From
 http://www.theregister.co.uk/2005/10/19/daugman_multi_biometrics/

"A regular correspondent (thanks, you know who you are) points us to some calculations by John Daugman, originator of the Daugman algorithms for iris recognition. These ought to provide disturbing reading for Home Office Ministers who casually claim that by using multiple biometrics you'll get a better result than by using just the one. Although that may seem logical, it turns out that it it isn't, necessarily."

And
""The key to resolving the apparent paradox," writes Daugman, "is that when two tests are combined, one of the resulting error rates (False Accept or False Reject rate) becomes better than that of the stronger of the two tests, while the other error rate becomes worse even than that of the weaker of the tests. If the two biometric tests differ significantly in their power, and each operates at its own cross-over point, then combining them gives significantly worse performance than relying solely on the stronger biometric.

This is of particular relevance to the Home Office's current case for use of multiple biometrics, because its argument is based on the use of three types of biometric, fingerprint, facial and iris, which are substantially different in power."

read the article for more skinny on it
here
 http://www.theregister.co.uk/2005/10/19/daugman_multi_biometrics/
and
 http://www.theregister.co.uk/2005/10/17/mcnulty_fingers_id_problem/