The defence have to make an application to the judge to have previous convictions made known to the jury if they wish to show "bad character". This provision was only introduced about three years ago. Prior to that they could not be put at all.
It is because the jury are supposed to judge the single offence before them on the evidence presented and not be swayed by anything the defendant may have done previously.
If someone was in court charged with armed robbery and you were on the jury and found that he had 12 previous convictions for shop lifting, you could think that he obviously has a criminal record as long as your arm so is obviously guilty of this offence as well. Even if they didn't do it.
If they had 12 previous convictions for the same offence then you would think the same but it would be much more likely that they had continued in their life of crime.
Personally I think that only related offences should be disclosed. Maybe this is already the case, I don't know - before anyone shouts at me.
What if someone was up for rape, no previous convictions were mentioned during the trial and the person was found not guilty. Then after the trial is it over, it is disclosed that he has been accused of rape on 4 previous occasions but been acquitted on every occasion?
Surely you would then think 'Hang on, perhaps he did do it after all...'
ye, but I guess it's a case of 'innocent until proven guilty', and that means 'innocent of this particular crime', and you have to prove otherwise, and having a previous record doesn't show you're guilty of this crime. You're proving they did this crime, not that they're a rapist.
To expand a bit and pick up on some previous points:
If the prosecution wants to introduce evidence of �bad character� they have to get the judge�s (or magistrates�) permission. They have to show that the defendant has �the propensity to commit similar offences� (that is, similar to the one with which they are currently charged). They cannot simply introduce the defendant�s previous record in full.
As I said earlier, the principle behind a criminal trial is that the prosecution has to produce sufficient evidence that the defendant committed the offence with which they are currently charged. It is not sufficient to say �he has done it before, therefore he must have done it this time�.
There are strict rules governing the admissibility of previous offence history to the jury or magistrates and it is up to the prosecution to make the argument. In a Crown Court the argument is heard by the judge without the jury being present.
In a magistrates� court the matter is more complicated in that the magistrates are the arbiters of the argument and of the trial proper. Sometimes the �bad character� evidence is ruled upon by a bench of magistrates in advance of the trial. They rule simply on that matter and the trial proper is heard by a new bench. On other occasions the trial bench hear the bad character evidence. If they decide it is not to be admitted they must �put the evidence out of their mind� when reaching their verdict. Often on these occasions the defence will make a representation to have the trial heard by a new bench.