The demand for greater security at borders, government buildings, and companies has meant boom times for biometrics-technologies that measure biological traits to identify individuals. Systems that digitally fingerprint people, read the patterns of their irises, measure the unique dimensions of their faces, or verify their voices are expected to become a $1 billion business in 2004, quadruple what it was just five years ago.

But there's a problem: no single measurement works for everyone. As many as 3 percent of people lack readable fingerprints, and perhaps 7 percent have eye pigmentation that interferes with iris scans. Face recognition software can be thwarted by veils or thrown off by changes in hairstyle or lighting. And biometrics can be tricked: a fingerprint left on a sensor can potentially be lifted and used by someone else; many face recognition systems can be fooled by photographs or video clips. "No biometric has proven to be the ultimate," says Gary Strong, the manager of behavioral and biometrics programs in the U.S. Department of Homeland Security's science and technology office.