Erin Murphy, Relative Doubt: Partial Match or “Familial” Searches of DNA Databases

Erin Murphy, Relative Doubt:  Partial Match or “Familial” Searches of DNA Databases

Comment by: Peter Winn

PLSC 2009

Published version available here:

Workshop draft abstract:

This paper sets forth an architecture for considering the relevant legal standards for familial searches.  Familial searches are searches of a DNA database, using a crime-scene sample profile, that intend to look not for a complete match but rather for partial matches.  Using principles of heritability, such partial matches may allow investigators to identify relatives of the perpetrator in cases in which the perpetrator herself is not in the database.  California recently adopted governing rules for conducting familial searches, and many states and the federal government are contemplating following suit. This article is a collaboration with Dr. Yun Song (Statistics and Computer Science) and Dr. Montgomery Slatkin (Integrative Biology), both of UC Berkeley, who have calculated a formula for determining the likely results (in terms of number of hits) for various partial match searches.  Currently, there is very little legal literature about familial searching (as it is a relatively new idea), and there is virtually no statistical work contemplating the number of profiles likely returned by various levels of searches.  Moreover, in the rush to embrace “familial searching,” legal actors overlook the probabilistic sensitivity of various approaches.  Dr. Song’s formulas provide a springboard from which to examine important legal questions, such as how close a match ought to be to justify:  brief detention (reasonable articulable suspicion); a search warrant or an arrest warrant (probable cause), or perhaps even a subpoena for an evidentiary sample (relevance).