Say a mentally incompetent person is questioned and :
a> Spontaneously confesses to something even before the Miranda rights are read.
b> Confesses to something after the Miranda rights are read (assumingly waving the rights)
Later suppose the attorney establishes that the person is mentally incompetent (and does not understand what it means to implicate oneself), is evidence obtained either by a or b above automatically suppressed ?
If police suspect mental incompetency in anyway, are they required to stop questioning a suspect ?
There was a case in Massachusetts some years back (Commonwealth v. Hilton, 443 Mass. 597 (2005)) that dealt with roughly this issue. It involved a mentally retarded defendant who 1) made an unsolicited (inculpatory) statement to a court deputy and 2) waived Miranda and confessed. The court concluded that she was not competent to waive Miranda, so those statements were excluded. The unsolicited remarks were admitted.
I don’t think a confession by a mentally incompetent person alone would ever lead to a conviction since the state would still have to prove their case. Most likely the statement would be deemed inadmissible by a judge.
In many/most states these people are tested to see if they are competent to stand trial, and if aren’t the case is handled completely differently.
One of the funniest lines they ever gave Capt. Deakins on L&O: Criminal Intent was in reference to a mentally-challenged witness being interrogated. He asks if he was Mirandized and is told he waived. He replies, “Oh, that’ll hold up.”
If the person is mentally incompetent, given it doesn’t matter.
The real question is when the person is marginal and the court might consider them competent. Then it boils down to exactly what they understand, I suppose.