Busting 10 Myths About DNA Evidence in Florida Criminal Trials

September 10, 2025 Don Pumphrey, Jr. Criminal Defense, Sex Crimes, Violent Crimes Social Share

In Florida and throughout the U.S., DNA evidence often serves as a key aspect of a criminal case. But there are clear limits to the value of DNA evidence that are not widely understood. This blog will bust 10 common myths about the use of DNA evidence in Florida criminal trials.

#1 – A “DNA match” means someone is guilty

This is perhaps one of the most pervasive myths – and deserves special attention. The belief that a DNA match automatically means someone is guilty is often called “the prosecutor’s fallacy.” It posits (for example) that if there is a 1 in 10,000 chance of a random DNA match, there is a 99.99% chance the defendant is guilty.

But this is inaccurate. Let’s say there is a city of five million people. A DNA swab is taken from the scene of a crime and a defendant is arrested. Police say that there is a 1 in 10,000 chance of a random DNA match. The jury interprets this as a 1 in 10,000 chance of innocence, leading to a conviction.

However, a 1 in 10,000 chance of a random match means that in a city of 5 million, 500 other people would randomly match the DNA sample if tested. Obviously, 500 people cannot be guilty of the crime. So, the defendant’s odds of guilt would actually be 1 in 500 unless additional evidence is provided that establishes their specific connection to the offense.

#2 – If the defendant’s DNA is found on a victim, they committed the crime

This is another common myth. The presence of a person’s DNA on another person can be strong evidence of guilt – but only under specific circumstances. 

For example, if the defendant in a sexual battery case claims to have never met the victim – but their semen is discovered in the victim with a 1 in 10 billion chance of a random match – this is very powerful evidence of guilt. But often, DNA evidence is not so clear cut.

DNA is very easily transferred (shed) – especially if it is skin (epithelial) cells. Thus, if someone is commonly in a location or has physical contact with an alleged victim (especially if they live with them), their DNA is almost certain to be found on or around the alleged victim.

DNA can also be transferred via common items, such as a shared object or commonly touched surfaces – including in public. A stranger’s DNA may be found on someone they have never even met in some situations. The presence of DNA does not automatically prove guilt.

#3 – DNA tells you when it was left behind

DNA evidence does not show the exact time it was deposited. As a result, an aged DNA sample may be mistakenly viewed as evidence that someone touched an object or was in a location quite recently, when this is not the case.

For example, let’s say the old owner of a home is accused of burglarizing it. There isn’t strong evidence, but law enforcement tests the window sill and finds a DNA match of the ex-owner, leading to their arrest. This has little to no evidentiary value as the DNA could have been there from years ago.

#4 – DNA alone is enough to convict of a crime

In the absence of any other evidence, the presence of DNA alone is insufficient to convict a defendant of a crime if it fails to exclude every reasonable hypothesis of innocence. This is a key principle in Florida case law, as DNA is considered circumstantial evidence. Brown v. State, 672 So.2d 648, 650 (Fla. 4th DCA 1996). 

In certain cases, DNA may be sufficient to convict. An example may include:

• B is charged with sexual battery and claims he has never met C, but his semen is found inside C with a 1 in 1 trillion odds of a random match
• G claims he has never been to Florida or owned a gun, but his DNA is found on a murder weapon in Miami with a 1 in 10 trillion chance of a random match
• T is charged with burglary and claims he has never been inside the victim’s home – but his blood is found on broken glass from a window used to gain entry, with a 1 in 100 billion chance of a random match

In many cases, however, DNA fails to exclude every reasonable hypothesis of innocence. For example, someone’s skin cells being found on someone they live with cannot be the sole evidence of a crime, as there are many ways it could have gotten there.

#5 – It’s hard for DNA to get on a person or a surface

This is a common misconception that ties into Myth #2. Though it is significantly harder for certain types of DNA (e.g. seminal or vaginal fluid) to get on a surface or another person by accident, other forms of DNA (such as skin cells) can very easily shed. 

Because of this, the presence of DNA alone is not generally strong evidence of guilt unless it is of a type relevant to the commission of the offense, and found in a location that excludes every reasonable hypothesis of innocence (e.g. semen inside a victim).

#6 – DNA is always admissible in court

Another common myth is that if law enforcement recovers DNA, it is certain to be admitted in court. But there are various grounds upon which a defense attorney may move to exclude a DNA sample from being introduced in criminal court. These include:

• 90.403 challenges: If the probative (evidentiary) value of DNA evidence is substantially outweighed by the danger of unfair prejudice, confusion of issues, needlessly cumulative presentation of evidence, or misleading the jury – it is to be excluded (Fla. Stat. 90.403).
• Daubert challenges: The methods by which DNA is collected and tested, and expert testimony surrounding DNA, must be sufficiently scientifically reliable. If it is not, this evidence may be excluded pursuant to Fla. Stat. 90.702.
• Chain of custody challenges: A chain of custody documents where a DNA sample is and who has it from the time police collect it to its use in court. If the defense shows evidence of probable tampering or contamination of the DNA sample, the State must disprove this, or the DNA is to be excluded. Lapses in the chain of custody can assist in this.
• Constitutional challenges: If a defendant’s DNA/the sample was obtained in violation of the Fourth Amendment (e.g. threatening the defendant if they didn’t provide DNA, illegally searching a house to get the sample), DNA evidence can be suppressed as “fruit of the poisonous tree.” (Wong Sun v. U.S.)

As you can see, DNA is not always admissible in a court of law, even in cases where the DNA evidence seems reliable.

#7 – DNA testing is 100% accurate

DNA testing is often quite accurate – but not always. If police recover a partial sample or use a testing method that is not generally accepted in the scientific community, this can lead to serious questions about the veracity of the conclusions drawn from the testing of that DNA. 

In some cases, DNA samples are mixed (e.g. more than one person’s DNA recovered in the same sample). Other cases involve partial profiles. Especially in these cases, DNA evidence is far from guaranteed to be 100 percent accurate.

#8 – DNA databases contain everyone’s DNA

This is another incorrect assumption. DNA databases (e.g. CODIS) do not contain the DNA of everyone in Florida, America, or the world. Instead, they contain the profiles of only a fraction of the population (disproportionately those with prior arrests/convictions).

As not everyone’s DNA profile is available in these databases, this can complicate calculating the odds of a “random match.” This is because random match probabilities are assessed based on a fairly small number of DNA samples (relative to the size of the population) that do not entirely capture the genetic diversity of the broader community.

#9 – Police can’t get my DNA without my consent

This is not totally correct. While it is true that you can refuse to consent to providing a DNA sample if asked (and you should do so), police can compel someone to provide a DNA sample if they seek and obtain a search warrant.

Moreover, law enforcement may collect someone’s DNA off of an item if the person no longer had a “reasonable expectation of privacy” as to that item. For example, if someone sips from a coffee cup and throws it in a public trash can, police can perform a DNA swab on that cup. California v. Greenwood, 486 U.S. 35 (1988). 

Critics have argued that abandoned DNA should not be considered “fair game” for police, as it is seen as a way to get around consent without a warrant. But Greenwood and subsequent cases have held that the Fourth Amendment does not apply to personal items if there’s no longer a reasonable expectation of privacy as to those items (e.g. a trash bag on the curb).

#10 – A jury will automatically understand the limits of DNA evidence

This is not true. The “CSI Effect” is a documented phenomenon where the presence of DNA evidence is believed by many jurors (due to shows like “CSI”) to establish guilt in a criminal case. In the minds of some jurors, hearing of a “DNA match” is equivalent to proof beyond a reasonable doubt of the defendant’s guilt.

This makes it crucial to find an experienced and aggressive Florida criminal defense attorney to challenge the introduction of DNA evidence in court, as well as explain to the jury the limits of DNA evidence at trial. Without this, even unreliable DNA evidence may lead to a conviction.

If someone is arrested and formally charged in Florida with an offense and is concerned about DNA evidence, it is critical to find experienced and trusted legal representation as soon as possible. This decision could make the difference in whether or not someone faces a lengthy prison term and hefty fines.

Criminal Defense Attorney in Tallahassee, FL

Don Pumphrey, Jr. is a Former Prosecutor, Former State Police Officer, Lifetime Member of the Florida Association of Criminal Defense Lawyers; for over 25 years as a private defense attorney who is Trusted, Experienced, Aggressive in Criminal Defense as a Trial Attorney, Criminal Lawyer, Criminal Defense Lawyer for the accused in Florida State Courts located in Tallahassee, Florida but handling cases throughout the State of Florida.

Don Pumphrey, Jr. and the Tallahassee criminal defense lawyers at Pumphrey Law have decades of experience fighting drug charges on behalf of clients and winning. Call Pumphrey Law now at (850) 681-7777 to learn more about what we can do for you. Our lawyers will be happy to provide you with a free consultation.

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