An experienced criminal defense attorney specializing in white-collar cases has given one of the best analyses ever of why it is often a bad idea for criminal defendants to testify at their trials and why those criminal defendants still do it.
Writing a post dated June 12, 2012 on the White Collar Crime Prof Blog, Lawrence S. Goldman pointed out that criminal defendants who testify are forced to undergo cross-examination, and that cross-examination gives the prosecution the equivalent of another closing argument. Writing about an insider trading case, Goldman writes:
"Interrogation about these repeated events would allow the prosecutors in effect an extra summation to hammer on these facts, indeed perhaps even better than a summation since the defendant would have to respond directly to each of the allegations, whereas in summation an attorney would have the option of ignoring, glossing over or generalizing about all or portions of the evidence."
Goldman's insights regarding the effect of a criminal
defendant's testimony are particularly valuable (indeed, they
are so valuable that they justify the funky look of the rest of
this post) :
"In any case, white-collar or not, I believe that when a
defendant testifies, the standard of proof beyond a
reasonable doubt is diluted. Jurors, rather than
asking themselves whether the prosecutor has proved
the case beyond a reasonable doubt, focus more on
whether the defendant probably told the truth.
"The decision whether to testify is one of the very few
that virtually all lawyers, and all ethics rules, decree
belongs ultimately to the client. It is often difficult to
convince white collar clients, especially those whose
egos have become enlarged because of their extreme
success, that they will be unable to convince a jury."
Also worth reading is Goldman's post of May 29, 2012 on that
same blog entitled "DOJ 'Punishment' of Stevens Prosecutors
Too Lenient," which includes the lines "If a truck driver
causes serious personal injury by reckless driving, is there
any doubt he would be fired. The injury to Senator Stevens
was serious; the punishment was far too gentle."