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The defendant is never called to testify in a hearing pursuant to his 5th amendment rights, but he does bear the burden of proving that the attorney did know that an expert should have been used and he would do that by presenting another legal expert to testify and to testify that the attorney's actions were not up to legally accepted standards and the expert would have likely made a difference in the outcome of the case.
Once a defendant gets on the stand, they have to testify to pretty much everything asked and this is why usually another attorney is used to testify to what should have been done by the first attorney.
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