In the courtroom, there are really two “prongs” on which to secure a conviction, or argue against one. The first is forensic evidence [direct DNA evidence, fingerprints etc]. The second is circumstantial evidence [indirect evidence, witness testimonies, alibis or lack of]. Sometimes one prong is emphasized over the other. Other times they’re used to complement a particular argument.
Examples of cases where circumstantial evidence was more important include the murder of Laci Peterson and triple axe murderer Henri van Breda. In the Peterson case there was virtually no trace of Laci other than a single hair in a needle-nose pliers in Peterson’s boat, and cadaver odors.
In the Van Breda case there was an Armageddon-like crime scene overflowing with blood. In the latter case there was so much blood and DNA it was slippery to sort out and somewhat confusing. The main thrust of the defense case in the Van Breda trial focused on nitpicking DNA samples that were inconclusive.The Judge in the Van Breda case said if all the DNA evidence was excluded [from the prosecution side and the defense side] the circumstantial evidence remaining was still compelling, and overwhelming.
Examples of cases where DNA evidence was more important [and arguably too important] include the Amanda Knox case, OJ Simpson and Madeleine McCann. If DNA can be used to convict, any uncertainty around its scientific veracity means it can also become the fulcrum around which a defense can secure an acquittal.
Examples of cases where DNA evidence was used to argue a case but shouldn’t have include the JonBenet Ramsey case and arguably the Madeleine McCann case. In both these cases circumstantial evidence ought to have been sufficient.
Where a crime scene is severely contaminated or compromised, reasonable doubt exists surrounding forensic samples that are typically not quite good enough to qualify as sufficient. The law requires forensic samples to be 100% accurate, and the protocols in collecting them to be professional and beyond reproach.
The third dimension to the true crime pitchfork is psychology, or in the parlance of the genre, “motive”. Unfortunately, modern criminal law no longer puts much value in motive, just intention [or Dolus]. As a result, more than a few trials conclude with the court unable to explain why the crime even took place [Chris Watts, Casey Anthony, Amanda Knox, Oscar Pistorius, Henri van Breda etc.].
The courts say knowing why doesn’t matter, just that knowing when, what, where and how is sufficient. It may be sufficient for the law, but it’s not adequate for human beings.