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

interrogation.' [Citation.]" [Citation.]' [Citation.] [] A finding of coercive police activity is a prerequisite to a finding that a confession was involuntary under the federal and state Constitutions. [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.]" {People v. Maury (2003) 30 Cal.4th 342, 404-405.)

"'[M]ere 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.' [Citation.] In terms of assessing inducements assertedly offered to a suspect, '"[w]hen the benefit pointed out by the police... is merely that which flows naturally from a truthful and honest course of conduct," the subsequent statement will not be considered involuntarily made. [Citation.]' [Citation.]" (People v. Howard (1988) 44 Cal.3d 375, 398.)