Expert testimony - mental handicap.
Where expert testimony on the mental capacity of the defendant can undercut the reliability of a confession, it is admissible on those grounds.
Non-expert testimony subject to the discretion of the court.
http://media.ca7.uscourts.gov/cgi-bin/rssExec.pl?Submit=Display&Path=Y2015/D12-30/C:14-2514:J:Sykes:aut:T:fnOp:N:1679527:S:0
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.