Early in the term, the Supreme Court has already heard arguments in two cases related to issues of great importance in criminal law: the use of eyewitness testimony and the use GPS without a warrant. Evaluating these issues individually, the answers, from a policy perspective at least, seem pretty simple. If eyewitness testimony isn't reliable we shouldn't use it; and our privacy interests should trump such 1984-style investigations by the police*. But once we start evaluating these positions as companions, it reveals in an interesting tension.
The more we learn about eyewitness testimony, the less reliable we find it. Indeed, Prof. Garrett's study of the first 250 wrongfully-convicted people exonerated with DNA evidence found that 190 of 250 cases involved faulted eyewitness testimony. Not surprisingly, pressure against using it has increased--the New Jersey Supreme Court, for instance, just mandated a number of procedural safeguards it will now require when its used in criminal trials.
If we start discounting the value of eyewitness testimony, though, we have no reason not apply similar scrutiny to other forms of evidence. And once we do that, we'll probably find out much of it is also unreliable. The could easily dismantle the entire facade of the litigation process.
Without the use of traditional forms of evidence, police will push for the ability to use technology that lets them access quite reliable forms of it. They can, for instance, monitor e-mails or use a GPS to track our every move. But clearly this raises privacy concerns.
Taken to its conclusion, then, this puts the legal system in a rather perilous position. If we want privacy, we lose reliability. If we want reliability, we lose privacy. And if we want reliability and privacy, we lose a functioning legal system.**
* I suppose one could have a reasonable debate about this point, but I'd imagine most liberals would err on the side of protecting privacy. Even if the justices don't necessarily agree the Constitution protects against these police actions, they all seemed troubled by the idea of extended periods of round-the-clock surveillance by GPS.
** Again, this sort of depends on your view underlying views of a legal system. This would essentially prevent the government from every depriving citizens of rights at the cost of losing its ability to protect the rights of citizens from being deprived by other citizens.