally, in 2002 the Court explicitly overruled Walton. See Ring, 536 U.S. at 609, 122 S.Ct. 2428. In the wake of Ring, Supreme Court precedent now firmly establishes that the mental culpability and aggravating factors required by the FDPA must — in addition to being included in the government’s notice to seek the death penalty — be presented to a grand jury, charged in the indictment, and proved beyond a reasonable doubt. But as we shall explain, even though this eliminates a background assumption against which the FDPA was framed, it does not render the statute unconstitutional. Sampson contends both that there is a conflict between the FDPA and Ring, and that curing the problem would require the rewriting of the statute, which is a legislative function. This contention is unpersuasive. The courts that have considered this thesis uniformly have rejected it. See United States v. Brown, 441 F.3d 1330, 1367 (11th Cir.2006), cert, denied, — U.S. —, 127 S.Ct. 1149, — L.Ed.2d - (2007); United States v. Allen, 406 F.3d 940, 949 (8th Cir.2005), cert, denied, — U.S.—, 127 S.Ct. 826, 166 L.Ed.2d 665 (2006); United States v. Barnette, 390 F.3d 775, 788-90 (4th Cir.2004), vacated on other grounds, - U.S.-, 126 S.Ct. 92, 163 L.Ed.2d 32 (2005); United States v. Robinson, 367 F.3d 278, 290 (5th Cir.2004); cf. United States v. Collazo-Aponte, 281 F.3d 320, 324-25 (1st Cir.2002) (rejecting a facial challenge to 21 U.S.C. § 841 because “there is nothing in the statutory language that explicitly defies Apprendi ”). We agree with this line of cases; there is no irredeemable conflict between the FDPA and Ring. The FDPA does not, as Sampson suggests, grant to prosecutors exclusive authority for determining the likely existence of aggravating factors. No provision of the FDPA prohibits a grand jury from considering those factors necessary for imposition of a death sentence. The statute simply is silent with respect to the function of the grand jury. It thus is not rendered facially unconstitutional by Ring. Sampson also presents a variation on this theme, arguing that the application of Ring to the FDPA requires impermissible judicial or executive redrafting of the statute. We think not. What is involved in the application of Ring is a matter of procedure, not of substantive definition regarding death-penalty eligibility. After all, in the habeas context, Apprendi and Ring regularly have been held to announce a new rule of criminal procedure, not a new rule of substantive law. See United States v. Brown, 305 F.3d 304, 308-09 (5th Cir.2002); Cannon v. Midlin, 297 F.3d 989, 994 (10th Cir.2002); United States v. Warden, 286 F.3d 1059, 1063 (8th Cir.2002); United States v. Sanchez-Cervantes, 282 F.3d 664, 668 (9th Cir. 2002); McCoy v. United States, 266 F.3d 1245, 1257 n. 16 (11th Cir.2001); see also Sepulveda v. United States, 330 F.3d 55, 59 (1st Cir.2003) (treating Apprendi as having announced a rule of criminal procedure); United States v. McAllister, 272 F.3d 228, 232 (4th Cir.2001) (holding, in the context of a constitutional challenge to 21 U.S.C. § 841, that Apprendi announced a procedural rule). To cinch matters, the Jones Court made pellucid that “[t]he constitutional guarantees that g[a]ve rise to [its] concern in no way restrict the ability of legislatures to identify the conduct they wish to characterize as criminal or to define the facts whose proof is essential to the establishment of criminal liability.” 526 U.S. at 243 n. 6,119 S.Ct. 1215. It follows, then, that the rule against massive judicial rewriting of statutes simply is not implicated here. Adhering to a court-crafted rule of criminal procedure when applying the FDPA does not constitute impermissible statutory redrafting. In this respect, the instant case is unlike United States v. Jackson, 390 U.S. 570, 88 S.Ct. 1209, 20 L.Ed.2d 138 (1968), on which Sampson relies. In Jackson, the Supreme Court invalidated the death penalty provision of the Federal Kidnaping Act, which permitted only a jury to impose a death sentence. Id. at 591, 88 S.Ct. 1209. The objection was that it infringed on the right to a jury trial by encouraging a defendant to plead guilty or to waive that right in order to avoid the possibility of a death sentence. Id. at 572-73, 88 S.Ct. 1209. The government encouraged the Supreme Court to save the statute by reading it to allow a judge — in the event of a guilty plea or bench trial — to convene a “special jury” to determine whether the death penalty was warranted. Id. The Court rejected this suggestion, stating that it could not “create from whole cloth a complex and completely novel procedure and ... thrust it upon unwilling defendants for the sole purpose of rescuing a statute from a charge of unconstitutionality.” Id. at 580, 88 S.Ct. 1209. This case, however, does not require us to “create from whole cloth a complex and completely novel procedure.” As the district court perspicaciously noted, see Sampson I, 245 F.Supp.2d at 337, the role of the grand jury jn charging the elements of an offense has long been established. See, e.g., Hamling v. United States, 418 U.S. 87, 117-18, 94 S.Ct. 2887, 41 L.Ed.2d 590 (1974) (ruling on a challenge to the sufficiency of an indictment); Russell v. United States, 369 U.S. 749, 763-64, 82 S.Ct. 1038, 8 L.Ed.2d 240 (1962) (same). Here, the government honored that role; it simply presented the grand jury with evidence of possible aggravating factors. This case also is unlike Blount v. Rizzi, 400 U.S. 410, 91 S.Ct. 423, 27 L.Ed.2d 498 (1971), and United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005), on which Sampson further relies. In Blount, the Supreme Court invalidated two provisions of the Postal Reorganization Act because they did not conform to the constitutional requirements of an administrative censorship scheme. 400 U.S. at 421-22, 91 S.Ct. 423. The Court rejected a proposed severing construction of the statute that itself failed to comply with the Constitution and then noted that “it [was] for Congress, not [the] Court, to rewrite the statute.” Id. at 419, 91 S.Ct. 423. In Booker, the Court refused to graft onto the federal sentencing guidelines a set of procedures for presenting to a jury facts necessary for sentence enhancements. 543 U.S. at 246, 125 S.Ct. 738. Instead, the Court severed from the guidelines the provision making mandatory the effect of sentencing factors not found by a jury. Id. at 245, 125 S.Ct. 738. To do otherwise, it held, “would [have] so transform[ed] the scheme that Congress created that Congress likely would not have intended the Act as so modified to stand.” Id. at 249, 125 S.Ct. 738. The statutes in question in Blount and Booker, like the statute in Jackson, were incompatible with constitutional requirements. See id. at 227-29, 125 S.Ct. 738; Blount, 400 U.S. at 417, 91 S.Ct. 423. Saving either statute, as proposed by the government, while at the same time complying with constitutional mandates, would have required the Court to perform a complete statutory rewrite, which is a legislative and not a judicial function. In contrast, allowing a grand jury to consider and charge aggravating factors under the FDPA does not have any effect either on the substantive aspects of the statute or on the discrete roles that the statute assigns to the judge, the prosecutor, and the jury, respectively. To sum up, Ring does not render the FDPA unconstitutional either on its face or as applied in this case. B. Arbitrary Operation. Sampson makes two arguments in support of his claim that the FDPA is unconstitutional because it is arbitrary in its operation. First, he argues that because the federal death penalty is infrequently sought and even more infrequently carried out, its imposition is arbitrary, capricious, and therefore unconstitutional. Sampson centers this argument around Furman v. Georgia, 408 U.S. 238, 92 S.Ct. 2726, 33 L.Ed.2d 346 (1972), in which the Supreme Court struck down as unconstitutional the Georgia capital punishment statute then in effect. In a concurring opinion, Justice Stewart stated that the death sentences at issue were cruel and unusual in the same way that being struck by lightning is cruel and unusual. For, of all the people convicted of rapes and murders in 1967 and 1968, many just as reprehensible as [those committed by petitioners], the petitioners are among a capriciously selected random handful upon whom the sentence of death has in fact been imposed. Id. at 309-10, 92 S.Ct. 2726 (Stewart, J., concurring) (footnote omitted). Sampson argues that Justice Stewart’s statement constitutes the essence of Furman. See Appellant’s Br. at 238 (characterizing Fur-man as viewing “arbitrariness and caprice ... as the inevitable side-effects of a rarely-imposed punishment of death”). Given this reading, he argues that because the federal death penalty is rarely sought or imposed, the FDPA is no different from the Georgia statute invalidated in Furman. This argument mistakes the nature of the arbitrariness concern in the Supreme Court’s jurisprudence. In the thirty-four years since Furman was decided, the Court has made clear that its decision was not based on the frequency with which the death penalty was sought or imposed. Rather, the primary emphasis of the Court’s death penalty jurisprudence has been the requirement that the discretion exercised by juries be guided so as to limit the potential for arbitrariness. Thus, in Gregg v. Georgia, 428 U.S. 153, 96 S.Ct. 2909, 49 L.Ed.2d 859 (1976), the Court, upholding the revised Georgia capital sentencing statute, described Furman as “mandatfing] that where discretion is afforded a sentencing body on a matter so grave as the determination of whether a human life should be taken or spared, that discretion must be suitably directed and limited so as to minimize the risk of wholly arbitrary and capricious action.” Id. at 189, 96 S.Ct. 2909 (opinion of Stewart, Powell, and Stevens, JJ.). Together, Furman and Gregg require that a death penalty statute “(1) rationally narrow the class of death-eligible defendants[ ] and (2) permit a jury to render a reasoned, individualized sentencing determination based on a death-eligible defendant’s record, personal characteristics, and the circumstances of his crime.” Kansas v. Marsh, - U.S. -, 126 S.Ct. 2516, 2524-25, 165 L.Ed.2d 429 (2006). As the Supreme Court recognized in a different context, these requirements “further an essential need of the Anglo-American criminal justice system — to balance the desirability of a high degree of uniformity against the necessity for the exercise of discretion.” McCleskey v. Kemp, 481 U.S. 279, 312 n. 35, 107 S.Ct. 1756, 95 L.Ed.2d 262 (1987). Like the statute upheld in Gregg, the FDPA fully meets the requirements of guided discretion, suitably directing and limiting the leeway afforded to the decisionmakers. Nor does the frequency with which the federal death penalty is sought render the FDPA unconstitutional. To the extent that there may be an independent constitutional concern as to the decisional process by which the government decides if it will seek the death penalty, that process contains numerous safeguards built into an articulated death penalty protocol. See United States Attorneys’ Manual § 9-10.000, http://www.usdoj.gov/usao/eousa/ foia — reading—room/usam/title9/10m crm. htm. The process, therefore, is not arbitrary. See Gregg, 428 U.S. at 195, 96 S.Ct. 2909. Sampson’s second argument is that the FDPA is unconstitutional because there is no principled basis for distinguishing between those cases in which the federal death penalty is imposed and those in which it is not. His argument is premised on Eddings v. Oklahoma, 455 U.S. 104, 102 S.Ct. 869, 71 L.Ed.2d 1 (1982), in which the Supreme Court stated that “capital punishment [must] be imposed fairly, and with reasonable consistency, or not at all.” Id. at 112, 102 S.Ct. 869. This argument ignores the remainder of the Eddings Court’s discussion of consistency, in which the Court recognized that “a consistency produced by ignoring individual differences is a false consistency.” Id. Indeed, the thrust of Eddings is that those who make sentencing decisions must be permitted to focus on the individual characteristics of the defendant and the circumstances of the crime. Id. And, finally, the argument cannot survive McCleskey, in which the Court stated that “[t]he Constitution is not offended by inconsistency in results based on the objective circumstances of the crime. Numerous legitimate factors may influence ... a defendant’s ultimate sentence, even though they may be irrelevant to his actual guilt.” 481 U.S. at 307 n. 28, 107 S.Ct. 1756. In all events, the “evidence” that Sampson submits is wholly inadequate to prove that the death penalty has been imposed in an arbitrary manner. The summaries on which Sampson relies to demonstrate inconsistency are devoid of details and fail to account for the objective circumstances of the underlying crimes. Even the more detailed verdict sheets that he submitted to the district court fail to establish arbitrary imposition of the death penalty. On this record and mindful of the teachings of McCleskey, we decline Sampson’s invitation to ignore individual differences across offenders and offenses. Consequently, there is no principled basis for finding that similar cases are treated differently. C. Other Challenges to the FDPA. Sampson’s remaining challenges to the constitutionality of the FDPA are those related to race, geography, and innocence. Sampson (who is white) raises no argument that he was sentenced to death because of his race, the race of his victims, or the geographic location in which he was sentenced. Nor does he claim to be actually innocent. What, then, is his claim? In essence, Sampson attempts to assert the rights of other capital defendants. His claim is that, although he himself was not disadvantaged by race or geography and although he is not innocent, the FDPA is unconstitutional because it has these untoward effects elsewhere. And if the FDPA is unconstitutional, his thesis runs; he may not be sentenced under it. It is questionable whether Sampson has standing to challenge the constitutionality of the federal death penalty on these grounds. See County Court v. Allen, 442 U.S. 140, 155, 99 S.Ct. 2213, 60 L.Ed.2d 777 (1979) (“As a general rule, if there is no constitutional defect in the application of the statute to a litigant, he does not have standing to argue that it would be unconstitutional if applied to third parties in hypothetical situations.”); see also Broadrick v. Oklahoma, 413 U.S. 601, 610, 93 S.Ct. 2908, 37 L.Ed.2d 830 (1973) (citing cases). Because there is no Supreme Court precedent directly on point and because “death is ... different,” Gardner v. Florida, 430 U.S. 349, 357, 97 S.Ct. 1197, 51 L.Ed.2d 393 (1977), we will assume ar-guendo that Sampson has standing to pursue these arguments. He nonetheless fails to prove the unconstitutionality of the FDPA. 1. Race-and-Geography-Related Claims. We take first Sampson’s claims that the FDPA is unconstitutional because the death penalty is sought based on the race of the defendant and victim and on the locale in which the defendant is charged. Sampson’s race-based claims implicate the Fifth and Eighth Amendments. His geography-related claim implicates the Eighth Amendment. Sampson’s challenges are based primarily on a 2000 DOJ study of the administration of the federal death penalty from 1988 to 2000, and on a 2001 supplemental report. He reads the studies as demonstrating a pattern of discrimination against minority defendants and against defendants in the South. For example, he notes that over 70% of federal defendants for whom the death penalty was sought were nonwhites. Similarly, between 1995 and 2000, only slightly more than half of the districts in the federal system submitted a case to the Attorney General with a recommendation for capital prosecution. Sampson also cites evidence of a white-victim effect, including one study showing that the death penalty authorization rate is 37% in white-victim cases, but only 21% in minority-victim cases. Bare statistical discrepancies are insufficient to prove a Fifth Amendment violation with respect to the implementation of a statute. This principle is firmly established by McCleskey. 481 U.S. at 292,107 S.Ct. 1756 (stating that “to prevail under the Equal Protection Clause, [a defendant] must prove that the decisionmak-ers in his case acted with discriminatory purpose”). It applies here: because Sampson has presented no specific evidence of purposeful discrimination either against himself or against those southern and minority defendants upon whom he purports to base his claim, his Fifth Amendment challenge fails. By like token, Sampson’s Eighth Amendment claims cannot succeed. The McCleskey Court, in rejecting an Eighth Amendment claim based on a statistical study indicating race-based discrepancies in capital sentencing, stated: Apparent disparities in sentencing are an inevitable part of our criminal justice system.... [0]ur consistent rule has been that constitutional guarantees are met when “the mode [for determining guilt or punishment] itself has been surrounded with safeguards to make it as fair as possible.” Where the discretion that is fundamental to our criminal process is involved, we decline to assume that what is unexplained is invidious. 481 U.S. at 312-13, 107 S.Ct. 1756 (second alteration in original) (citation omitted) (quoting Singer v. United States, 380 U.S. 24, 35, 85 S.Ct. 783, 13 L.Ed.2d 630 (1965)). The statistics submitted by Sampson are no more probative than those rejected in McCleskey. The DOJ study provides no basis for attributing the statistical discrepancies with respect to geography and race in FDPA prosecutions to discrimination rather than to other factors, such as differences in the nature of the crimes involved. McCleskey prohibits us from assuming that “what is unexplained is invidious.” Id. 2. Innocence-Related Claims. Sampson’s final claims regarding the FDPA’s constitutionality concern the risk of executing innocent defendants. Again, we assume arguendo that Sampson, who has admitted his guilt by pleading to the charges against him, has standing to assert the interests of the innocent. Sampson appears to make two closely related Fifth Amendment arguments, both of which derive from his reliance on United States v. Quinones, 205 F.Supp.2d 256 (S.D.N.Y.), rev’d, 313 F.3d 49 (2d Cir. 2002). Sampson’s first argument is that because factual studies establish that innocent defendants have been convicted and sentenced to death, the Fifth Amendment right to procedural due process demands that defendants be permitted to attempt to prove their innocence throughout their natural lives. His second argument is that the risk of executing the innocent offends Fifth Amendment substantive due process protections. As with Sampson’s other constitutional challenges to the FDPA, these arguments are foreclosed by Supreme Court precedent. For years, a perennial debate has raged over the propriety of capital punishment in view of the omnipresent risk of executing innocent defendants. See Quinones, 313 F.3d at 63-64. Withal, the Court has declined, for over two centuries, to hold the death penalty unconstitutional per se. In Furman, for example, both Justices Marshall and Brennan explicitly recognized that the risk of executing the innocent is inherent in capital punishment. See 408 U.S. at 364, 366-68, 92 S.Ct. 2726 (Marshall, J., concurring); id. at 290, 92 S.Ct. 2726 (Brennan, J., concurring). Despite this stark reality, no majority of Justices ever has held the death penalty unconstitutional on those grounds. The Court squarely addressed the issue of the risk of executing the innocent in Herrera v. Collins, 506 U.S. 390, 113 S.Ct. 853, 122 L.Ed.2d 203 (1993). It affirmed the denial of a petition for a writ of habeas corpus notwithstanding a claim by the petitioner that new evidence could prove his actual innocence. Id. at 393,113 S.Ct. 853. The Court noted that “[cjlaims of actual innocence based on newly discovered evidence have never been held to state a ground for federal habeas relief absent an independent constitutional violation occurring in the underlying state criminal proceeding.” Id. at 400, 113 S.Ct. 853. The Court continued: We may assume, for the sake of argument in deciding this case, that in a capital case a truly persuasive demonstration of “actual innocence” made after trial would render the execution of a defendant unconstitutional, and warrant federal habeas relief if there were no state avenue open to process such a claim. But ... the threshold showing for such an assumed right would necessarily be extraordinarily high. Id. at 417, 113 S.Ct. 853. We understand Herrera to leave open the possibility that, in a particular instance of newly discovered, highly persuasive evidence of innocence, emerging at a time when no state remedy remains available, a federal court might be able to issue a writ of habeas corpus under the Constitution to prohibit execution. That is a far cry, however, from saying that the FDPA is unconstitutional. Indeed, Herrera supports the constitutionality of the statute. Despite the “unalterable fact that our judicial system, like the human beings who administer it,, is fallible,” id. at 415, 113 S.Ct. 853, the Herrera Court could not say that the “refusal to entertain petitioner’s newly discovered evidence eight years after his conviction” transgressed his due process rights. Id. at 411, 113 S.Ct. 853. This is entirely consistent with the Court’s opinion in Chapman v. United States, 500 U.S. 453, 111 S.Ct. 1919, 114 L.Ed.2d 524 (1991), which prevents inferi- or federal courts from holding capital punishment per se violative of due process. See id. at 465, 111 S.Ct. 1919. The Chapman Court held that under the Due Process Clause, [e]very person has a fundamental right to liberty in the sense that the Government may not punish him unless and until it proves his guilt beyond a reasonable doubt at a criminal trial conducted in accordance with the relevant constitutional guarantees. But a person who has been so convicted is eligible for, and the court may impose, whatever punishment is authorized by statute for his offense, so long as that penalty is not cruel and unusual, and so long as the penalty is not based on an arbitrary distinction that would violate the Due Process Clause of the Fifth Amendment. Id. (citations omitted); see United States v. Inglesi, 988 F.2d 500, 503 (4th Cir.1993) (quoting Chapman for the proposition that “the relevant due process inquiry on [a constitutional challenge in the sentencing context] is only whether the sentence at issue is ‘based on an arbitrary distinction,’ or, instead, on ‘a rational sentencing scheme’ ”). The Supreme Court has held that capital punishment is not per se cruel and unusual, see Gregg, 428 U.S. at 187, 96 S.Ct. 2909 (opinion of Stewart, Powell, and Stevens, JJ.); id. at 226, 96 S.Ct. 2909 (opinion of White, J., joined by Burger, C.J., and Rehnquist, J.), and there is no allegation here that the execution of innocent individuals results from arbitrary distinctions or application of the FDPA. We are bound by this Supreme Court precedent. Accordingly, we join those courts of appeals that have rebuffed similar arguments, see Robinson, 367 F.3d at 290; Quinones, 313 F.3d at 61-69, and reject Sampson’s claims that the federal death penalty is unconstitutional because it necessarily entails a risk of executing the innocent. D. The Death Penalty Simpliciter. In a final catchall argument, Sampson asks us to declare the death penalty unconstitutional as cruel and unusual punishment and a per se denial of due process in all cases. Sampson concedes that under Supreme Court precedent the argument fails. Yet, he urges this court to rule to the contrary because the death penalty is “racist to its very core,” Appellant’s Br. at 291; leads to the execution of innocent people; and vests an unacceptable level of unreviewable discretion in prosecutors. He adds, moreover, his belief that the American public eventually will determine that capital punishment is wrong and immoral. In Gregg, the Supreme Court held that “the death penalty is not a form of punishment that may never be imposed, regardless of the circumstances of the offense, regardless of the character of the offender, and regardless of the procedure followed in reaching the decision to impose it.” 428 U.S. at 187, 96 S.Ct. 2909 (opinion of Stewart, Powell, and Stevens, JJ.); see id. at 226, 96 S.Ct. 2909 (opinion of White, J., joined by Burger, C.J., and Rehnquist, J.); accord Roberts v. Louisiana, 428 U.S. 325, 331, 96 S.Ct. 3001, 49 L.Ed.2d 974 (1976) (opinion of Stewart, Powell, and Stevens, JJ.); id. at 350-56, 96 S.Ct. 3001 (White, J., joined by Burger, C.J., Blackmun, J. and Rehnquist, J., dissenting). The Chapman Court laid out the test, under which Sampson cannot prevail, for determining whether a particular form of punishment violates due process. Because Gregg, Roberts, and Chapman are binding upon us, we reject Sampson’s bedrock claim and hold that the death penalty itself is not unconstitutional. III. CLAIMS OF TRIAL ERROR We turn next to Sampson’s manifold claims of trial error. We begin by evaluating alleged errors in the charge and the jury selection process. We then proceed to Sampson’s evidence-related claims, including challenges to several of the district court’s rulings and to evidentiary sufficiency. We conclude by addressing a potpourri of other claims, including Sampson’s invocation of the cumulative error doctrine. A. Jury Instructions. We begin with the district court’s charge to the jury. Sampson alleges three strains of instructional error. Because each of them presents a properly preserved question of law, we afford de novo review, taking into,account the charge as a whole and the body of evidence presented at trial. See United States v. Woodward, 149 F.3d 46, 68-69 (1st Cir.1998); United States v. Alzanki, 54 F.3d 994, 1001 (1st Cir.1995). 1. Weighing. Sampson’s primary complaint of instructional error relates to the district court’s charge on the weighing of aggravating and mitigating factors. In Sampson’s view, these instructions violated the FDPA and, in the bargain, infringed upon constitutional protections. Specifically, he argues that the instructions (i) erroneously invited jurors to apply their own idiosyncratic standards to the weighing process and (ii) failed to require that they find beyond a reasonable doubt that aggravating factors outweighed mitigating factors before voting to impose the death penalty. Because our assessment of Sampson’s plaints must take into account the weighing instructions as a whole, Woodward, 149 F.3d at 69, we reprint the pertinent portions here: [Y]ou are called upon to decide if the proven aggravating factors or factor sufficiently outweigh the proven mitigating factors. This is not a matter of arithmetic. You’re not being asked to simply count the total number of aggravating and mitigating factors and reach a decision based on which number is greater. Instead, you must consider the weight and value that you feel should be given to each factor. Different factors may be given different weights or values by different jurors. You might find that a single aggravating factor is serious enough to outweigh several mitigating factors. Similarly, a single mitigating factor might outweigh several aggravating factors. If you find that the government has not proven that the aggravating factor or factors outweigh the mitigating factors at all, you may not vote to impose the death penalty on the count that you’re considering. If, however, you decide that the prosecution has proven that the aggravating factor or factors outweigh the mitigating factors, you must decide if the prosecution has also proven beyond a reasonable doubt that those aggravating factors sufficiently outweigh the mitigating factors to make death the appropriate penalty for Mr. Sampson’s crime rather than life in prison without possibility of release. The law does not define what is sufficient to make death the appropriate penalty. Here, the law relies on each of you as a representative of our community to consult your conscience and determine what is sufficient to justify Mr. Sampson’s execution. Thus, your decision as to what the appropriate sentence is will depend in part on what is sufficient for you. If you find that the government has proven that the aggravating factors slightly outweigh the mitigating factors and that is sufficient for you to find that death is the appropriate penalty, you may properly vote for death. On the other hand, even if the government has proven to you that the aggravating factors greatly outweigh the mitigating factors, you may properly decide that this is not sufficient to justify a sentence of death because, for you, even more is required for you to find that a man should die. However you personally define sufficiency, the prosecution must convince you beyond a reasonable doubt that the aggravating factor or factors sufficiently outweigh the mitigating factors to make death the appropriate penalty in this case. As I told you earlier, this is a heavy burden. More than a strong probability is required. You must be certain beyond any reasonable doubt that a death sentence should be imposed before voting for it. Death is, of course, the ultimate irreversible punishment. You must not sentence Gary Sampson to die unless you are convinced beyond a reasonable doubt that death is the appropriate punishment. As I’ve told you previously, the law never requires that any or all of you find that the death penalty is justified. Any one of you may decline to impose a death sentence. If you decide that the prosecution has not proven beyond a reasonable doubt that the death penalty is justified, you do not have to give a reason for that decision. The law does require that you follow the process that I’ve explained and then make a reasoned moral judgment. Sampson argues initially that the sufficiency language embedded in this part of the charge violates both the FDPA and the Eighth Amendment. He asserts that, by inviting each juror to use his or her own definition of sufficiency, the district court “offered its own idiosyncratic definition of the language of the statute,” Appellant’s Br. at 62-63, and failed to provide adequate sentencing guidance as required by the Eighth Amendment in capital cases. To determine whether the sufficiency language comported with the FDPA, we start by consulting the statute’s text; we then proceed, if necessary, to examine its structure and surrounding provisions. See Green, 407 F.3d at 441-43. The relevant section of the FDPA, 18 U.S.C. § 3593(e), which we set out in the margin, provides no definition of the phrase “sufficiently outweigh.” Thus, the court’s definition of sufficiency does not explicitly contradict the statutory text. At any rate, the court’s definition is fully consistent with the immediately preceding paragraph, which provides that “any member of the jury who finds the existence of a mitigating factor may consider such factor established.” Id. § 3593(d). For jurors to consider different mitigating factors, they would necessarily need to engage in the type of individualized weighing described by the district court. Given the statutory phrase “sufficiently outweigh,” Sampson’s attack under the FDPA fails. We hold, as well, that the sufficiency instruction does not violate the Eighth Amendment. Sampson argues strenuously that the Eighth Amendment, if not the statute, requires that the jury be instructed to follow a uniform method of weighing, precluding individual approaches. We disagree. The Supreme Court repeatedly has emphasized that key to harmonizing a capital sentencing scheme with the proscription against cruel and unusual punishment is an individualized determination by the jury of whether, taking into account all the relevant aggravating and mitigating factors, a death sentence is appropriate in a particular case. See, e.g., Tuilaepa v. California, 512 U.S. 967, 973, 114 S.Ct. 2630, 129 L.Ed.2d 750 (1994); Zant v. Stephens, 462 U.S. 862, 879, 103 S.Ct. 2733, 77 L.Ed.2d 235 (1983). Sampson correctly points out that the jury’s discretion in sentencing must be “suitably directed and limited so as to minimize the risk of wholly arbitrary and capricious action.” Gregg, 428 U.S. at 189, 96 S.Ct. 2909 (opinion of Stewart, Powell, and Stevens, JJ.). However, once a jury has found a defendant death-eligible, it may be given unfettered discretion in the weighing process. See Tuilaepa, 512 U.S. at 979, 114 S.Ct. 2630 (explaining that “[a] capital sentencer need not be instructed how to weigh any particular fact in the capital sentencing decision”); see also Ayers v. Belmontes, - U.S. -, 127 S.Ct. 469, 479, 166 L.Ed.2d 334 (2006). Sampson’s argument that the jury needed more guidance in order to weigh aggrava-tors against mitigators is foreclosed by these precedents. Sampson also alleges that the weighing instructions failed to require jurors to find beyond a reasonable doubt that aggravating factors outweighed mitigating factors, and thus violated the FDPA and the Fifth and Sixth Amendments. We can readily dismiss the statutory argument. The FDPA makes no mention of the reasonable doubt standard in the context of weighing aggravating and mitigating factors, see 18 U.S.C. § 3593(e), but it does reference the reasonable doubt standard in two proximate sections, see id. §§ 3591(a)(2), 3593(c). Because the inclusion of a term in one part of a statute is persuasive evidence that its omission elsewhere is deliberate, see Green, 407 F.3d at 443, we hold that Congress did not intend the reasonable doubt standard to apply to the weighing process. That being so, Sampson cannot have been harmed by the instruction given. Sampson’s constitutional arguments on this front are no more persuasive. Relying on a procession of Supreme Court eases, see, e.g., Booker, 543 U.S. at 244, 125 S.Ct. 738; Blakely v. Washington, 542 U.S. 296, 305, 124 S.Ct. 2531, 159 L.Ed.2d 403 (2004); Ring, 536 U.S. at 609, 122 S.Ct. 2428; Apprendi, 530 U.S. at 490, 120 S.Ct. 2348, he contends that the balance between aggravating and mitigating factors is a “fact” that should have been found by the jury beyond a reasonable doubt. The district court’s alleged failure to instruct that the reasonable doubt standard applied to this “fact” was, therefore, error. Sampson’s attempt to draw an analogy between this weighing determination and the sentencing determinations found unconstitutional in the Apprendi line of cases lacks force. In Blakely, for example, the Court invalidated a sentence that had been elevated above the statutory maximum based upon a finding of “deliberate cruelty” because the facts supporting that finding were found by a judge, not by the jury beyond a reasonable doubt. See 542 U.S. at 803, 124 S.Ct. 2531. The case at hand is different in kind because, under the FDPA, the jury already had found beyond a reasonable doubt the facts needed to support a sentence of death — the presence of aggravating factors and the requisite intent — before it reached the weighing stage. See 18 U.S.C. §§ 3591(a)(2), 3593(c)-(e). Sampson tries to circumvent this logic by pointing out that the weighing determination itself is essential to imposing a death sentence. He repeatedly refers to Justice Scalia’s statement “that all facts essential to imposition of the level of punishment that the defendant receives— whether the statute calls them elements of the offense, sentencing factors, or Mary Jane — must be found by the jury beyond a reasonable doubt.” Ring, 536 U.S. at 610, 122 S.Ct. 2428 (Scalia J., joined by Thomas, J., concurring). This argument founders, however, because it assumes, without the slightest support, that the weighing of aggravating and mitigating factors is a fact. This assumption is incorrect. As other courts have recognized, the requisite weighing constitutes a process, not a fact to be found. See United States v. Purkey, 428 F.3d 738, 750 (8th Cir.2005) (characterizing the weighing process as “the lens through which the jury must focus the facts that it has found” to reach its individualized determination), cert, denied, - U.S.-, 127 S.Ct. 433, 166 L.Ed.2d 307 (2006); see also Ford v. Strickland, 696 F.2d 804, 818 (11th Cir.1983); Gray v. Lucas, 685 F.2d 139, 140 (5th Cir.1982) (per curiam). The outcome of the weighing process is not an objective truth that is susceptible to (further) proof by either party. Hence, the weighing of aggrava-tors and mitigators does not need to be “found.” We hold, therefore, that the district court’s instructions were free from Apprendi error. Sampson has a residual argument: that the district court’s simultaneous and alternating references to both a personal notion of sufficiency and a reasonable doubt standard were confusing and inconsistent. We evaluate this argument by determining whether there exists a reasonable likelihood that the jury interpreted the instructions in a way that would violate the law. See Boyde v. California, 494 U.S. 370, 380, 110 S.Ct. 1190, 108 L.Ed.2d 316 (1990); see also Jones v. United States, 527 U.S. 373, 390, 119 S.Ct. 2090, 144 L.Ed.2d 370 (1999). Under this approach, a defendant cannot rely solely on how a single hypothetical juror might have interpreted a challenged instruction. See Boyde, 494 U.S. at 380, 110 S.Ct. 1190 (noting the “strong policy against retrials ... where the claimed error amounts to no more than speculation”). After careful assessment of the instructions as a whole, we reject Sampson’s contention that the challenged instruction is likely to have caused harmful juror confusion. Sampson places great emphasis on the language telling jurors that they could impose the death penalty if the aggravating factors “slightly outweigh[ed]” the mitigating factors. This phrase was followed immediately, however, by an assurance that jurors could refuse to vote for a death sentence “even if the government ha[d] proven to [them] that the aggravating factors greatly outweigh[ed] the mitigating factors.” The court further instructed that however jurors defined sufficiency, the prosecution had to convince them “beyond a reasonable doubt that the aggravating factor or factors sufficiently outweigh the mitigating factors to make death the appropriate penalty in [the] case.” Viewed in its entirety, this instruction clearly communicated to jurors the relatively straightforward proposition that they, as individuals, had to be certain that death was the appropriate punishment before imposing it. Even if we assume (favorably to Sampson) that the instruction might have been mildly confusing, any error was benign. In a capital case, as in any other case, a confusing instruction may be harmless. See Boyde, 494 U.S. at 383-84, 110 S.Ct. 1190. Here, we discern no reasonable likelihood that jurors may have inter-preted the instruction in a way that could have harmed Sampson. There are only two possibilities: either the jurors eschewed the reasonable doubt standard visa-vis the weighing process (which, as we have held, would have comported fully with the law) or they applied the reasonable doubt standard (which would have benefitted Sampson by imposing a more onerous burden on the government). Any error was, therefore, patently harmless. 2. Vulnerable Victim. The FDPA contemplates that the government must show the presence of at least one enumerated aggravating factor in order to render a defendant death-eligible. See 18 U.S.C. § 3593(e). In this case, the jury unanimously found several statutory aggravating factors: that Sampson committed both the McCloskey and Rizzo murders in an “especially heinous, cruel, or depraved manner,” id. § 3592(c)(6); that McCloskey was “particularly vulnerable due to ... infirmity,” id. § 3592(e)(ll); and that Rizzo was murdered “after substantial planning and premeditation,” id. § 3592(c)(9). With respect to certain of these factors, Sampson objects to the lower court’s jury instructions. We turn next to his challenge anent the “vulnerable victim” instructions. The FDPA permits the vulnerability of a victim to be used as an aggravating factor and defines that quality as a showing that the victim was “particularly vulnerable due to old age, youth, or infirmity.” Id. § 3592(c)(ll). In this case, the government contended that Phillip McCloskey was a vulnerable victim. After finding the evidence sufficient to support a vulnerable victim instruction vis-a-vis McCloskey, the district court instructed as follows: Question 3B asks whether it’s been proven to each and every one of you beyond a reasonable doubt that Philip McCloskey was particularly vulnerable due to infirmity. In essence, the Federal Death Penalty statute provides that a defendant is especially blameworthy if he murders someone who is particularly vulnerable to being killed because he has an infirmity which made him less able to escape or resist attack than most people. In this context, to be vulnerable means to be subject to being attacked or injured because of some weakness. To be particularly vulnerable means to be especially or significantly vulnerable or to be vulnerable to a particularly high degree. An infirmity is a physical or mental weakness or flaw. To prove this aggravating factor, it must also be proven beyond a reasonable doubt to each and every one of you that there was a connection between Mr. McCloskey’s alleged vulnerability and his death. This means that any infirmity which you find made Mr. McCloskey particularly vulnerable must somehow have contributed to his death. However, the requirement of a connection between any proven infirmity and a person’s death does not mean that the prosecution must prove that the defendant knew of Mr. McClos-key’s alleged vulnerability and targeted him because of it. Rather, it means that the prosecution must prove that, once targeted, Mr. McCloskey was significantly more vulnerable to being killed because he had an infirmity. Sampson alleges that this instruction violated his Eighth Amendment rights because it did not require the jury to find that he knew McCloskey was particularly vulnerable due to infirmity and targeted McCloskey on that account. To support this contention, he argues that this aggravating factor, absent an element of scienter, was based on the “mere happenstance” of the victim’s infirmity, Appellant’s Br. at 179, and did not adequately narrow the class of persons eligible for the death penalty. See Arave v. Creech, 507 U.S. 463, 474, 113 S.Ct. 1534, 123 L.Ed.2d 188 (1993). Sampson’s remonstrance notwithstanding, it is clear that the challenged instruction complies with the FDPA. The statutory provision describing this aggravating factor does not contain a scienter element. See 18 U.S.C. § 3592(c)(ll). By contrast, other statutory aggravating factors contain explicit scienter requirements. See, e.g., id. § 3592(c)(5) (requiring a showing that “[t]he defendant ... knowingly created a grave risk of death”); id. § 3592(d)(4) (requiring a showing that the defendant either used a firearm or “knowingly directed ... another to use a firearm”); id. § 3592(d)(8) (requiring a showing that the defendant was aware of the presence of a potentially lethal adulterant). Because “the intentional inclusion of a[n] [element] in one part of the statute persuasively indicates that the exclusion of such a[n] [element] in another part of the same statute , was intentional,” Green, 407 F.3d at 443, we conclude, without serious question, that Congress did not intend that application of the vulnerable victim aggravating factor would depend upon proof of scien-ter. Sampson’s fallback position is that any version of the vulnerable victim aggravator that omits a scienter requirement is unconstitutional. We think not. The Supreme Court has stated that an aggravating factor must satisfy two criteria in order to comport with the Eighth Amendment. First, the statutory language must be clear and specific enough to furnish guidance to the factfinder. Arave, 507 U.S. at 470-74, 113 S.Ct. 1534. Second, the factor must provide a principled basis for distinguishing between those who deserve capital punishment and those who do not. Id. at 474, 113 S.Ct. 1534. Viewing the FDPA’s vulnerable victim factor through the prism of these requirements, we agree with the Fifth Circuit’s conclusion that the aggravator, even without a scienter requirement, satisfies both criteria. See United States v. Bourgeois, 423 F.3d 501, 510-11 (5th Cir.2005), cert, denied, - U.S. -, 126 S.Ct. 2020, 164 L.Ed.2d 786 (2006). Since vulnerability and infirmity are concepts that are easily understandable by the average juror, the plain language in which the factor is couched supplies adequate guidance to jurors and judges. Cf. Arave, 507 U.S. at 471-72, 113 S.Ct. 1534 (concluding that the words “cold-blooded” and “pitiless” contained adequate content to guide a capital jury). By the same token, the factor effectively narrows the class of persons eligible for the death penalty so that not all victims can be considered particularly vulnerable due to age or infirmity. See id. at 474, 113 S.Ct. 1534 (explaining the importance of such a lack of universality). This narrowing effect is well-illustrated in the case at hand by the fact that the government did not charge the vulnerable victim factor with respect to the Rizzo murder. In the final analysis, then, the vulnerable victim factor provides adequate guidance to the jury and effectively circumscribes the class of defendants to whom it applies. Consequently, we hold that it passes Eighth Amendment muster. In an effort to blunt the force of this reasoning, Sampson asks us to extrapolate from another line of death penalty cases the additional requirement that an aggravating factor must directly relate to a defendant’s moral culpability (and, thus, his knowledge). See Tison v. Arizona, 481 U.S. 137, 149, 107 S.Ct. 1676, 95 L.Ed.2d 127 (1987) (explaining that a criminal sentence must be “directly related to the personal culpability of the criminal offender”); Enmund v. Florida, 458 U.S. 782, 801, 102 S.Ct. 3368, 73 L.Ed.2d 1140 (1982) (stating that punishment must be must be “tailored to ... personal responsibility and moral guilt”). On this basis, Sampson strives to convince us that the absence of a mental state requirement in the vulnerable victim factor renders it unconstitutional. We are not persuaded. The Enmund Court invalidated, as violative of the Eighth Amendment, the death sentence of a getaway-car driver who, although participating in an armed robbery, did not intend that lethal force be used. 458 U.S. at 801, 102 S.Ct. 3368. The Court reasoned that the punishment was disproportionate to the conduct and, thus, violated the Eighth Amendment. Id. at 797-98, 102 S.Ct. 3368. In Tison, however, the Court held that death is not a disproportionate sentence for a defendant who, while not intending to kill his victims, is a major p