What is the test for determining whether a confession is involuntary?

"A statement is involuntary if it is not the product of '"a rational intellect and free will."'" (People v. Maury (2003) 30 Cal.4th 342, 404.) The test for determining voluntariness is whether the defendant's will was "'overborne'" at the time of the statement. (Ibid.) "A finding of coercive police activity is a prerequisite to a finding that a confession was involuntary under the federal and state Constitutions. [Citation.] 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.] . . . The statement and the inducement must be causally linked." (Id. at p. 404-405, italics added.) On appeal, we defer to the trial court's findings of historical fact if they are supported by substantial evidence, but independently review its determination that the defendant's statements were voluntary. (People v. Holloway (2004) 33 Cal.4th 96, 121.)

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