What is the test for whether a confession is voluntary or coercive?

" 'In assessing allegedly coercive police tactics, "[t]he courts have prohibited only those psychological ploys which, under all the circumstances, are so coercive that they tend to produce a statement that is both involuntary and unreliable." [Citation.]' [Citation.] [] A confession is not involuntary unless the coercive police conduct and the defendant's statement are causally related. [Citations.]" (People v. Williams, supra, 49 Cal.4th at p. 436.)

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"We review independently a trial court's determinations as to whether coercive police activity was present and whether the statement was voluntary. [Citation.] We review the trial court's findings as to the circumstances surrounding the confession, including the characteristics of the accused and the details of the interrogation, for substantial evidence. [Citation.] '[T]o the extent the facts conflict, we accept the version favorable to the People if supported by substantial evidence.' [Citation.]" (People v. Guerra (2006) 37 Cal.4th 1067, 1093.)