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.' " [Citation.] The test for determining whether a confession is voluntary is whether the defendant's "will was overborne at the time he confessed." [Citation.] " 'The question posed by the due process clause in cases of claimed psychological coercion is whether the influences brought to bear upon the accused were "such as to overbear petitioner's will to resist and bring about confessions not freely self-determined." [Citation.]' [Citation.] In determining whether or not an accused's will was overborne, 'an examination must be made of "all the surrounding circumstances both the characteristics of the accused and the details of the interrogation." ' " ' " (People v. McWhorter (2009) 47 Cal.4th 318, 346-347.) " '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.] 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.' " (Id. at p. 347.)