What You'll Learn
The Concept
The purpose of a criminal investigation should not be defined simply as building a case against a suspect. A more accurate definition is: a systematic effort to determine what occurred and identify what conclusion is supported by reliable evidence. That distinction is fundamental.
Once investigators form an early theory, there is a risk that subsequent information will be interpreted primarily through that theory. Strong investigative practice therefore requires investigators to seek corroboration while remaining alert to contradictory and exculpatory evidence.
Modern investigations may incorporate interviews, surveillance video, documents, digital evidence, cellular records, DNA, fingerprints, firearms evidence, financial data, geographic information, crime analysis, and numerous other forms of information. NIJ defines forensic science broadly as the application of scientific disciplines to matters of law.
DNA provides one of the clearest examples of forensic science's power. DNA evidence may associate an individual with biological evidence, exclude an individual as its source, identify unknown remains, connect previously unrelated crimes, and reopen investigations that were once considered unsolvable.
Yet forensic evidence should not be treated as infallible simply because it appears scientific. NIJ research concerning wrongful convictions has examined cases involving misleading forensic evidence alongside mistaken identification, false confession, official misconduct, inadequate defense, and false accusation.
Eyewitness evidence provides another important example. Human memory is not equivalent to a video recording. Research into eyewitness identification has led to reforms involving lineup procedures, witness instructions, documentation of confidence, and administrator practices intended to reduce unnecessary influence.
For criminal justice students, the larger lesson is methodological: evidence should be evaluated according to its reliability, context, limitations, and relationship to other evidence — not according to how persuasive it initially appears. The strongest investigators ask two questions simultaneously: What evidence supports my theory? And what evidence would demonstrate that my theory is wrong? That second question is one of the best protections against investigative error.
Real-World Application
A detective investigating a residential burglary develops an early theory that a known offender in the neighborhood is responsible. Strong investigative practice requires the detective to pursue that theory while simultaneously asking: what evidence would show this person is not responsible? Fingerprints, cell phone records, and witness accounts should be evaluated for what they actually show — not filtered through the initial theory. If the evidence points elsewhere, the theory must change.
Why It Matters
For Police Officers
Investigators who understand confirmation bias and the limitations of forensic evidence are less likely to pursue the wrong person and more likely to build cases that hold up in court. Understanding what evidence can and cannot prove is a core professional competency for any investigator.
For Students
Wrongful convictions represent one of the most serious failures of the criminal justice system. Understanding how they occur — through eyewitness error, misleading forensic evidence, and confirmation bias — is essential for any student who wants to understand the system's strengths and vulnerabilities.
Think About It
Why is 'building a case against a suspect' a problematic definition of criminal investigation? What is a more accurate definition, and why does the distinction matter?
Research has shown that eyewitness identification is less reliable than people assume. What reforms have been developed to improve eyewitness procedures, and why are they important?
How does confirmation bias affect criminal investigations? What practices can investigators adopt to reduce its influence?
Sources & Further Reading
National Institute of Justice. Forensic Science. nij.gov.
National Institute of Justice. Wrongful Convictions. nij.gov.
National Institute of Justice. Eyewitness Identification. nij.gov.
National Commission on Forensic Science. (2016). Views of the Commission. U.S. Department of Justice.