"'The Fourteenth Amendment to the federal Constitution and article I, section 15, of the state Constitution bar the prosecution from using a defendant's involuntary confession. These provisions require the prosecution to establish, by a preponderance of the evidence, that a defendant's confession was voluntary. . . .Under both state and federal law, courts apply a "totality of circumstances" test to determine the voluntariness of a confession. . . . On appeal, the trial court's findings as to the circumstances surrounding the confession are upheld if supported by substantial evidence, but the trial court's finding as to the voluntariness of the confession is subject to independent review. In determining whether a confession was voluntary, "the question is whether defendant's choice to confess was not 'essentially free' because his will was overborne."' " (People v. Holloway (2004) 33 Cal.4th 96, 114.) "'It is well settled that a confession is involuntary and therefore inadmissible if it was elicited by any promise of benefit or leniency whether express or implied. However, mere advice or exhortation by the police that it would be better for the accused to tell the truth when unaccompanied by either a threat or a promise does not render a subsequent confession involuntary. . . . Thus, "when the benefit pointed out by the police to a suspect is merely that which flows naturally from a truthful and honest course of conduct," the subsequent statement will not be considered involuntarily made. On the other hand, "if . . . the defendant is given to understand that he might reasonably expect benefits in the nature of more lenient treatment at the hands of the police, prosecution or court in consideration of making a statement, even a truthful one, such motivation is deemed to render the statement involuntary and inadmissible . . . ."' " (Holloway, supra, 33 Cal.4th at p. 115.) As explained in People v. Tully (2012) 54 Cal.4th 952, 993 (Tully), a confession is involuntary and inadmissible if it was elicited by any express or implied promise of benefit or leniency. Not every inducement to speak renders a confession involuntary; if the benefit pointed out by the police flows naturally from the truth and honesty, a subsequent confession is not involuntary. (Ibid.; People v. Howard (1988) 44 Cal.3d 375, 398.) "A confession is not rendered involuntary by coercive police activity that is not the 'motivating cause' of the defendant's confession. (People v. Williams (1997) 16 Cal.4th 635, 661.)" (People v. Linton (2013) 56 Cal.4th 1146, 1176.) The burden is on the prosecution to establish the voluntariness of a confession by a preponderance of the evidence. (Tully, supra, at p. 993; People v. Carrington (2009) 47 Cal.4th 145, 169 (Carrington).) The focus of the inquiry is on whether defendant's choice to confess was not essentially free because his will was overborne, which is determined by examination of the totality of the circumstances. (Tully, supra, at p. 993; Carrington, supra, at p. 169.) We review the trial court's factual findings for substantial evidence, but conduct a de novo review of the trial court's finding that the confession is voluntary. (Arizona v. Fulminante (1991) 499 U.S. 279, 287, 303 (Fulminante); Tully, supra, 54 Cal.4th at p. 993; Carrington, supra, 47 Cal.4th at p. 169.) Whether conduct constituted a promise of leniency and whether it operated as an inducement is subject to independent review. (People v. Jones (1998) 17 Cal.4th 279, 296.) An interrogator's statement that he has no authority to make promises does not, standing alone, undermine a defendant's contention that his confession was involuntary. "The viewpoint is too narrow, for promises can be implicit as well as explicit, and an assertion that no promises are being made may be contradicted by subsequent conversation. (People v. Hill (1967) 66 Cal.2d 536, 549-550.)" (People v. Andersen (1980) 101 Cal.App.3d 563, 579.) "'In carrying out their interrogations the police must avoid threats of punishment for the suspect's failure to admit or confess particular facts and must avoid false promises of leniency as a reward for admission or confession . . . . The police are authorized to interview suspects who have been advised of their rights, but they must conduct the interview without the undue pressure that amounts to coercion and without the dishonesty and trickery that amounts to false promise.' (People v. Andersen, supra, at p. 576.)" (People v. Holloway (2004) 33 Cal.4th 96, 115.) When statements are obtained in violation of Miranda, the error is reviewed under the federal "harmless beyond a reasonable doubt" standard set forth in Chapman v. California (1967) 386 U.S. 18 (Chapman). (Arizona v. Fulminante (1991) 499 U.S. 279, 297, 302 (Arizona) use of coerced confession not harmless error; People v. Cahill (1993) 5 Cal.4th 478, 510 (Cahill) federal harmless error standard applicable to inadmissible confession admitted in a California trial; see People v. Thomas (2011) 51 Cal.4th 449, 498 admission of confession harmless error where cumulative to stronger evidence; In re Z.A. (2012) 207 Cal.App.4th 1401, 1422-1423) highly inculpatory statements admitted in violation of Miranda not harmless beyond reasonable doubt.) In applying this standard: "'the question is whether there is a reasonable possibility that the evidence complained of might have contributed to the conviction.'" (Chapman, supra, 386 U.S. at p. 23.) As the court held in Arizona, in finding the state failed to meet its burden of proving that use of a coerced confession was harmless beyond a reasonable doubt, "a confession is like no other evidence. Indeed, 'the defendant's own confession is probably the most probative and damaging evidence that can be admitted against him....'" (Arizona, supra, at p. 296; Cahill, supra, 5 Cal.4th at p. 497 "'the confession operates as a kind of evidentiary bombshell which shatters the defense'".)