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Can DNA evidence be challenged?

DNA evidence has become a powerful tool in criminal investigations and prosecutions over the past few decades. DNA can conclusively link a suspect to a crime scene or victim. However, like any forensic evidence, DNA is not infallible. Defense attorneys have several strategies to challenge DNA evidence in court.

What is DNA evidence?

DNA (deoxyribonucleic acid) is the genetic material found in all human cells. DNA contains the instructions that make each person unique. Forensic scientists can extract DNA from blood, semen, saliva, skin cells, and other biological evidence found at a crime scene or on a victim. By analyzing and comparing DNA samples, investigators can sometimes connect a suspect to the crime.

There are two main ways DNA is used as evidence:

  • Direct matching – The suspect’s DNA sample directly matches the DNA left at the crime scene
  • Statistical probability – The suspect’s DNA profile matches the crime scene DNA at certain genetic markers. Experts testify about the extremely low odds that a random person’s DNA would match.

How can defense lawyers challenge DNA evidence?

Simply finding a DNA match does not automatically guarantee a conviction. Skilled defense attorneys can often raise reasonable doubts about DNA evidence. Possible challenges include:


Crime scene evidence is easily contaminated. Fingerprints, sweat, saliva, and other DNA sources may compromise the original sample. Evidence could also be tainted during collection, transport, or testing in the lab.

Defense lawyers scrutinize the chain of custody paperwork to identify any broken links. They look for signs of sloppy handling procedures. Lab testing areas may lack quality controls to prevent contamination.

Errors in testing and analysis

Humans conduct DNA testing, so mistakes can happen. Technical issues with equipment may skew results. Software glitches during data interpretation have occurred. In some cases, overworked lab technicians have even fabricated or falsified test outcomes.

Personnel qualifications and lab accreditation are fair game for any credible defense. Opposing experts can be hired to critically review procedures and re-run tests on samples.

Statistically misleading DNA results

When only mixed or partial DNA profiles can be obtained, analysts rely more heavily on statistics to make matches. Statistical weight derives from comparing genetic markers at different loci. The odds of a coincidental match become astronomically small.

Defense experts argue that these statistical models contain flawed assumptions. Jurors may not properly grasp how statistics are calculated and applied to the case at hand. Juries can be swayed by extremely low odds, even when other factors create reasonable doubt.

Illegal evidence collection

Police must strictly follow evidence handling protocols and respect a suspect’s constitutional rights. DNA samples taken without informed consent or a warrant may be inadmissible in court. Any hints of coercion of deception could spoil the credibility of associated DNA evidence.

Actual innocence

Even if the DNA evidence itself appears unassailable, defense lawyers can point to other factors that create reasonable doubt. Eyewitness mistakes and false confessions happen. Evidence planting and misconduct by overzealous police have led to wrongful convictions.

The defendant may have a convincing alibi. Other potential suspects exist. Physical evidence could contradict circumstances of the DNA match. Juries must weigh the full body of evidence, not just the DNA.

Using DNA evidence to exonerate

DNA analysis has helped reverse many wrongful convictions in recent years. The Innocence Project and similar organizations have successfully used DNA testing to expose flaws in older cases pre-dating advanced forensic methods.

DNA evidence has conclusively proved innocence and secured releases for over 375 wrongfully convicted persons in the US to date. Approximately 70% of their successful exonerations relied critically on re-examining DNA evidence.

DNA analysis on archived evidence can also produce new leads by identifying alternate suspects. Post-conviction DNA testing combined with other legal appeals provides a powerful means to challenge dubious older convictions.

Famous DNA exoneration cases

Here are a few high-profile examples where re-testing DNA helped exonerate wrongfully convicted individuals:

Kirk Bloodsworth – First death row inmate exonerated by DNA

Kirk Bloodsworth received a death sentence for rape and murder of a child in Maryland in 1985. After nearly 9 years in prison, improved DNA testing of crime scene evidence excluded him as the perpetrator. He was released in 1993, and the real killer was later identified by DNA. Bloodsworth was officially pardoned in 2004.

The Central Park Five

Five teenage boys were wrongly convicted of a brutal 1989 rape in NYC based on coerced confessions. After serving 6-13 years in prison each, they were exonerated when a convicted serial rapist named Matias Reyes confessed in 2002. His DNA matched evidence from the crime scene.

Timothy Cole – First DNA exoneration posthumously

Timothy Cole was wrongfully convicted of a 1985 rape in Texas. He died in prison in 1999 while appealing his conviction. In 2009, DNA tests implicated another man, and Cole received the state’s first posthumous pardon. Tim’s brother led efforts culminating in Tim Cole Act reforms in Texas.

Steven Avery – Exonerated but questionable case

Steven Avery served 18 years for a 1985 sexual assault in Wisconsin before improved DNA testing exonerated him in 2003. In a bizarre twist, Avery was charged 2 years later with murdering a woman, Teresa Halbach. That highly controversial case got national attention in the 2015 “Making a Murderer” Netflix documentary series.

Forensic DNA analysis continues advancing

DNA evidence carries tremendous weight with juries because it is grounded in science. But like any evidence, its validity depends on collection methods and handling. Skilled defense lawyers can raise enough doubt to overcome otherwise damning DNA matches. Improvements in forensic analysis and anti-contamination controls make DNA evidence progressively more trustworthy.

Thanks to organizations like the Innocence Project, DNA analysis has provided irrefutable proof that wrongful convictions do occur. Re-testing old evidence has also brought many perpetrators to justice after original investigations stalled. Continuing advances in sensitivity of DNA techniques will further strengthen justice in the courtroom.