Defendant Robert Lyons was indicted on twelve counts of knowingly and intentionally securing controlled narcotics by misrepresentation, fraud, deception and subterfuge in violation of 21 U.S.C. § 843(a)(3) (1976) and 18 U.S.C. § 2 (1976). Before trial Lyons informed the Assistant United States Attorney that he intended to rely on a defense of insanity: that he had lacked substantial capacity to conform his conduct to the requirements of the law because of drug addiction. See Fed.R.Crim.P. 12.2(a). Lyons proffered evidence1 that in 1978 he began to suffer from several painful ailments, that various narcotics were prescribed to be taken as needed for his pain, and that he became addicted to these drugs. He also offered to present expert witnesses who would testify that his drug addiction affected his brain both physiologically and psychologically and that as a result he lacked substantial capacity to conform his conduct to the requirements of the law.
In response to the government’s motion in limine, the district court excluded any evidence of Lyon’s drug addiction, apparently on the ground that such an addiction could not constitute a mental disease or defect sufficient to support an insanity defense. A panel of this Court reversed, holding that it was the jury’s responsibility *245to decide whether involuntary drug addiction could constitute a mental disease or defect depriving Lyons of substantial capacity to conform his conduct to the requirements of the law. United States v. Lyons, 704 F.2d 743 (5th Cir. 1983). We agreed to rehear the case en banc. Id. at 748.2
I.
For the greater part of two decades our Circuit has followed the rule that a defendant is not to be held criminally responsible for conduct if, at the time of that conduct and as a result of mental disease or defect, he lacked substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law. Blake v. United States, 407 F.2d 908, 916 (5th Cir.1969) (en banc).
Today the great weight of legal authority clearly supports the view that evidence of mere narcotics addiction, standing alone and without other physiological or psychological involvement, raises no issue of such a mental defect or disease as can serve as a basis for the insanity defense. Bailey v. United States, 386 F.2d 1, 3-4 (5th Cir.1967), cert. denied, 392 U.S. 946, 88 S.Ct. 2300, 20 L.Ed.2d 1408 (1968). Accord, United States v. Coffman, 567 F.2d 960, 963 (10th Cir.1977); United States v. Moore, 486 F.2d 1139, 1181 (D.C.Cir.) (en banc), cert. denied, 414 U.S. 980, 94 S.Ct. 298, 38 L.Ed.2d 224 (1973); United States v. Stevens, 461 F.2d 317, 321 (7th Cir.1972); Gaskins v. United States, 410 F.2d 987, 989 (D.C.Cir.1967); Green v. United States, 383 F.2d 199, 201 (D.C.Cir.1967), cert. denied, 390 U.S. 961, 88 S.Ct. 1061,19 L.Ed.2d 1158 (1968); United States v. Freeman, 357 F.2d 606, 625 (2d Cir.1966); Berry v. United States, 286 F.Supp. 816, 820 (E.D.Pa.1968), rev’d on other grounds, 412 F.2d 189 (3d Cir.1969). Cf. United States v. Romano, 482 F.2d 1183, 1196 (5th Cir.1973), cert. denied sub nom. Yassen v. United States, 414 U.S. 1129, 94 S.Ct. 866, 38 L.Ed.2d 753 (1974) (being involuntarily under the influence of drugs at the time of the crime is not a legal equivalent of insanity). See also Fingarette, Addiction and Criminal Responsibility, 84 Yale L.J. 413, 424-25 (1975) (“there is no consensus in the medical profession that addiction is a mental disease”).3
There are a number of reasons why. In the first place, there is an element of reasoned choice when an addict knowingly acquires and uses drugs; he could instead have participated in an addiction treatment program. Moore, 486 F.2d at 1183 (opinion of Leventhal, J.). A person is not to be excused for offending “simply because he wanted to very, very badly.” Bailey, 386 F.2d at 4. Second, since the defense of insanity is “essentially an acknowledgement on the part of society that because of mental disease or defect certain classes of wrongdoers are not properly the subjects of criminal punishment,” Freeman, 357 F.2d at 625, it seems anomalous to immunize narcotics addicts from other criminal sanctions when Congress has decreed severe penalties for mere possession and sale of narcotics. Id. In addition, Congress has dealt with the problem of responsibility of narcotics addicts for their crimes by providing for civil commitment and treatment of addicts in lieu of prosecution or sentencing. Bailey, 386 F.2d at 4. See, *246e.g., 18 U.S.C. §§ 4251-4255 (1976); 28 U.S.C. §§ 2901-2906 (1976).
Finally, what definition of “mental disease or defect” is to be employed by courts enforcing the criminal law is, in the final analysis, a question of legal, moral and policy — not of medical — judgment.4 Among the most basic purposes of the criminal law is that of preventing a person from injuring others or, perhaps to a lesser degree, himself. This purpose and others appropriate to law enforcement are not necessarily served by an uncritical application of definitions developed with medical considerations of diagnosis and treatment foremost in mind. Cf. Powell v. Texas, 392 U.S. at 540-41, 88 S.Ct. at 2158-59 (Black, J., concurring). Indeed, it would be coincidental indeed should concepts deriving from such disparate sources correspond closely, one to the other. Thus it is, for example, that the law has not greatly concerned itself with medical opinion about such mental states as accompany the commission of crimes of passion or of those done while voluntarily intoxicated; whatever that opinion may be, policy considerations have been thought to forbid its cutting much of a figure in court.
Contravening the broad thrust of the authorities cited above, the panel opinion appears to suggest that “involuntary” drug addiction can constitute a “mental disease or defect” bearing on the defendant’s criminal responsibility. 704 F.2d at 747. The panel believed itself bound to that rule by such a holding in United States v. Bass, 490 F.2d 846 (5th Cir.1974). In so concluding the panel acted with obvious reluctance but with fidelity to the principle that one panel of our court does not overrule another. Today, sitting en bane, we overrule Bass insofar as it may be read to hold that mere drug addiction, voluntary or involuntary, can be a mental disease for legal purposes. Insofar, however, as it countenanced the receipt of evidence of drug addiction in connection with Bass’s genuine mental disease — chronic anxiety — to which it contributed, we find no fault with the opinion.
Although mere narcotics addiction is not itself to be acknowledged as a mental disease or defect, evidence of narcotics addiction has been received by some courts as evidence of such an underlying condition. Green v. United States, 383 F.2d 199, 201 (D.C.Cir.1967), cert. denied, 390 U.S. 961, 88 S.Ct. 1061, 19 L.Ed.2d 1158 (1968). In addition, if addiction has caused actual physical damage to the structures of a defendant’s body, evidence of that addiction has been admitted to show any mental defect resulting. from that damage. Cf. Brinkley v. United States, 498 F.2d 505, 511-12 (8th Cir.1974) (remanding to explore possible physiological and psychological effects of long term LSD use on appellant and whether these effects might amount to insanity).
We view the reasoning of such rulings as Green with profound misgivings. To us it seems to rest on the proposition that, assuming drug addiction itself is neither a mental disease nor a defect, yet the two are often to be found in association, so that an addicted person is more likely to suffer from some mental disorder than is one who *247is not addicted.5 By a parity of reasoning, since combat veterans as a group are self-evidently more likely to have suffered the loss of a physical member than is the populace at large, evidence of whether a party is a combat veteran should be received on the issue whether he has lost a leg. Or, to take a less extreme example, since because of light skin pigmentation persons of Scandinavian ancestry are more subject to skin cancer than are others, the family tree of a suitor should be received in evidence when his skin cancer is at legal issue. The flaw in both illustrations seems evident: where evidence bearing directly on a legal question is available, that involving tangential matters, even though perhaps logically relevant in theory, is of small practical value.6
Our review of numerous records over the course of years has revealed no dearth of experts ready and willing to testify squarely on the issue of insanity in criminal trials: direct evidence on the issue seems all but too readily available. Since this is so, receiving evidence of drug addiction in addition seems to us an exercise seldom likely to prove more probative than prejudicial in practice. See Rule 403, Federal Rules of Evidence.7
Nor do we see how matters are clarified by reference to the condition of addiction as one involving “psychological damage” to the addict, e.g., Brinkley v. United States, supra. As nearly as we can determine, the psychological condition so described is simply one of drug addiction to one degree or another, a condition that we have already declined to view as a mental disease or defect for legal purposes. A.n actual drug-induced or drug-aggravated psychosis, or physical damage to the brain or nervous system would, however, be another matter.
We do not doubt that actual physical damage to the brain itself falls within the ambit of “mental disease or defect.” To refuse to recognize that a congenital micro-cephalic, or one who has suffered, say, extensive brain damage from a gunshot wound or other physical' trauma, may be thereby rendered unable to appreciate the character of his conduct as wrongful would be presumptuous. Here, within the limits of appropriate legal and policy considerations, the medical model must have its day. The same is true of the question whether such organic brain pathology or psychosis can be caused by drugs.
Lyons asserted by his proffer of evidence that his drug addiction caused physiological damage to his brain and that this damage caused him to lack substantial capacity to conform his conduct to the requirements of the law. 704 F.2d at 746. Since he did so, he should — under our subsisting Blake test — have been allowed to introduce evidence of any physical brain damage and consequent mental disease or defect. Because the proffer offers evidence tending to suggest such damage, that evidence should have been submitted to the jury. Blake, 407 F.2d at 911. And although we today withdraw our recognition of the volitional prong of Blake — that as to which such evidence has usually been advanced — we also conclude that should Lyons wish to offer such evidence in an attempt to satisfy the remaining cognitive prong, fairness demands that we afford him an opportunity to do so.
II.
Because the concept of criminal responsibility in the federal courts is a congeries of judicially-made rules of decision based on common law concepts, it is usual*248ly appropriate for us to reexamine and reappraise these rules in the light of new policy considerations. Wion v. United States, 325 F.2d 420, 425 (10th Cir.1963). We last examined the insanity defense in Blake v. United States, 407 F.2d 908 (5th Cir.1969) (en banc), where we adopted the ALI Model Penal Code definition of insanity: that a person is not responsible for criminal conduct if, at the time of such conduct and as a result of mental disease or defect, he lacks substantial capacity either to appreciate the wrongfulness of his conduct or to conform his conduct to the requirements of the law. Id. at 916. Following the example of sister circuits, we embraced this standard in lieu of our former one, defined in Howard v. United States, 232 F.2d 274, 275 (5th Cir.1956) (en banc),8 because we concluded that then current knowledge in the field of behavioral science supported such a result. 407 F.2d at 909, 914-15. Unfortunately, it now appears our conclusion was premature — that the brave new world that we foresaw has not arrived.
Reexamining the Blake standard today, we conclude that the volitional prong of the insanity defense — a lack of capacity to conform one’s conduct to the requirements of the law — does not comport with current medical and scientific knowledge, which has retreated from its earlier, sanguine expectations. Consequently, we now hold that a person is not responsible for criminal conduct on the grounds of insanity only if at the time of that conduct, as a result of a mental disease or defect, he is unable to appreciate the wrongfulness of that conduct.9
We do so for several reasons. First, as we have mentioned, a majority of psychiatrists now believe that they do not possess sufficient accurate scientific bases for measuring a person’s capacity for self-control or for calibrating the impairment of that capacity. Bonnie, The Moral Basis of the Insanity Defense, 69 ABA J. 194, 196 (1983).10 “The line between an irresistible impulse and an impulse not resisted is probably no sharper than between twilight and dusk.” American Psychiatric Association Statement on the Insanity Defense, 11 (1982) [APA Statement]. Indeed, Professor Bonnie states:
There is, in short, no objective basis for distinguishing between offenders who were undeterrable and those who were merely undeterred, between the impulse that was irresistible and the impulse not resisted, or between substantial impairment of capacity and some lesser impairment.
Bonnie, supra, at 196.11
*249In addition, the risks of fabrication and “moral mistakes” in administering the insanity defense are greatest “when the experts and the jury are asked to speculate whether the defendant had the capacity to ‘control’ himself or whether he could have ‘resisted’ the criminal impulse.” Bonnie, supra, at 196. Moreover, psychiatric testimony about volition is more likely to produce confusion for jurors than is psychiatric testimony concerning a defendant’s appreciation of the wrongfulness of his act. APA Statement at 12. It appears, moreover, that there is considerable overlap between a psychotic person’s inability to understand and his ability to control his behavior. Most psychotic persons who fail a volitional test would also fail a cognitive test, thus rendering the volitional test superfluous for them. Id.12 Finally, Supreme Court authority requires that such proof be made by the federal prosecutor beyond a reasonable doubt, an all but impossible task in view of the present murky state of medical knowledge. Davis v. United States, 160 U.S. 469, 16 S.Ct. 353, 40 L.Ed. 499 (1895).13
One need not disbelieve in the existence of Angels in order to conclude that the present state of our knowledge regarding them is not such as to support confident conclusions about how many can dance on the head of a pin.14 In like vein, it may be that some day tools will be discovered with which reliable conclusions about human volition can be fashioned. It appears to be all but a certainty, however, that despite earlier hopes they do not lie in our hands today. When and if they do, it will be time to consider again to what degree the law should adopt the sort of conclusions that they produce. But until then, we see no prudent course for the law to follow but to treat all criminal impulses — including those not resisted — as resistible. To do otherwise in the present state of medical knowledge would be to cast the insanity defense adrift upon a sea of unfounded scientific speculation, with the palm awarded ease by case to the most convincing advocate of that which is presently unknown — and may remain so, because unknowable.
III.
Thus, Lyons’ claim that he lacked substantial capacity to conform his conduct to the requirements of the law will not raise the insanity defense. It would be unfair, however, to remit him retroactively *250to our newly restricted insanity defense without allowing him the opportunity to plan a defense bearing its contours in mind. Consequently, we vacate his conviction and remand for a new trial in accordance with our new insanity standard. As for other cases, today’s holding shall have prospective application only, commencing thirty days from the date of its publication.
VACATED and REMANDED.
. Lyons’ proffer of evidence is reproduced in its entirety in the panel opinion. 704 F.2d at 744-47. We merely summarize it here.
. For the en banc hearing we invited interested groups to submit amicus briefs. Several were received, including briefs from the American Bar Association, American Psychological Association, and the National Association of Criminal Defense Lawyers, for all of which we are obliged.
. This rule is consistent with holdings that use of narcotics does not per se render a defendant incompetent to stand trial, Lewis v. United States, 542 F.2d 50, 51 (8th Cir.), cert. denied, 429 U.S. 837, 97 S.Ct. 105, 50 L.Ed.2d 103 (1976); United States v. Williams, 468 F.2d 819, 820 (5th Cir.1972); Grennett v. United States, 403 F.2d 928, 931 (D.C.Cir.1968), and that mere alcoholism does not constitute a mental disease or defect warranting an insanity instruction, Powell v. Texas, 392 U.S. 514, 535, 88 S.Ct. 2145, 2155, 20 L.Ed.2d 1254 (1968); United States v. Shuckahosee, 609 F.2d 1351, 1355 (10th Cir. 1979), cert. denied, 445 U.S. 919, 100 S.Ct. 1283, 63 L.Ed.2d 605 (1980); United States v. Malafronte, 357 F.2d 629, 632 n. 8 (2d Cir.1966).
. Speaking of the recent American Psychiatric Association Statement on the Insanity Defense, Professor Phillip E. Johnson notes:
The APA has not adopted the extreme views of Thomas Szasz, but it has definitely repudiated the ideology of Karl Menninger. The psychiatrists no longer want the criminal law to change to conform to deterministic psychiatric concepts; instead, they regard it as vital to the integrity of their own discipline that "legal or moral constructs such as free will" be understood as outside the domain of psychiatry. They emphatically affirm that most people, including those with sociopathic personality disorders, should be held accountable for what they do. They are not washing their hands of the legal problems, and they believe that the law still needs them, but they understand that legal and moral decisions are ultimately to be made by citizens, not experts. I regard this newly found modesty as evidence of the profession's increasing maturity, not as a sign of its failure.
Johnson, Book Review, 50 U.Chi.L.Rev. 1534, 1548 (1983) (reviewing N. Morris, Madness and the Criminal Law (1982)).
. See, e.g., Gerard & Kornetsky, Adolescent Opiate Addiction: A Study of Control and Addict Subjects, 29 Psychiatric Q. 457 (1955); Sutker, Personality Differences and Sociopathy in Heroin Addicts and Nonaddict Prisoners, 78 J. Abnormal Psychology, 247 (1971).
. Indeed, it may be counter-productive. One might well view with suspicion a claim to have lost a leg made by one who supported it only with evidence that he had served in combat, rather than by lifting his trouser cuff.
. We do not suggest that references in testimony to drug use as the cause of or as aggravating particular brain pathology should be viewed as taboo, only that attempts to characterize addiction as itself a mental disease or defect are not to be countenanced.
. The Howard standard provided that insanity constituted either the “incapacity from some mental disease or defect to distinguish between right and wrong with respect to the act, or the inability from such disease or defect to refrain from doing wrong in the commission of the act.” 232 F.2d at 275.
. We employ the phrase "is unable" in preference to our earlier formulation "lacks substantial capacity" for reasons well stated in the Commentary of the American Bar Association Standing Committee:
Finally, it should be pointed out that the standard employs the term "unable" in lieu of the "substantial capacity" language of the ALI test. This approach has been taken both to simplify the formulation and to reduce the risk that juries will interpret the test too loosely. By using the "substantial capacity” language, the drafters of the ALI standard were trying to avoid the rigidity implicit in the M’Naughten formulation. They correctly recognize that it is rarely possible to say that a mentally disordered person was totally unable to "know” what he was doing or to "know" that it was wrong; even a psychotic person typically retains some grasp of reality. However, the phrase "substantial capacity” is not essential to take into account these clinical realities. Sufficient flexibility is provided by the term “appreciate."
Commentary (revised November, 1983) to Standards 7-6.1(a) and 7-6.9(b), ABA Standing Committee on Association Standards for Criminal Justice (to be published).
. See also H. Fingarette, The Meaning of Insanity 166 (1972); Wootton, Book Review, 77 Yale L.J. 1019, 1026-27 (1968); Statement of David Robinson, Jr., The Insanity Defense, Hearings Before the Senate Comm, on the Judiciary, 97th Cong., 2d Sess. 72-73 (1982); Testimony of Stephen Morse, Insanity Defense in Federal Courts, Hearings Before the Subcomm. on Criminal Justice of the House Comm, on the Judiciary, 97th Cong., 2d Sess. 211 (1982).
. One commentator has noted that no one has ever observed the process of a person losing the *249capacity for self-control, and "that no one can.” Fingarette, supra, at 160.
. See abo Statement of Stephen Morse, Insanity Defense in Federal Courts, Hearings Before the Subcomm. on Criminal Justice of the House Comm, on the Judiciary, 97th Cong., 2d Sess. 231 (1982).
. John Hinckley is the young man who attempted to assassinate President Reagan in order to attract attention to himself and to impress a movie actress whom he admired from a distance. The subsequent proceedings called into question not only the insanity defense but the rationality of our adversarial jury-trial system. After more than a year of expensive pretrial maneuvering and psychiatric examinations, the lawyers jousted for eight weeks of trial, examining and cross-examining expert witnesses who naturally gave conflicting and confusing testimony on whether Hinckley's obviously warped mentality amounted to legal insanity. The judge instructed the jury to return a verdict of not guilty unless they could agree “beyond a reasonable doubt” that Hinckley was sane. If taken literally, the instruction amounted to a directed verdict of not guilty, considering the deadlock of expert opinion and the difficulty of certifying the sanity of a young man who shot the President to impress a movie star. Juries usually ignore such unpopular legal standards, but the Hinckley jury surprised everybody by taking the law seriously and finding him not guilty. Hinckley will now be confined to a mental hospital indefinitely because he is “dangerous,” although there is no reliable way to predict what he would do if released and no reliable test to determine if he has been "cured.”
Johnson, Book Review, 50 U.Chi.L.Rev. 1534, 1536 (1983) (reviewing N. Morris, Madness and the Criminal Law (1982)).
. "What Song the Syrens sang, or what name Achilles assumed when he hid himself among women, though puzzling questions, are not beyond all conjecture.” Sir Thomas Browne, URN BURIAL, v.