I found the following article to be very interesting. This of course is just an excerpt.
Psychological evaluations and the competency to waive Miranda rights
By I. Bruce Frumkin; Alfredo Garcia
Psychology and the law of confessions share a symbiotic relationship. As the “queen of proofs,” confessions play a prominent role in the investigation and prosecution of crimes. The question that has plagued the law is whether an individual who faces a “police-dominated” atmosphere can withstand the psychological, emotional and physical pressures that inhere in the confessional “box.” Before the advent of Miranda, the Supreme Court applied the abstruse and indeterminate “voluntariness” standard to determine the admissibility of a confession. That criterion, as two thoughtful scholars have observed, “required inquiry into metaphysical states of mind that, by the 1960s, were believed to be inherently unknowable.”This metaphysical inquiry represented the apogee of the intersection between law and psychology.
[November 2003, Page p12]