Justice Thomas, delivered the opinion of the Court.
The National Firearms Act makes it unlawful for any person to possess a machinegun that is not properly registered with the Federal Government. Petitioner contends that, to convict him under the Act, the Government should have been required to prove beyond a reasonable doubt that he knew the weapon he possessed had the characteristics that brought it within the statutory definition of a machinegun. We agree and accordingly reverse the judgment of the Court of Appeals.
The National Firearms Act (Act), 26 U. S. C. §§ 5801-5872, imposes strict registration requirements on statutorily defined "firearms." The Act includes within the term "firearm" a machinegun, § 5845(a)(6), and further defines a machinegun as "any weapon which shoots, . . . or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger," § 5845(b). Thus, any fully automatic weapon is a "firearm" within the meaning of the Act.
Upon executing a search warrant at petitioner's home, local police and agents of the Bureau of Alcohol, Tobacco and Firearms (BATF) recovered, among other things, an AR-15 rifle. The AR-15 is the civilian version of the military's M-16 rifle, and is, unless modified, a semiautomatic weapon. The M-16, in contrast, is a selective fire rifle that allows the operator, by rotating a selector switch, to choose semiautomatic or automatic fire. Many M-16 parts are interchangeable with those in the AR-15 and can be used to convert the AR-15 into an automatic weapon. No doubt to inhibit such conversions, the AR-15 is manufactured with a metal stop on its receiver that will prevent an M-16 selector switch, if installed, from rotating to the fully automatic position. The metal stop on petitioner's rifle, however, had been filed away, and the rifle had been assembled with an M-16 selector switch and several other M-16 internal parts, including a hammer, disconnector, and trigger. Suspecting that the AR-15 had been modified to be capable of fully automatic fire, BATF agents seized the weapon. Petitioner subsequently was indicted for unlawful possession of an unregistered machinegun in violation of § 5861(d).
At trial, BATF agents testified that when the AR-15 was tested, it fired more than one shot with a single pull of the trigger. It was undisputed that the weapon was not registered as required by § 5861(d). Petitioner testified that the rifle had never fired automatically when it was in his possession. He insisted that the AR-15 had operated only semiautomatically, and even then imperfectly, often requiring manual ejection of the spent casing and chambering of the next round. According to petitioner, his alleged ignorance of any automatic firing capability should have shielded him from criminal liability for his failure to register the weapon. He requested the District Court to instruct the jury that, to establish a violation of § 5861(d), the Government must prove
The District Court rejected petitioner's proposed instruction and instead charged the jury as follows:
Petitioner was convicted and sentenced to five years' probation and a $5,000 fine.
The Court of Appeals affirmed. Relying on its decision in United States v. Mittleider, 835 F.2d 769 (CA10 1987), cert. denied, 485 U.S. 980 (1988), the court concluded that the Government need not prove a defendant's knowledge of a weapon's physical properties to obtain a conviction under § 5861(d). 971 F.2d 608, 612-613 (CA10 1992). We granted certiorari, 508 U.S. 939 (1993), to resolve a conflict in the Courts of Appeals concerning the mens rea required under § 5861(d).
Whether or not § 5861(d) requires proof that a defendant knew of the characteristics of his weapon that made it a "firearm" under the Act is a question of statutory construction. As we observed in Liparota v. United States, 471 U.S. 419 (1985), "[t]he definition of the elements of a criminal offense is entrusted to the legislature, particularly in the case of federal crimes, which are solely creatures of statute." Id. , at 424 (citing United States v. Hudson, 7 Cranch 32
The language of the statute, the starting place in our inquiry, see Connecticut Nat. Bank v. Germain, 503 U.S. 249, 253-254 (1992), provides little explicit guidance in this case. Section 5861(d) is silent concerning the mens rea required for a violation. It states simply that "[i]t shall be unlawful for any person . . . to receive or possess a firearm which is not registered to him in the National Firearms Registration and Transfer Record." 26 U. S. C. § 5861(d). Nevertheless, silence on this point by itself does not necessarily suggest that Congress intended to dispense with a conventional mens rea element, which would require that the defendant know the facts that make his conduct illegal. See Balint, supra, at 251 (stating that traditionally, "scienter " was a necessary element in every crime). See also n. 3, infra. On the contrary, we must construe the statute in light of the background rules of the common law, see United States v. United States Gypsum Co., 438 U.S. 422, 436-437 (1978), in which the requirement of some mens rea for a crime is firmly embedded. As we have observed, "[t]he existence of a mens rea is the rule of, rather than the exception to, the principles of Anglo-American criminal jurisprudence." Id., at 436 (internal quotation marks omitted). See also Morissette v. United States, 342 U.S. 246, 250 (1952) ("The contention that an injury can amount to a crime only when inflicted by intention is no provincial or transient notion. It is as universal and persistent in mature systems of law as belief in freedom of the human will and a consequent ability and duty of the normal individual to choose between good and evil").
There can be no doubt that this established concept has influenced our interpretation of criminal statutes. Indeed, we have noted that the common-law rule requiring mens rea
According to the Government, however, the nature and purpose of the Act suggest that the presumption favoring mens rea does not apply to this case. The Government argues that Congress intended the Act to regulate and restrict the circulation of dangerous weapons. Consequently, in the Government's view, this case fits in a line of precedent concerning what we have termed "public welfare" or "regulatory" offenses, in which we have understood Congress to impose a form of strict criminal liability through statutes that do not require the defendant to know the facts that make his conduct illegal. In construing such statutes, we have inferred from silence that Congress did not intend to require proof of mens rea to establish an offense.
For example, in Balint, we concluded that the Narcotic Act of 1914, which was intended in part to minimize the spread of addictive drugs by criminalizing undocumented sales of certain narcotics, required proof only that the defendant knew that he was selling drugs, not that he knew the specific items he had sold were "narcotics" within the ambit of the statute. See Balint, supra, at 254. Cf. United States v. Dotterweich, 320 U.S. 277, 281 (1943) (stating in dicta that a statute criminalizing the shipment of adulterated or misbranded drugs did not require knowledge that the items were misbranded or adulterated). As we explained in Dotterweich, Balint dealt with "a now familiar type of legislation whereby penalties serve as effective means of regulation. Such legislation dispenses with the conventional
Such public welfare offenses have been created by Congress, and recognized by this Court, in "limited circumstances." United States Gypsum, supra, at 437. Typically, our cases recognizing such offenses involve statutes that regulate potentially harmful or injurious items. Cf. United States v. International Minerals & Chemical Corp., 402 U.S. 558, 564-565 (1971) (characterizing Balint and similar cases as involving statutes regulating "dangerous or deleterious devices or products or obnoxious waste materials"). In such situations, we have reasoned that as long as a defendant knows that he is dealing with a dangerous device of a character that places him "in responsible relation to a public danger," Dotterweich, supra, at 281, he should be alerted to the probability of strict regulation, and we have assumed that in such cases Congress intended to place the burden on the defendant to "ascertain at his peril whether [his conduct] comes within the inhibition of the statute." Balint, supra, at 254. Thus, we essentially have relied on the nature of the statute and the particular character of the items regulated to determine whether congressional silence concerning the mental element of the offense should be interpreted as dispensing with conventional mens rea requirements. See generally Morissette, supra, at 252-260.
The Government argues that § 5861(d) defines precisely the sort of regulatory offense described in Balint . In this view, all guns, whether or not they are statutory "firearms," are dangerous devices that put gun owners on notice that they must determine at their hazard whether their weapons come within the scope of the Act. On this understanding, the District Court's instruction in this case was correct, because a conviction can rest simply on proof that a defendant knew he possessed a "firearm" in the ordinary sense of the term.
The Government seeks support for its position from our decision in United States v. Freed, 401 U.S. 601 (1971), which involved a prosecution for possession of unregistered grenades under § 5861(d).
As the Government concedes, Freed did not address the issue presented here. In Freed, we decided only that § 5861(d) does not require proof of knowledge that a firearm is unregistered. The question presented by a defendant who possesses a weapon that is a "firearm" for purposes of the Act, but who knows only that he has a "firearm" in the general sense of the term, was not raised or considered. And our determination that a defendant need not know that his weapon is unregistered suggests no conclusion concerning whether § 5861(d) requires the defendant to know of the features that make his weapon a statutory "firearm"; different elements of the same offense can require different mental states. See Liparota, 471 U. S., at 423, n. 5; United States v. Bailey, 444 U.S. 394, 405-406 (1980). See also W. LaFave & A. Scott, Handbook on Criminal Law 194-195 (1972). Moreover, our analysis in Freed likening the Act to the public welfare statute in Balint rested entirely on the assumption that the defendant knew that he was dealing with hand grenades—that is, that he knew he possessed a particularly dangerous type of weapon (one within the statutory definition of a "firearm"), possession of which was not entirely "innocent" in and of itself. 401 U. S., at 609. The predicate for that analysis is eliminated when, as in this case, the very question to be decided is whether the defendant must know of the particular characteristics that make his weapon a statutory firearm.
Notwithstanding these distinctions, the Government urges that Freed `s logic applies because guns, no less than grenades,
Neither, in our view, can all guns be compared to hand grenades. Although the contrast is certainly not as stark as that presented in Liparota, the fact remains that there is a long tradition of widespread lawful gun ownership by private individuals in this country. Such a tradition did not apply to the possession of hand grenades in Freed or to the selling of dangerous drugs that we considered in Balint. See also International Minerals, 402 U. S., at 563-565; Balint, 258 U. S., at 254. In fact, in Freed we construed § 5861(d) under the assumption that "one would hardly be surprised to learn that possession of hand grenades is not an innocent act." Freed, supra, at 609. Here, the Government essentially suggests that we should interpret the section under the altogether different assumption that "one would hardly be surprised to learn that owning a gun is not an innocent act." That proposition is simply not supported by common experience. Guns in general are not "deleterious devices or products or obnoxious waste materials," International Minerals,
The Government protests that guns, unlike food stamps, but like grenades and narcotics, are potentially harmful devices.
If we were to accept as a general rule the Government's suggestion that dangerous and regulated items place their owners under an obligation to inquire at their peril into compliance with regulations, we would undoubtedly reach some untoward results. Automobiles, for example, might also be termed "dangerous" devices and are highly regulated at both the state and federal levels. Congress might see fit to criminalize the violation of certain regulations concerning automobiles, and thus might make it a crime to operate a vehicle without a properly functioning emission control system. But we probably would hesitate to conclude on the basis of silence that Congress intended a prison term to apply to a car owner whose vehicle's emissions levels, wholly unbeknownst to him, began to exceed legal limits between regular inspection dates.
Here, there can be little doubt that, as in Liparota, the Government's construction of the statute potentially would impose criminal sanctions on a class of persons whose mental state—ignorance of the characteristics of weapons in their
We concur in the Fifth Circuit's conclusion on this point: "It is unthinkable to us that Congress intended to subject such law-abiding, well-intentioned citizens to a possible tenyear term of imprisonment if . . . what they genuinely and reasonably believed was a conventional semi-automatic [weapon] turns out to have worn down into or been secretly modified to be a fully automatic weapon." Anderson, supra, at 1254. As we noted in Morissette, the "purpose and obvious effect of doing away with the requirement of a guilty intent is to ease the prosecution's path to conviction." 342 U. S., at 263.
The potentially harsh penalty attached to violation of § 5861(d)—up to 10 years' imprisonment—confirms our reading of the Act. Historically, the penalty imposed under a statute has been a significant consideration in determining whether the statute should be construed as dispensing with mens rea. Certainly, the cases that first defined the concept of the public welfare offense almost uniformly involved statutes that provided for only light penalties such as fines or short jail sentences, not imprisonment in the state penitentiary. See, e. g., Commonwealth v. Raymond, 97 Mass. 567 (1867) (fine of up to $200 or six months in jail, or both); Commonwealth v. Farren, 91 Mass. 489 (1864) (fine); People v. Snowburger, 113 Mich. 86, 71 N. W. 497 (1897) (fine of up to $500 or incarceration in county jail).
As commentators have pointed out, the small penalties attached to such offenses logically complemented the absence of a mens rea requirement: In a system that generally requires
In rehearsing the characteristics of the public welfare offense, we, too, have included in our consideration the punishments imposed and have noted that "penalties commonly are relatively small, and conviction does no grave damage to an
Our characterization of the public welfare offense in Morissette hardly seems apt, however, for a crime that is a felony, as is violation of § 5861(d).
We need not adopt such a definitive rule of construction to decide this case, however. Instead, we note only that where, as here, dispensing with mens rea would require the defendant to have knowledge only of traditionally lawful conduct, a severe penalty is a further factor tending to suggest that Congress did not intend to eliminate a mens rea requirement.
In short, we conclude that the background rule of the common law favoring mens rea should govern interpretation of § 5861(d) in this case. Silence does not suggest that Congress dispensed with mens rea for the element of § 5861(d) at issue here. Thus, to obtain a conviction, the Government should have been required to prove that petitioner knew of the features of his AR-15 that brought it within the scope of the Act.
We emphasize that our holding is a narrow one. As in our prior cases, our reasoning depends upon a commonsense evaluation of the nature of the particular device or substance Congress has subjected to regulation and the expectations that individuals may legitimately have in dealing with the regulated items. In addition, we think that the penalty attached to § 5861(d) suggests that Congress did not intend to eliminate a mens rea requirement for violation of the section. As we noted in Morissette: "Neither this Court nor,
For the foregoing reasons, the judgment of the Court of Appeals is reversed, and the case is remanded for further proceedings consistent with this opinion.
The statute petitioner Harold E. Staples is charged with violating, 26 U. S. C. § 5861(d), makes it a crime for any person to "receive or possess a firearm which is not registered to him." Although the word "knowingly" does not appear in the statute's text, courts generally assume that Congress, absent a contrary indication, means to retain a mens rea requirement. Ante, at 606; see Liparota v. United States, 471 U.S. 419, 426 (1985); United States v. United States Gypsum Co., 438 U.S. 422, 437-438 (1978).
Conviction under § 5861(d), the Government accordingly concedes, requires proof that Staples "knowingly" possessed the machinegun. Brief for United States 23. The question before us is not whether knowledge of possession is required, but what level of knowledge suffices: (1) knowledge simply of possession of the object; (2) knowledge, in addition, that the object is a dangerous weapon; (3) knowledge, beyond dangerousness, of the characteristics that render the object subject to regulation, for example, awareness that the weapon is a machinegun.
Recognizing that the first reading effectively dispenses with mens rea, the Government adopts the second, contending that it avoids criminalizing "apparently innocent conduct," Liparota, supra, at 426, because under the second reading, "a defendant who possessed what he thought was a toy or a violin case, but which in fact was a machinegun, could not be convicted." Brief for United States 23. The Government, however, does not take adequate account of the "widespread lawful gun ownership" Congress and the States have allowed to persist in this country. See United States v. Harris, 959 F.2d 246, 261 (CADC) (per curiam), cert. denied, 506 U.S. 932 (1992). Given the notable lack of comprehensive regulation, "mere unregistered possession of certain types of [regulated weapons]—often [difficult to distinguish]
The Nation's legislators chose to place under a registration requirement only a very limited class of firearms, those they considered especially dangerous. The generally "dangerous" character of all guns, the Court therefore observes, ante, at 611-612, did not suffice to give individuals in Staples' situation cause to inquire about the need for registration. Cf. United States v. Balint, 258 U.S. 250 (1922) (requiring reporting of sale of strictly regulated narcotics, opium and cocaine). Only the third reading, then, suits the purpose of the mens rea requirement—to shield people against punishment for apparently innocent activity.
The indictment in Staples' case charges that he "knowingly received and possessed firearms." 1 App. to Brief for Appellant in No. 91-5033 (CA10), p. 1.
For these reasons, I conclude that conviction under § 5861(d) requires proof that the defendant knew he possessed not simply a gun, but a machinegun. The indictment in this case, but not the jury instruction, properly described this knowledge requirement. I therefore concur in the Court's judgment.
To avoid a slight possibility of injustice to unsophisticated owners of machineguns and sawed-off shotguns, the Court has substituted its views of sound policy for the judgment Congress made when it enacted the National Firearms Act (or Act). Because the Court's addition to the text of 26 U. S. C. § 5861(d) is foreclosed by both the statute and our precedent, I respectfully dissent.
The Court is preoccupied with guns that "generally can be owned in perfect innocence." Ante, at 611. This case, however, involves a semiautomatic weapon that was readily convertible into a machinegun—a weapon that the jury found to be "`a dangerous device of a type as would alert one to the likelihood of regulation.' " Ante, at 604. These are not guns "of some sort" that can be found in almost "50 percent of American homes." Ante, at 613-614.
The question presented is whether the National Firearms Act imposed on the Government the burden of proving beyond a reasonable doubt not only that the defendant knew he possessed a dangerous device sufficient to alert him to
Contrary to the assertion by the Court, the text of the statute does provide "explicit guidance in this case." Cf. ante, at 605. The relevant section of the Act makes it "unlawful for any person . . . to receive or possess a firearm which is not registered to him in the National Firearms Registration and Transfer Record." 26 U. S. C. § 5861(d). Significantly, the section contains no knowledge requirement, nor does it describe a common-law crime.
The common law generally did not condemn acts as criminal unless the actor had "an evil purpose or mental culpability," Morissette v. United States, 342 U.S. 246, 252 (1952), and was aware of all the facts that made the conduct unlawful, United States v. Balint, 258 U.S. 250, 251-252 (1922). In interpreting statutes that codified traditional common-law offenses, courts usually followed this rule, even when the text of the statute contained no such requirement. Ibid. Because the offense involved in this case is entirely a creature of statute, however, "the background rules of the common law," cf. ante, at 605, do not require a particular construction, and critically different rules of construction apply. See Morissette v. United States, 342 U. S., at 252-260.
In Morissette, Justice Jackson outlined one such interpretive rule:
The provision's place in the overall statutory scheme, see Crandon v. United States, 494 U.S. 152, 158 (1990), confirms this intention. In 1934, when Congress originally enacted the statute, it limited the coverage of the 1934 Act to a relatively narrow category of weapons such as submachine-guns and sawed-off shotguns—weapons characteristically used only by professional gangsters like Al Capone, Pretty Boy Floyd, and their henchmen.
In addition, at the time of enactment, this Court had already construed comparable provisions of the Harrison Anti-Narcotic Act not to require proof of knowledge of all the facts that constitute the proscribed offense. United States v. Balint, 258 U.S. 250 (1922).
Like the 1934 Act, the current National Firearms Act is primarily a regulatory measure. The statute establishes
As the Court acknowledges, ante, at 607, to interpret statutory offenses such as § 5861(d), we look to "the nature of the statute and the particular character of the items regulated" to determine the level of knowledge required for conviction. An examination of § 5861(d) in light of our precedent dictates that the crime of possession of an unregistered machinegun is in a category of offenses described as "public welfare" crimes.
"Public welfare" offenses share certain characteristics: (1) they regulate "dangerous or deleterious devices or products
Public welfare statutes render criminal "a type of conduct that a reasonable person should know is subject to stringent public regulation and may seriously threaten the community's health or safety." Liparota v. United States, 471 U.S. 419, 433 (1985). Thus, under such statutes, "a defendant can be convicted even though he was unaware of the circumstances of his conduct that made it illegal." Id., at 443, n. 7 (White, J., dissenting). Referring to the strict criminal sanctions for unintended violations of the food and drug laws, Justice Frankfurter wrote:
The National Firearms Act unquestionably is a public welfare statute. United States v. Freed, 401 U.S. 601, 609 (1971) (holding that this statute "is a regulatory measure in the interest of the public safety"). Congress fashioned a legislative scheme to regulate the commerce and possession of certain types of dangerous devices, including specific kinds of weapons, to protect the health and welfare of the citizenry. To enforce this scheme, Congress created criminal penalties for certain acts and omissions. The text of some of these offenses—including the one at issue here— contains no knowledge requirement.
The Court recognizes:
Both the Court and Justice Ginsburg erroneously rely upon the "tradition[al]" innocence of gun ownership to find that Congress must have intended the Government to prove knowledge of all the characteristics that make a weapon a statutory "firear[m]." Ante, at 610-612; ante, at 621-622 (Ginsburg, J., concurring in judgment). We held in Freed, however, that a § 5861(d) offense may be committed by one with no awareness of either wrongdoing or of all the facts that constitute the offense.
Cases arise, of course, in which a defendant would not know that a device was dangerous unless he knew that it was a "firearm" as defined in the Act. Freed was such a case; unless the defendant knew that the device in question was a hand grenade, he would not necessarily have known that it was dangerous. But given the text and nature of the statute, it would be utterly implausible to suggest that Congress intended the owner of a sawed-off shotgun to be criminally liable if he knew its barrel was 17.5 inches long but not if he mistakenly believed the same gun had an 18inch barrel. Yet the Court's holding today assumes that Congress intended that bizarre result.
The enforcement of public welfare offenses always entails some possibility of injustice. Congress nevertheless has repeatedly decided that an overriding public interest in health or safety may outweigh that risk when a person is dealing with products that are sufficiently dangerous or deleterious to make it reasonable to presume that he either knows, or should know, whether those products conform to special regulatory requirements. The dangerous character of the product is reasonably presumed to provide sufficient notice of the probability of regulation to justify strict enforcement against those who are merely guilty of negligent, rather than willful, misconduct.
The National Firearms Act is within the category of public welfare statutes enacted by Congress to regulate highly dangerous items. The Government submits that a conviction under such a statute may be supported by proof that the
The history and interpretation of the National Firearms Act supports the conclusion that Congress did not intend to
Congress subsequently amended the statute twice, once in 1968 and again in 1986. Both amendments added knowledge requirements to other portions of the Act,
In short, petitioner's knowledge that he possessed an item that was sufficiently dangerous to alert him to the likelihood of regulation would have supported a conviction during the first half century of enforcement of this statute. Unless application of that standard to a particular case violates the Due Process Clause,
On the premise that the purpose of the mens rea requirement is to avoid punishing people "for apparently innocent activity," Justice Ginsburg concludes that proof of knowledge that a weapon is "`a dangerous device of a type as would alert one to the likelihood of regulation' " is not an adequate mens rea requirement, but that proof of knowledge that the weapon possesses "`every last characteristic' " that subjects it to regulation is. Ante, at 622-623, and n. 5 (Ginsburg, J., concurring in judgment) (quoting the trial court's jury instruction).
First, a defendant may know that he possesses a weapon with all of the characteristics that make it a "firearm" within the meaning of the statute and also know that it has never been registered, but be ignorant of the federal registration requirement. In such a case, we presume knowledge of the law even if we know the defendant is "innocent" in the sense that Justice Ginsburg uses the word. Second, a defendant may know that he possesses a weapon with all of the characteristics of a statutory firearm and also know that the law requires that it be registered, but mistakenly believe that it is in fact registered. Freed squarely holds that this defendant's "innocence" is not a defense. Third, a defendant
Justice Ginsburg treats the first, second, and third alternatives differently from the fourth and fifth. Her acceptance of knowledge of the characteristics of a statutory "firearm" as a sufficient predicate for criminal liability—despite ignorance of either the duty to register or the fact of nonregistration, or both—must rest on the premise that such knowledge would alert the owner to the likelihood of regulation, thereby depriving the conduct of its "apparen[t] innocen[ce]." Yet in the fourth alternative, a jury determines just such knowledge: that the characteristics of the weapon known to the defendant would alert the owner to the likelihood of regulation.
In short, Justice Ginsburg's reliance on "the purpose of the mens rea requirement—to shield people against punishment for apparently innocent activity," ante, at 622, neither explains why ignorance of certain facts is a defense although
This case presents no dispute about the dangerous character of machineguns and sawed-off shotguns. Anyone in possession of such a weapon is "standing in responsible relation to a public danger." See Dotterweich, 320 U. S., at 281 (citation omitted). In the National Firearms Act, Congress determined that the serious threat to health and safety posed by the private ownership of such firearms warranted the imposition of a duty on the owners of dangerous weapons to determine whether their possession is lawful. Semiautomatic weapons that are readily convertible into machineguns are sufficiently dangerous to alert persons who knowingly possess them to the probability of stringent public regulation. The jury's finding that petitioner knowingly possessed "a dangerous device of a type as would alert one to the likelihood of regulation" adequately supports the conviction.
Accordingly, I would affirm the judgment of the Court of Appeals.
But the dissent apparently does not conceive of the mens rea requirement in terms of specific categories of weapons at all, and rather views it as a more fluid concept that does not require delineation of any concrete elements of knowledge that will apply consistently from case to case. The dissent sees no need to define a class of items the knowing possession of which satisfies the mens rea element of the offense, for in the dissent's view the exact content of the knowledge requirement can be left to the jury in each case. As long as the jury concludes that the item in a given case is "sufficiently dangerous to alert [the defendant] to the likelihood of regulation," post, at 637, the knowledge requirement is satisfied. See also post, at 624, 639, 640. But the mens rea requirement under a criminal statute is a question of law, to be determined by the court. Our decisions suggesting that public welfare offenses require that the defendant know that he stands in "responsible relation to a public danger," Dotterweich, 320 U. S., at 281, in no way suggest that what constitutes a public danger is a jury question. It is for courts, through interpretation of the statute, to define the mens rea required for a conviction. That task cannot be reduced to setting a general "standard," post, at 637, that leaves it to the jury to determine, based presumably on the jurors' personal opinions, whether the items involved in a particular prosecution are sufficiently dangerous to place a person on notice of regulation.
Moreover, as our discussion above should make clear, to determine as a threshold matter whether a particular statute defines a public welfare offense, a court must have in view some category of dangerous and deleterious devices that will be assumed to alert an individual that he stands in "responsible relation to a public danger." Dotterweich, supra, at 281. The truncated mens rea requirement we have described applies precisely because the court has determined that the statute regulates in a field where knowing possession of some general class of items should alert individuals to probable regulation. Under the dissent's approach, however, it seems that every regulatory statute potentially could be treated as a public welfare offense as long as the jury—not the court—ultimately determines that the specific items involved in a prosecution were sufficiently dangerous.
"[A] person is knowingly in possession of a thing if his possession occurred voluntarily and intentionally and not because of mistake or accident or other innocent reason. The purpose of adding the word `knowingly' is to insure that no one can be convicted of possession of a firearm he did not intend to possess. The Government need not prove the defendant knows he's dealing with a weapon possessing every last characteristic [which subjects it] to the regulation. It would be enough to prove he knows that he is dealing with a dangerous device of a type as would alert one to the likelihood of regulation. If he has such knowledge and if the particular item is, in fact, regulated, then that person acts at his peril. Mere possession of an unregistered firearm is a violation of the law of the United States, and it is not necessary for the Government to prove that the defendant knew that the weapon in his possession was a firearm within the meaning of the statute, only that he knowingly possessed the firearm." Id., at 465.
"Neither this Court nor, so far as we are aware, any other has undertaken to delineate a precise line or set forth comprehensive criteria for distinguishing between crimes that require a mental element and crimes that do not. We attempt no closed definition, for the law on the subject is neither settled nor static." Id., at 260 (footnotes omitted).
"But where a criminal statute involves regulation of a highly hazardous substance—and especially where it penalizes a failure to act or to comply with a registration scheme—the defendant's knowledge that he was dealing with such a substance and that it was likely to be subject to regulation provides sufficient intent to support a conviction." Id., at 17-18. "Rather, absent contrary congressional direction, knowledge of the highly dangerous nature of the articles involved and the likelihood that they are subject to regulation takes the place of the more rigorous knowledge requirement applicable where apparently innocent and harmless devices are subject to regulation." Id., at 20. "But the instruction did not require the government to prove that petitioner knew his weapon `possess[ed] every last characteristic [which subjects it] to regulation'; he need only have `know[n] that he [was] dealing with a dangerous device of a type as would alert one to the likelihood of regulation.' Tr. 465.
"That instruction accurately describes the mental state necessary for a violation of Section 5861(d)." Id., at 23. "[P]roof that a defendant was on fair notice that the item he possessed was highly dangerous and likely to be regulated is sufficient to support a conviction." Id., at 24.
And, as I have already noted, United States v. Freed, 401 U.S. 601 (1971), was consistent with the Government's position here. Although the Government accepted the burden of proving that Freed knew that the item he possessed was a hand grenade, the possessor of an unfamiliar object such as a hand grenade would not know that it was "a dangerous item of a type likely to be subject to regulation," Brief for United States 16; see also id., at 20, 23, 24, unless he knew what it was.