In principle, it’s the witness encroaching on the province of the jury. Truth of a witness’ testimony is for the jury to decide, not for another witness to decide.
>> Truth of a witness’ testimony is for the jury to decide, not for another witness to decide.
On the face of it, and in most cases, that makes perfect sense.
But what you have here could be framed as testimony from an “expert witness” whose specialty, and indeed part of whose job, is to assess the truthfulness of the defendant.
Is the jury to believe that his conclusion isn’t a valid part of that testimony? Or, as another poster asked, what if Serino’s conclusion was that Z was lying? Would *that* opinion be admissible? Because that’s clearly how the prosecution was seeking to use Serino’s testimony, to impeach Z’s credibility.
(I realize it’s water under the bridge though.)
So is Zimmerman a witness or the defendant?
It's not for the police to say whether the defendant was truthful?
-PJ