Scent Identification in Criminal Investigations and Prosecutions: New Protocol Designs Improve Forensic Reliability

83 Pages Posted: 24 Aug 2010 Last revised: 19 Oct 2010

See all articles by John J. Ensminger

John J. Ensminger

John J. Ensminger, Esq.

Tadeusz Jezierski

Polish Academy of Sciences (PAS)

Michael McCulloch

Pine Street Foundation

Date Written: August 24, 2010


Scent lineups are a powerful tool in the investigation of crimes. With proper procedures, both forensic and judicial, scent lineups can be valuable evidence for a jury to consider. Unfortunately, many courts have been willing to admit poorly conducted procedures, even if giving lip service to the fact that the scent lineup was deficient by saying that its admission was harmless error. The tendency of some courts to view scent lineups as an extension of scent tracking has resulted in admission of scent lineup evidence under inappropriate standards. Tracking cases have set foundational requirements from long-held social and judicial assumptions about the accuracy of dogs. Although no specific set of training procedures or testing protocols need be imposed for the conduct of scent lineups, a protocol with elements that have produced a diagnostic ratio greater than 10 should be required for admission of scent lineup evidence in a criminal prosecution. Dogs should be trained in a series of stages and should not be approved for working trials until they have high proficiency ratios in test trials. Samples should be prepared under rigorous standards that eliminate contamination to the extent possible. Cloth or metal tubes may be used, but the choice of such items will affect the frequency with which objects in the lineups are changed. Handlers and experimenters visible to the dog should be blind as to the location of target scents. Control trials should determine if a dog is willing to work on a particular day or is overly attracted to a suspect’s scent, and failure in control trials should preclude the dog’s participation in a final trial. Some control trials should probably be zero trials in which no choice is correct. At least two, but preferably three dogs should alert to a suspect’s scent in a final trial for the evidence to be advanced as trial evidence (and the number of dogs not alerting correctly should be at least two or three less than the number of dogs alerting correctly). A failure to reach a trial evidence level does not preclude a record of an alert being kept as part of the investigation of a crime. Rigorous research should continue both as to the science and behavior of dogs in identification settings and in designing optimal procedures for such settings. Better coordination between FBI resources and state and local police departments could lead to more solid scent lineup evidence, which can be particularly useful in cases where witnesses tend to disappear or refuse to cooperate with visual lineups. Nevertheless, because the possibility of a false identification cannot be completely eliminated, corroboration by other evidence should be required, probably at a clear and convincing level.

Suggested Citation

Ensminger, John J. and Jezierski, Tadeusz and McCulloch, Michael, Scent Identification in Criminal Investigations and Prosecutions: New Protocol Designs Improve Forensic Reliability (August 24, 2010). Available at SSRN: or

John J. Ensminger (Contact Author)

John J. Ensminger, Esq. ( email )

21 Oakwood Ln.
Rhinebeck, NY 12572
United States
917-613-4960 (Phone)


Tadeusz Jezierski

Polish Academy of Sciences (PAS) ( email )

3 Maja str. 2 m 164
Warsaw, 00-391

Michael McCulloch

Pine Street Foundation ( email )

124 Pine Street
San Anselmo, CA 94960-2674
United States

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