In countless cop shows and detective movies, juries routinely convict suspects based on fingerprints left at the crime scene.
The verdict always seems so simple - a doorframe, a window and the murder weapon all bear the invisible mark of the suspect's distinctive fingerprints.
And just as in real life, these simple prints often are considered enough hard evidence to put someone behind bars.
"Fingerprinting is a routine forensic tool," said Law Prof. Jennifer Mnookin, a forensic evidence specialist.
In a paper Mnookin wrote for a national symposium on DNA profiling, she explains how courts historically have considered fingerprints unquestionable markers of guilt or innocence.
Unlike other forensic experts, such as handwriting analysts who merely give opinions or probabilities of a positive match between evidence and a suspect, fingerprint experts have used an "all or nothing" standard to evaluate their evidence, Mnookin said.
Many lawyers believe that fingerprinting experts stubbornly insist on the accuracy of their methods without giving any clear scientific evidence.
This new attitude toward fingerprinting has come about partly because of recent advances in DNA profiling.
"If people follow the protocols [of DNA profiling] that have been developed over the years, then the probability of two things matching up is quite small," said Ralph Allen, chemistry and environmental science professor.
Just how small is that probability? It depends on several factors, but the probability always is presented as a concrete number, Allen said.
For example, the probability that the infamous stain on Monica Lewinsky's dress originated from someone other than former President Bill Clinton is about 1 in 5 billion, Allen added.
Fingerprint experts usually cannot claim such precision in their results, but DNA profiling has come under controversy as well.
Critics cite weaknesses in the technique, such as how easily samples are contaminated both at the crime scene and at the lab.
In order to overcome this problem, crime lab scientists submit their own DNA for testing to make sure crime scene samples have not been contaminated, Allen said.
These challenges to DNA profiling have forced DNA analysts to perfect their methods, causing fingerprint evidence to come under fierce criticism.
DNA evidence now is easier, more reliable and shows matches more clearly than fingerprint evidence, Allen said.
Additionally, most fingerprints are smeared and crime scene investigators generally recover only partial prints.
The beauty of DNA analysis is that a small sample, such as a drop of blood or saliva left on a beer can, can be amplified in a process that some call "molecular Xeroxing," Allen said.
In this process, crime lab technicians replicate a small amount of DNA many times until there is a sufficient quantity for testing.
Once the DNA has been amplified, technicians determine the length of certain sections of "junk" DNA.
These sections have no known function in the human body, but are different lengths in almost every person. The DNA sections essentially form a molecular fingerprint that is unique for every person.
Once the sections are measured, it is just a matching game between numbers - there is no ambiguity or subjectivity in this process, Allen said.
Historically, juries have believed that fingerprint evidence undeniably proves the guilt or innocence of suspected criminals.
For nearly a century this attitude toward fingerprint evidence has remained unchallenged, but in a recent case, a Pennsylvania court ruled that fingerprinting evidence is not reliable.
The court based its ruling on a 1993 Supreme Court decision that said expert evidence should be rejected if it fails to meet standards of scientific proof.
So why the big controversy over fingerprinting? There simply is not enough of a well-developed model for fingerprinting evidence, Mnookin said.
The Pennsylvania "decision was a step in the right direction," she said.
The ruling allows for experts to testify on similarities between fingerprints, but not to assert they are identical.
The controversy over fingerprinting centers around the basic question of how reliable evidence should be before it's allowed in court, Mnookin said.
For years courts accepted expert evidence on the basis of a 1923 precedent, which states forensic techniques must be "generally accepted scientific practices" before being allowed in court.
For instance, an astrologer would not be accepted as an expert witness under this precedent because astrology is not generally accepted as science.
Mnookin wrote in her paper that the very sureness with which fingerprinting experts affirm or deny matches is what criminal courts no longer can stomach.
According to Mnookin, the experts have failed to answer an important question.
"If we get a partially smudged print at the scene of a crime, how likely is it that someone else matches that print," Mnookin said.
DNA profiling has established new standards in forensics that are challenging the long established technique of fingerprinting.
During the next several years criminal courts may discard types of forensic evidence as simply too unscientific, while other evidence undoubtedly will become more reliable as experts work to improve their methods.
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