States, 219 U. S. 79, and Jin Fuey Moy v. United States, 254 U. S. 189, overruled on this point. Pp. 290 U. S. 373, 290 U. S. 386.
2. In the absence of a federal statute governing the subject, the competency of witnesses in criminal trials in federal courts is determined by the common law. P. 290 U. S. 379.
3. In the taking of testimony in criminal cases, the federal courts are not bound by the rules of the common law as they existed at a specified time in the respective states; they are to apply those rules as they have been modified by changed conditions. P. 290 U. S. 379.
4. The reasons anciently assigned for disqualifying a wife as a witness in behalf of her husband in criminal cases can no longer be accepted in the federal courts in view of modern thought and legislation touching the subject. P. 290 U. S. 380.
5. The public policy of one generation may not, under changed conditions, be the public policy of another. P. 290 U. S. 381.
6. The federal courts have no power to amend or repeal a rule of the common law; but they have the power, and it is their duty, in the absence of any congressional legislation on the subject, to disregard an old rule which is contrary to modern experience and thought and is opposed in principle to the general current of legislation and judicial opinion, and to declare and apply what is the present rule in the light of the new conditions. Pp. 290 U. S. 381-383.
7. The common law is not immutable, but flexible, and by its own principles adapts itself to varying conditions. P. 290 U. S. 383.
v. United States, 254 U. S. 189. Petitioner contends that these cases, if not directly contrary to the decisions in Benson v. United States, 146 U. S. 325, and Rosen v. United States, 245 U. S. 467, are so in principle. We shall first briefly review these cases, with the exception of the Hendrix case and the Jin Fuey Moy case, which we leave for consideration until a later point in this opinion.
P. 53 U. S. 365. The court concluded that this could not be the common law as it existed at the time of the emigration of the colonists or the rule which then prevailed in England, and [therefore] the only known rule which could be supposed to have been in the mind of Congress was that which was in force in the respective states when the federal courts were established by the Judiciary Act of 1789. Applying this rule, it was decided that the witness was incompetent.
P. 144 U. S. 303.
Nor can the exclusion of the wife's testimony, in the face of the broad and liberal extension of the rules in respect of the competency of witnesses generally, be any longer justified, if it ever was justified, on any ground of public policy. It has been said that to admit such testimony is against public policy because it would endanger the harmony and confidence of marital relations, and, moreover, would subject the witness to the temptation to commit perjury. Modern legislation, in making either spouse competent to testify in behalf of the other in criminal cases, has definitely rejected these notions, and, in the light of such legislation and of modern thought, they seem to be altogether fanciful. The public policy of one generation may not, under changed conditions, be the public policy of another. Patton v. United States, 281 U. S. 276, 281 U. S. 306.
Compare Holden v. Hardy, 169 U. S. 366, 169 U. S. 385-387.