The trial of Scott Peterson for the murders of his wife Laci and their unborn child Conner is a classic example of prosecution based almost solely on circumstantial evidence.

Circumstantial evidence is evidence that has no direct proof but is instead based on a certain provable fact or facts used to form a credible theory of the events of a case. Even the most credible eye-witness testimony is only circumstantial because there are so many influences that can have an impact on human recall.

In cases lacking direct evidence, the prosecution must attempt to provide evidence of the circumstances from which the judge and jury can logically deduct, or reasonably infer, a factual theory of the case that cannot be proven directly. It's up to the prosecutors to show through a set of circumstances that their theory of what took place is the only logical deduction—that the circumstances can be explained by no other possible theory.

Conversely, in cases of circumstantial evidence, the job of the defense is to show that the same circumstances might be explained by an alternative theory. In order to avoid conviction, all a defense attorney must do is create reasonable doubt. If even one juror is convinced strongly enough that the prosecution's explanation of the circumstances is flawed, the case may be dismissed.

No Direct Evidence in Peterson Case

In the trial of Scott Peterson, there was very little, if any, direct evidence connecting Peterson to the murder of his wife and the death of their unborn child. It became the prosecution's mandate to prove that the circumstances surrounding her death and the disposal of her body could be linked to no one other than her husband.

In the sixth week of the trial, Defense attorney Mark Geragos was able to cast doubt on two key pieces of evidence that supported the prosecution's theory that Peterson had dumped his wife's body in San Francisco Bay: the homemade anchors Peterson allegedly used to sink the body and a hair collected from his boat that was consistent with his wife's DNA.

Alternative Theories in the Peterson Case

Photos presented by police investigator Henry "Dodge" Hendee and subsequent questions from prosecutors were used to show the jury that Peterson had used a water pitcher found at his warehouse to mold five boat anchors—four of which were missing. Under cross-examination, however, Geragos was able to get Hendee to acknowledged to jurors that the prosecution's own expert witness had determined that the pitcher found in fertilizer salesman Peterson's warehouse could not have been used to make the cement boat anchor found in his boat.

One of the few forensic pieces of evidence the prosecution did have was a six-inch dark hair consistent with Laci Peterson's that was found on a pair of pliers in Peterson's boat. Geragos showed Hendee two police photos: one of a camouflage jacket in a duffle bag taken at Peterson's warehouse and the other showing it inside the boat.

Under Geragos' questioning, Hendee testified that the hair and pliers were collected as evidence after a crime scene technician took the second photo (of the jacket in the boat). Geragos was able to argue that it was possible that hair might have been transferred from Laci Peterson's head to her husband's coat to the pliers in the boat without her ever having been inside the boat.

Circumstantial Evidence Wins Over Direct Evidence

As with all circumstantial evidence cases, as the Scott Peterson trial progressed, Geragos continued to offer alternative explanations for each facet of the prosecution's case in hopes of creating reasonable doubt in at least one juror's mind. His efforts did not succeed. On November 12, 2004, a jury found Scott Peterson guilty of first-degree murder in the death of his wife, Laci, and of second-degree murder in the death of their unborn child, Conner.

Three members of the jury spoke to reporters about what led them to convict Peterson. "It was hard to narrow it down to one specific issue, there were so many," said jury foreman Steve Cardosi. "Collaboratively, when you add it all up, it doesn't appear to be any other possibility."

The jurors pointed to these deciding factors:

The bodies of Laci and their unborn child washed up close to where Peterson said he went fishing on the day she was reported missing.

Peterson was a proven liar.

Peterson showed no remorse for the loss of Laci and their unborn child, including continuing his romantic liaison with his girlfriend Amber Frey in the days following Laci's disappearance.

While Mark Geragos did manage to offer alternative explanations for much of the circumstantial evidence that prosecutions presented during the trial, there was little he could do to negate the effect Peterson's lack of emotions had on the jury. Peterson was sentenced to death by lethal injection in 2005. He is currently on death row in San Quentin State Prison.