Most human biologists are aware of controversies regarding the use of
DNA profiles in the courtroom. Much attention has been given to estima
ting the probability of obtaining matches between DNA samples from an
innocent suspect and those from a crime scene, but considerably less a
ttention has been given to the critical issue of determining the proba
bility of guilt given a match. Using Bayes' rule and simple algebra, w
e develop a measure of the strength of DNA evidence that indicates the
amount of incriminating evidence needed in combination with DNA match
evidence to meet a given conviction standard, Based on current standa
rds and practices, we use this measure to demonstrate that (1) the amo
unt of non-DNA evidence needed to convict, given a DNA match, generall
y is quite small, even if errors can occur in the processing of DNA ev
idence; (2) DNA match evidence alone is insufficient to convict, even
for the lowest recognized conviction standards; (3) failure to match D
NA evidence samples should be exculpatory unless laboratory proficienc
y is poor; and (4) if errors in handling evidentiary samples occur (ev
en rarely) that tend to produce a false DNA match, then the legal sign
ificance of DNA evidence is remarkably insensitive to estimates of cha
nce match probability.