MR. JUSTICE REHNQUIST delivered the opinion of the Court.

In the early morning hours of August 26, 1976, respondents Clifford Bailey, James T. Cogdell, Ronald C. Cooley, and Ralph Walker, federal prisoners at the District of Columbia jail, crawled through a window from which a bar had been removed, slid down a knotted bedsheet, and escaped from custody. Federal authorities recaptured them after they had remained at large for a period of time ranging from one month to three and one-half months. Upon their apprehension, they were charged with violating 18 U.S.C. § 751 (a), which governs escape from federal custody. At their trials, each of the respondents adduced or offered to adduce evidence as to various conditions and events at the District of Columbia jail, but each was convicted by the jury. The Court of Appeals for the District of Columbia Circuit reversed the convictions by a divided vote, holding that the District Court had improperly precluded consideration by the respective juries of respondents' tendered evidence. We granted certiorari, 440 U.S. 957, and now reverse the judgments of the Court of Appeals.

In reaching our conclusion, we must decide the state of mind necessary for violation of § 751(a) and the elements that constitute defenses such as duress and necessity. In explaining the reasons for our decision, we find ourselves in a position akin to that of the mother crab who is trying to teach her progeny to walk in a straight line, and finally in desperation exclaims: "Don't do as I do, do as I say." The Act of Congress we construe consists of one sentence set forth in the margin, n. 1, supra; our own pragmatic estimate, expressed infra, at 417, is that "[i]n general, trials for violations of § 751(a) should be simple affairs." Yet we have written, reluctantly but we believe necessarily, a somewhat lengthy opinion supporting our conclusion, because in enacting the Federal Criminal Code Congress legislated in the light of a long history of case law that is frequently relevant in fleshing out the bare bones of a crime that Congress may have proscribed in a single sentence. See Morissette v. United States, 342 U.S. 246 (1952).

I

All respondents requested jury trials and were initially scheduled to be tried jointly. At the last minute, however, respondent Cogdell secured a severance. Because the District Court refused to submit to the jury any instructions on respondents' defense of duress or necessity and did not charge the jury that escape was a continuing offense, we must examine in some detail the evidence brought out at trial.

The prosecution's case in chief against Bailey, Cooley, and Walker was brief. The Government introduced evidence that each of the respondents was in federal custody on August 26, 1976, that they had disappeared, apparently through a cell window, at approximately 5:35 a. m. on that date, and that they had been apprehended individually between September 27 and December 13, 1976.

Respondents' defense of duress or necessity centered on the conditions in the jail during the months of June, July, and August 1976, and on various threats and beatings directed at them during that period. In describing the conditions at the jail, they introduced evidence of frequent fires in "Northeast One," the maximum-security cellblock occupied by respondents prior to their escape. Construed in the light most favorable to them, this evidence demonstrated that the inmates of Northeast One, and on occasion the guards in that unit, set fire to trash, bedding, and other objects thrown from the cells. According to the inmates, the guards simply allowed the fires to burn until they went out. Although the fires apparently were confined to small areas and posed no substantial threat of spreading through the complex, poor ventilation caused smoke to collect and linger in the cellblock.

Respondents Cooley and Bailey also introduced testimony that the guards at the jail had subjected them to beatings and to threats of death. Walker attempted to prove that he was an epileptic and had received inadequate medical attention for his seizures.

Consistently during the trial, the District Court stressed that, to sustain their defenses, respondents would have to introduce some evidence that they attempted to surrender or engaged in equivalent conduct once they had freed themselves from the conditions they described. But the court waited for such evidence in vain. Respondent Cooley, who had eluded the authorities for one month, testified that his "people" had tried to contact the authorities, but "never got in touch with anybody." App. 119. He also suggested that someone had told his sister that the Federal Bureau of Investigation would kill him when he was apprehended.

Respondent Bailey, who was apprehended on November 19, 1976, told a similar story. He stated that he "had the jail officials called several times," but did not turn himself in because "I would still be under the threats of death." Like Cooley, Bailey testified that "the FBI was telling my people that they was going to shoot me." Id., at 169, 175-176.

Only respondent Walker suggested that he had attempted to negotiate a surrender. Like Cooley and Bailey, Walker testified that the FBI had told his "people" that they would kill him when they recaptured him. Nevertheless, according to Walker, he called the FBI three times and spoke with an agent whose name he could not remember. That agent allegedly assured him that the FBI would not harm him, but was unable to promise that Walker would not be returned to the D.C. jail. Id., at 195-200. Walker testified that he last called the FBI in mid-October. He was finally apprehended on December 13, 1976.

At the close of all the evidence, the District Court rejected respondents' proffered instruction on duress as a defense to prison escape. The court ruled that respondents had failed as a matter of law to present evidence sufficient to support such a defense because they had not turned themselves in after they had escaped the allegedly coercive conditions. After receiving instructions to disregard the evidence of the conditions in the jail, the jury convicted Bailey, Cooley, and Walker of violating § 751(a).

Two months later, respondent Cogdell came to trial before the same District Judge who had presided over the trial of his co-respondents. When Cogdell attempted to offer testimony concerning the allegedly inhumane conditions at the D.C. jail, the District Judge inquired into Cogdell's conduct between his escape on August 26 and his apprehension on September 28. In response to Cogdell's assertion that he "may have written letters," the District Court specified that Cogdell could testify only as to "what he did . . . [n]ot what he may have done." App. 230. Absent such testimony, however, the District Court ruled that Cogdell could not present evidence of conditions at the jail. Cogdell subsequently chose not to testify on his own behalf, and was convicted by the jury of violating § 751(a).

By a divided vote, the Court of Appeals reversed each respondent's conviction and remanded for new trials. See 190 U.S.App.D.C. 142, 585 F.2d 1087 (1978); 190 U.S. App.D.C. 185, 585 F.2d 1130 (1978). The majority concluded that the District Court should have allowed the jury to consider the evidence of coercive conditions in determining whether the respondents had formulated the requisite intent to sustain a conviction under § 751(a). According to the majority, § 751(a) required the prosecution to prove that a particular defendant left federal custody voluntarily, without permission, and "with an intent to avoid confinement." 190 U.S. App. D.C., at 148, 585 F.2d, at 1093. The majority then defined the word "confinement" as encompassing only the "normal aspects" of punishment prescribed by our legal system. Thus, where a prisoner escapes in order to avoid "non-confinement" conditions such as beatings or homosexual attacks, he would not necessarily have the requisite intent to sustain a conviction under § 751(a). According to the majority:

"When a defendant introduces evidence that he was subject to such `non-confinement' conditions, the crucial factual determination on the intent issue is . . . whether the defendant left custody only to avoid these conditions or whether, in addition, the defendant also intended to avoid confinement. In making this determination the jury is to be guided by the trial court's instructions pointing out those factors that are most indicative of the presence or absence of an intent to avoid confinement." 190 U.S. App. D.C., at 148, n. 17, 585 F.2d, at 1093, n. 17 (emphasis in original).

Turning to the applicability of the defense of duress or necessity, the majority assumed that escape as defined by § 751(a) was a "continuing offense" as long as the escapee was at large. Given this assumption, the majority agreed with the District Court that, under normal circumstances, an escapee must present evidence of coercion to justify his continued absence from custody as well as his initial departure. Here, however, respondents had been indicted for "flee[ing] and escap[ing]" "[o]n or about August 26, 1976," and not for "leaving and staying away from custody." 190 U.S. App. D.C., at 155, 585 F.2d, at 1100 (emphasis in original). Similarly, "[t]he trial court's instructions when read as a whole clearly give the impression that [respondents] were being tried only for leaving the jail on August 26, and not for failing to return at some later date." Id., at 155, n. 50, 585 F.2d, at 1100, n. 50. Under these circumstances, the majority believed that neither respondents nor the juries were acquainted with the proposition that the escapes in question were continuing offenses. This failure, according to the majority, constituted "an obvious violation of [respondents'] constitutional right to jury trial." Id., at 156, 585 F.2d, at 1101.

The dissenting judge objected to what he characterized as a revolutionary reinterpretation of criminal law by the majority. He argued that the common-law crime of escape had traditionally required only "general intent," a mental state no more sophisticated than an "intent to go beyond permitted limits." Id., at 177, 585 F.2d, at 1122 (emphasis deleted). The dissent concluded that the District Court had properly removed from consideration each respondent's contention that conditions and events at the D.C. jail justified his escape, because each respondent had introduced no evidence whatsoever justifying his continued absence from jail following that escape.

II

Criminal liability is normally based upon the concurrence of two factors, "an evil-meaning mind [and] an evil-doing hand. . . ." Morissette v. United States, 342 U.S., at 251. In the present case, we must examine both the mental element, or mens rea, required for conviction under § 751(a) and the circumstances under which the "evil-doing hand" can avoid liability under that section because coercive conditions or necessity negates a conclusion of guilt even though the necessary mens rea was present.

A

Few areas of criminal law pose more difficulty than the proper definition of the mens rea required for any particular crime. In 1970, the National Commission on Reform of Federal Criminal Laws decried the "confused and inconsistent ad hoc approach" of the federal courts to this issue and called for "a new departure." See 1 Working Papers of the National Commission on Reform of Federal Criminal Laws 123 (hereinafter Working Papers). Although the central focus of this and other reform movements has been the codification of workable principles for determining criminal culpability, see, e. g., American Law Institute, Model Penal Code §§ 2.01 — 2.13 (Prop. Off. Draft 1962) (hereinafter Model Penal Code); S. 1, 94th Cong., 2d Sess., §§ 301 — 303 (1976), a byproduct has been a general rethinking of traditional mens rea analysis.

At common law, crimes generally were classified as requiring either "general intent" or "specific intent." This venerable distinction, however, has been the source of a good deal of confusion. As one treatise explained:

"Sometimes `general intent' is used in the same way as `criminal intent' to mean the general notion of mens rea, while `specific intent' is taken to mean the mental state required for a particular crime. Or, `general intent' may be used to encompass all forms of the mental state requirement, while `specific intent' is limited to the one mental state of intent. Another possibility is that `general intent' will be used to characterize an intent to do something on an undetermined occasion, and `specific intent' to denote an intent to do that thing at a particular time and place." W. LaFave A. Scott, Handbook on Criminal Law § 28, pp. 201-202 (1972) (footnotes omitted) (hereinafter LaFave Scott).

This ambiguity has led to a movement away from the traditional dichotomy of intent and toward an alternative analysis of mens rea. See id., at 202. This new approach, exemplified in the American Law Institute's Model Penal Code, is based on two principles. First, the ambiguous and elastic term "intent" is replaced with a hierarchy of culpable states of mind. The different levels in this hierarchy are commonly identified, in descending order of culpability, as purpose, knowledge, recklessness, and negligence. See LaFave Scott 194; Model Penal Code § 2.02. Perhaps the most significant, and most esoteric, distinction drawn by this analysis is that between the mental states of "purpose" and "knowledge." As we pointed out in United States v. United States Gypsum Co., 438 U.S. 422, 445 (1978), a person who causes a particular result is said to act purposefully if "`he consciously desires that result, whatever the likelihood of that result happening from his conduct,'" while he is said to act knowingly if he is aware "`that that result is practically certain to follow from his conduct, whatever his desire may be as to that result.'"

In the case of most crimes, "the limited distinction between knowledge and purpose has not been considered important since `there is good reason for imposing liability whether the defendant desired or merely knew of the practical certainty of the results.'" United States v. United States Gypsum Co., supra, at 445, quoting LaFave Scott 197. Thus, in Gypsum we held that a person could be held criminally liable under § 1 of the Sherman Act if that person exchanged price information with a competitor either with the knowledge that the exchange would have unreasonable anticompetitive effects or with the purpose of producing those effects. 438 U.S., at 444-445, and n. 21.

In certain narrow classes of crimes, however, heightened culpability has been thought to merit special attention. Thus, the statutory and common law of homicide often distinguishes, either in setting the "degree" of the crime or in imposing punishment, between a person who knows that another person will be killed as the result of his conduct and a person who acts with the specific purpose of taking another's life. See LaFave Scott 196-197. Similarly, where a defendant is charged with treason, this Court has stated that the Government must demonstrate that the defendant acted with a purpose to aid the enemy. See Haupt v. United States, 330 U.S. 631, 641 (1947). Another such example is the law of inchoate offenses such as attempt and conspiracy, where a heightened mental state separates criminality itself from otherwise innocuous behavior. See Model Penal Code § 2.02, Comments, p. 125 (Tent. Draft No. 4, 1955) (hereinafter MPC Comments).

In a general sense, "purpose" corresponds loosely with the common-law concept of specific intent, while "knowledge" corresponds loosely with the concept of general intent. See ibid.; LaFave Scott 201-202. Were this substitution of terms the only innovation offered by the reformers, it would hardly be dramatic. But there is another ambiguity inherent in the traditional distinction between specific intent and general intent. Generally, even time-honored common-law crimes consist of several elements, and complex statutorily defined crimes exhibit this characteristic to an even greater degree. Is the same state of mind required of the actor for each element of the crime, or may some elements require one state of mind and some another? In United States v. Feola, 420 U.S. 671 (1975), for example, we were asked to decide whether the Government, to sustain a conviction for assaulting a federal officer under 18 U.S.C. § 111, had to prove that the defendant knew that his victim was a federal officer. After looking to the legislative history of § 111, we concluded that Congress intended to require only "an intent to assault, not an intent to assault a federal officer." 420 U.S., at 684. What Feola implied, the American Law Institute stated: "[C]lear analysis requires that the question of the kind of culpability required to establish the commission of an offense be faced separately with respect to each material element of the crime." MPC Comments 123. See also Working Papers 131; LaFave Scott 194.

Before dissecting § 751(a) and assigning a level of culpability to each element, we believe that two observations are in order. First, in performing such analysis courts obviously must follow Congress' intent as to the required level of mental culpability for any particular offense. Principles derived from common law as well as precepts suggested by the American Law Institute must bow to legislative mandates. In the case of § 751(a), however, neither the language of the statute nor the legislative history mentions the mens rea required for conviction.

Second, while the suggested element-by-element analysis is a useful tool for making sense of an otherwise opaque concept, it is not the only principle to be considered. The administration of the federal system of criminal justice is confided to ordinary mortals, whether they be lawyers, judges, or jurors. This system could easily fall of its own weight if courts or scholars become obsessed with hair-splitting distinctions, either traditional or novel, that Congress neither stated nor implied when it made the conduct criminal.

As relevant to the charges against Bailey, Cooley, and Walker, § 751(a) required the prosecution to prove (1) that they had been in the custody of the Attorney General, (2) as the result of a conviction, and (3) that they had escaped from that custody. As for the charges against respondent Cogdell, § 751(a) required the same proof, with the exception that his confinement was based upon an arrest for a felony rather than a prior conviction. Although § 751(a) does not define the term "escape," courts and commentators are in general agreement that it means absenting oneself from custody without permission. See, e. g., 190 U.S.App.D.C., at 148, 585 F.2d, at 1093; id., at 177, 585 F.2d, at 1122 (Wilkey, J., dissenting); United States v. Wilke, 450 F.2d 877 (CA9 1971), cert. denied, 409 U.S. 918 (1972). See also 2 J. Bishop, Criminal Law § 1103, p. 819 (9th ed. 1923); 1 W. Burdick, Law of Crime 462-463 (1946); R. Perkins, Criminal Law 429 (1957); 3 F. Wharton, Criminal Law § 2003, p. 2178 (11th ed. 1912).

Respondents have not challenged the District Court's instructions on the first two elements of the crime defined by § 751(a). It is undisputed that, on August 26, 1976, respondents were in the custody of the Attorney General as the result of either arrest on charges of felony or conviction. As for the element of "escape," we need not decide whether a person could be convicted on evidence of recklessness or negligence with respect to the limits on his freedom. A court may someday confront a case where an escapee did not know, but should have known, that he was exceeding the bounds of his confinement or that he was leaving without permission. Here, the District Court clearly instructed the juries that the prosecution bore the burden of proving that respondents "knowingly committed an act which the law makes a crime" and that they acted "knowingly, intentionally, and deliberately. . . ." App. 221-223, 231-233. At a minimum, the juries had to find that respondents knew they were leaving the jail and that they knew they were doing so without authorization. The sufficiency of the evidence to support the juries' verdicts under this charge has never seriously been questioned, nor could it be.

The majority of the Court of Appeals, however, imposed the added burden on the prosecution to prove as a part of its case in chief that respondents acted "with an intent to avoid confinement." While, for the reasons noted above, the word "intent" is quite ambiguous, the majority left little doubt that it was requiring the Government to prove that the respondents acted with the purpose — that is, the conscious objective — of leaving the jail without authorization. In a footnote explaining their holding, for example, the majority specified that an escapee did not act with the requisite intent if he escaped in order to avoid "`non-confinement' conditions" as opposed to "normal aspects of `confinement.'" 190 U.S. App. D.C., at 148, n. 17, 585 F.2d, at 1093, n. 17.

We find the majority's position quite unsupportable. Nothing in the language or legislative history of § 751(a) indicates that Congress intended to require either such a heightened standard of culpability or such a narrow definition of confinement. As we stated earlier, the cases have generally held that, except in narrow classes of offenses, proof that the defendant acted knowingly is sufficient to support a conviction. Accordingly, we hold that the prosecution fulfills its burden under § 751(a) if it demonstrates that an escapee knew his actions would result in his leaving physical confinement without permission. Our holding in this respect comports with parallel definitions of the crime of escape both in the Model Penal Code and in a proposed revision of the Federal Criminal Code. See Model Penal Code §§ 2.02(3), 242.6(1); Report of Senate Committee on the Judiciary to Accompany S. 1, S. Rep. No. 94-00, pp. 333-334 (Comm. Print 1976). Moreover, comments accompanying the proposed revision of the Federal Criminal Code specified that the new provision covering escape "substantially carrie[d] forward existing law. . . ." Id., at 332.

B

Respondents also contend that they are entitled to a new trial because they presented (or, in Cogdell's case, could have presented) sufficient evidence of duress or necessity to submit such a defense to the jury. The majority below did not confront this claim squarely, holding instead that, to the extent that such a defense normally would be barred by a prisoner's failure to return to custody, neither the indictment nor the jury instructions adequately described such a requirement. See 190 U.S. App. D.C., at 155-156, 585 F.2d, at 1100-1101.

Common law historically distinguished between the defenses of duress and necessity. Duress was said to excuse criminal conduct where the actor was under an unlawful threat of imminent death or serious bodily injury, which threat caused the actor to engage in conduct violating the literal terms of the criminal law. While the defense of duress covered the situation where the coercion had its source in the actions of other human beings, the defense of necessity, or choice of evils, traditionally covered the situation where physical forces beyond the actor's control rendered illegal conduct the lesser of two evils. Thus, where A destroyed a dike because B threatened to kill him if he did not, A would argue that he acted under duress, whereas if A destroyed the dike in order to protect more valuable property from flooding, A could claim a defense of necessity. See generally LaFave Scott 374-384.

Modern cases have tended to blur the distinction between duress and necessity. In the court below, the majority discarded the labels "duress" and "necessity," choosing instead to examine the policies underlying the traditional defenses. See 190 U.S. App. D.C., at 152, 585 F.2d, at 1097. In particular, the majority felt that the defenses were designed to spare a person from punishment if he acted "under threats or conditions that a person of ordinary firmness would have been unable to resist," or if he reasonably believed that criminal action "was necessary to avoid a harm more serious than that sought to be prevented by the statute defining the offense." Id., at 152-153, 585 F.2d, at 1097-1098. The Model Penal Code redefines the defenses along similar lines. See Model Penal Code § 2.09 (duress) and § 3.02 (choice of evils).

We need not speculate now, however, on the precise contours of whatever defenses of duress or necessity are available against charges brought under § 751(a). Under any definition of these defenses one principle remains constant: if there was a reasonable, legal alternative to violating the law, "a chance both to refuse to do the criminal act and also to avoid the threatened harm," the defenses will fail. LaFave Scott 379. Clearly, in the context of prison escape, the escapee is not entitled to claim a defense of duress or necessity unless and until he demonstrates that, given the imminence of the threat, violation of § 751(a) was his only reasonable alternative. See United States v. Boomer, 571 F.2d 543, 545 (CA10), cert. denied sub nom. Heft v. United States, 436 U.S. 911 (1978); People v. Richards, 269 Cal.App.2d 768, 75 Cal.Rptr. 597 (1969).

In the present case, the Government contends that respondents' showing was insufficient on two grounds. First, the Government asserts that the threats and conditions cited by respondents as justifying their escape were not sufficiently immediate or serious to justify their departure from lawful custody. Second, the Government contends that, once the respondents had escaped, the coercive conditions in the jail were no longer a threat and respondents were under a duty to terminate their status as fugitives by turning themselves over to the authorities.

Respondents, on the other hand, argue that the evidence of coercion and conditions in the jail was at least sufficient to go to the jury as an affirmative defense to the crime charged. As for their failure to return to custody after gaining their freedom, respondents assert that this failure should be but one factor in the overall determination whether their initial departure was justified. According to respondents, their failure to surrender "may reflect adversely on the bona fides of [their] motivation" in leaving the jail, but should not withdraw the question of their motivation from the jury's consideration. Brief for Respondents 67. See also n. 3, supra.

We need not decide whether such evidence as that submitted by respondents was sufficient to raise a jury question as to their initial departures. This is because we decline to hold that respondents' failure to return is "just one factor" for the jury to weigh in deciding whether the initial escape could be affirmatively justified. On the contrary, several considerations lead us to conclude that, in order to be entitled to an instruction on duress or necessity as a defense to the crime charged, an escapee must first offer evidence justifying his continued absence from custody as well as his initial departure and that an indispensable element of such an offer is testimony of a bona fide effort to surrender or return to custody as soon as the claimed duress or necessity had lost its coercive force.

First, we think it clear beyond peradventure that escape from federal custody as defined in § 751(a) is a continuing offense and that an escapee can be held liable for failure to return to custody as well as for his initial departure. Given the continuing threat to society posed by an escaped prisoner, "the nature of the crime involved is such that Congress must assuredly have intended that it be treated as a continuing one." Toussie v. United States, 397 U.S. 112, 115 (1970). Moreover, every federal court that has considered this issue has held, either explicitly or implicitly, that § 751(a) defines a continuing offense. See, e. g., United States v. Michelson, 559 F.2d 567 (CA9 1977); United States v. Cluck, 542 F.2d 728 (CA8), cert. denied, 429 U.S. 986 (1976); United States v. Joiner, 496 F.2d 1314 (CA5), cert. denied, 419 U.S. 1002 (1974); United States v. Chapman, 455 F.2d 746 (CA5 1972).

Respondents point out that Toussie calls for restraint in labeling crimes as continuing offenses. The justification for that restraint, however, is the tension between the doctrine of continuing offenses and the policy of repose embodied in statutes of limitations. See 397 U.S., at 114-115. This tension is wholly absent where, as in the case of § 751(a), the statute of limitations is tolled for the period that the escapee remains at large.

The remaining considerations leading to our conclusion are, perhaps ironically, derived from the same concern for the statutory and constitutional right of jury trial upon which the majority of the Court of Appeals based its reasoning. There was no significant "variance" in the indictment merely because respondents had not been indicted under a theory of escape as a continuing offense and because the District Court did not explain this theory to the juries. We have held on several occasions that "an indictment is sufficient if it, first, contains the elements of the offense charged and fairly informs the defendant of the charge against which he must defend, and, second, enables him to plead an acquittal or conviction in bar of future prosecutions for the same offense." Hamling v. United States, 418 U.S. 87, 117 (1974). These indictments, which track closely the language of § 751(a), were undoubtedly sufficient under this standard. See 418 U.S., at 117. As for the alleged failure of the District Court to elaborate for the benefit of the jury on the continuing nature of the charged offense, we believe that such elaboration was unnecessary where, as here, the evidence failed as a matter of law in a crucial particular to reach the minimum threshold that would have required an instruction on respondents' theory of the case generally.

The Anglo-Saxon tradition of criminal justice, embodied in the United States Constitution and in federal statutes, makes jurors the judges of the credibility of testimony offered by witnesses. It is for them, generally, and not for appellate courts, to say that a particular witness spoke the truth or fabricated a cock-and-bull story. An escapee who flees from a jail that is in the process of burning to the ground may well be entitled to an instruction on duress or necessity, "`for he is not to be hanged because he would not stay to be burnt.'" United States v. Kirby, 7 Wall. 482, 487 (1869). And in the federal system it is the jury that is the judge of whether the prisoner's account of his reason for flight is true or false. But precisely because a defendant is entitled to have the credibility of his testimony, or that of witnesses called on his behalf, judged by the jury, it is essential that the testimony given or proffered meet a minimum standard as to each element of the defense so that, if a jury finds it to be true, it would support an affirmative defense — here that of duress or necessity.

We therefore hold that, where a criminal defendant is charged with escape and claims that he is entitled to an instruction on the theory of duress or necessity, he must proffer evidence of a bona fide effort to surrender or return to custody as soon as the claimed duress or necessity had lost its coercive force. We have reviewed the evidence examined elaborately in the majority and dissenting opinions below, and find the case not even close, even under respondents' versions of the facts, as to whether they either surrendered or offered to surrender at their earliest possible opportunity. Since we have determined that this is an indispensable element of the defense of duress or necessity, respondents were not entitled to any instruction on such a theory. Vague and necessarily self-serving statements of defendants or witnesses as to future good intentions or ambiguous conduct simply do not support a finding of this element of the defense.

III

In reversing the judgments of the Court of Appeals, we believe that we are at least as faithful as the majority of that court to its expressed policy of "allowing the jury to perform its accustomed role" as the arbiter of factual disputes. 190 U.S. App. D.C., at 151, 585 F.2d, at 1096. The requirement of a threshold showing on the part of those who assert an affirmative defense to a crime is by no means a derogation of the importance of the jury as a judge of credibility. Nor is it based on any distrust of the jury's ability to separate fact from fiction. On the contrary, it is a testament to the importance of trial by jury and the need to husband the resources necessary for that process by limiting evidence in a trial to that directed at the elements of the crime or at affirmative defenses. If, as we here hold, an affirmative defense consists of several elements and testimony supporting one element is insufficient to sustain it even if believed, the trial court and jury need not be burdened with testimony supporting other elements of the defense.

These cases present a good example of the potential for wasting valuable trial resources. In general, trials for violations of § 751(a) should be simple affairs. The key elements are capable of objective demonstration; the mens rea, as discussed above, will usually depend upon reasonable inferences from those objective facts. Here, however, the jury in the trial of Bailey, Cooley, and Walker heard five days of testimony. It was presented with evidence of every unpleasant aspect of prison life from the amount of garbage on the cellblock floor, to the meal schedule, to the number of times the inmates were allowed to shower. Unfortunately, all this evidence was presented in a case where the defense's reach hopelessly exceeded its grasp. Were we to hold, as respondents suggest, that the jury should be subjected to this potpourri even though a critical element of the proffered defenses was concededly absent, we undoubtedly would convert every trial under § 751(a) into a hearing on the current state of the federal penal system.

Because the juries below were properly instructed on the mens rea required by § 751(a), and because the respondents failed to introduce evidence sufficient to submit their defenses of duress and necessity to the juries, we reverse the judgments of the Court of Appeals.

Reversed.

MR. JUSTICE MARSHALL took no part in the consideration or decision of these cases.