There are many different ways to prove that a person is guilty of a crime. For example, for a purse snatch, the victim or witnesses could identify the robber, the robbery could be caught on tape, or the police could stop the defendant shortly after the robbery, in possession of the victim's purse.
Suppose that the only evidence of the purse snatch is the last--the police stop the defendant, who has the purse. In many states, the jury might be told to consider the evidence that the defendant was found with the victim's purse by applying a special test. In these states, whenever the prosecutor relies solely or primarily on circumstantial evidence to prove that a defendant is guilty of a crime, the court must instruct the jury that circumstantial evidence can be considered as proof of guilt onlyif the evidence is not also consistent with innocence. To put it another way, circumstantial evidence that is consistent with either guilt or innocence is not strong enough to support a conviction.
To understand circumstantial evidence, we first need to understand direct evidence. Direct evidence is evidence that directly proves that face at issue. If you look out the window and see that it is raining, that is direct evidence. Direct evidence, standing alone – if believed – proves that a defendant is guilty of the charged crime. For example, confessions and eyewitness testimony identifying the defendant are direct evidence. If a witness says, "I saw Larry kill Susan," then that is direct evidence of Larry's guilt for Susan's murder.
In contrast, circumstantial evidence is evidence that proves a fact or event by inference. For example, if the skies have been overcast all day, and rain was predicted, and your spouse comes home from work soaking wet, that might be circumstantial evidence that it is raining. If a witness says, "Larry came home from Susan's house the night that she was killed, covered in blood," that is circumstantial evidence of Larry's guilt.
Prosecutors can rely on either direct or circumstantial evidence, and courts usually tell juries that neither type of evidence is necessarily superior. But, unlike direct evidence, circumstantial evidence may be consistent with other facts. For example, if Larry stopped on the site of an accident to help an injured motorist on his way home from Susan's house, that could explain his bloody state. However, the fact that one scenario is much more likely and reasonable than others does not convert circumstantial evidence into direct evidence. Larry's appearance at home covered in blood may be strong circumstantial evidence that he killed Susan (and, without some other explanation, would almost certainly be sufficient to convict him), but it is not direct evidence.
The circumstantial evidence jury instruction tells the jury that in order to convict a defendant based on circumstantial evidence, the jury must not only find that the circumstantial evidence is consistent with defendant's guilt, but also that the evidence is not reasonably consistent with innocence.
For example, California courts instruct juries that: "Before you may rely on circumstantial evidence to conclude that a fact necessary to find the defendant guilty has been proved, you must be convinced that the People [through the prosecutor] have proved each fact essential to that conclusion beyond a reasonable doubt. Also, before you may rely on circumstantial evidence to find the defendant guilty, you must be convinced that the only reasonable conclusion supported by the circumstantial evidence is that the defendant is guilty. If you can draw two or more reasonable conclusions from the circumstantial evidence, and one of those reasonable conclusions points to innocence and another to guilt, you must accept the one that points to innocence. However, when considering circumstantial evidence, you must accept only reasonable conclusions and reject any that are unreasonable." (CALCRIM 224.) So, if Larry offers a reasonable, alternate explanation for his blood-soaked clothes, the jury must accept it. But, if Larry suggests that he was covered in blood because he cut himself shaving, the jury may reject that explanation becauseit is not reasonable for a shaving cut to result in so much blood loss.
The states have different rules on when the circumstantial evidence instruction should be given.
For example, if a witness saw three unidentified people commit a burglary and get into a red car, and police stopped the defendant later in his red car with two friends and items stolen in the burglary, that would be circumstantial evidence of guilt. If this was the only evidence of guilt, in California, the court would have to give the instruction. In Georgia, the court would have to give the instruction if requested, and in Alabama, it would be up to the court whether to give the instruction.
If you are facing charges that depend, even in part, on circumstantial evidence, you should talk to a criminal defense attorney about your case. An attorney can explain the laws in your state and help you strategize. An attorney will under the rules that apply to circumstantial evidence in your state and will be able to tell you how to obtain the best possible outcome in your case.