>One exception I can think of though is when previous conviction changes the nature of whether or not a charge should involve premeditation. If you have been convicted of DUI previously, taking a drink without handing over your car keys is premeditation. So the charge might be different.
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To my knowledge, the only way you can introduce past convictions is when the defendant's attorney gives you an opening, like putting his client on the stand. Then it's wide open. You can bring in people who have known him and they can talk about him and his momma, not to mention all the past crimes.