What is the legal test for establishing whether a confession was involuntary?

"Both the state and federal Constitutions bar the prosecution from introducing a defendant's involuntary confession into evidence at trial. [Citations.] '"A statement is involuntary if it is not the product of '"a rational intellect and free will."' [Citation.] The test for determining whether a confession is voluntary is whether the defendant's 'will was overborne at the time he confessed.'"' [Citations.] [] '"A confession may be found involuntary if extracted by threats or violence, obtained by direct or implied promises, or secured by the exertion of improper influence. [Citation.] Although coercive police activity is a necessary predicate to establish an involuntary confession, it 'does not itself compel a finding that a resulting confession is involuntary.' [Citation.] The statement and the inducement must be causally linked. [Citation.]" [Citation].' [Citation.] A confession is not rendered involuntary by coercive police activity that is not the 'motivating cause' of the defendant's confession." (People v. Linton (2013) 56 Cal.4th 1146, 1176.)

"'The prosecution has the burden of establishing by a preponderance of the evidence that a defendant's confession was voluntarily made.' [Citation.] 'Whether a confession was voluntary depends upon the totality of the circumstances.' [Citations.] 'On appeal, we conduct an independent review of the trial court's legal determination and rely upon the trial court's findings on disputed facts if supported by substantial evidence.' [Citation.] The facts surrounding an admission or confession are undisputed to the extent the interview is tape-recorded, making the issue subject to our independent review." (People v. Linton, supra, 56 Cal.4th at pp. 1176-1177.)

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Under both state and federal law, relevant factors in determining the question of voluntariness of a confession, by applying the totality of circumstances test include "'"'the crucial element of police coercion [citation]; the length of the interrogation [citation]; its location [citation]; its continuity' as well as 'the defendant's maturity [citation]; education [citation]; physical condition [citation]; and mental health.'"'" (People v. Boyette (2002) 29 Cal.4th 381, 411.)