The U.S. forensic DNA database has expanded rapidly in recent years. While it was originally authorized to store the DNA profiles of only convicted violent felons, the FBI Combined DNA Index System (CODIS) now includes all federal offenders—including arrestees not yet convicted of any crime—as well as convicts from all 50 states and arrestees from many. Such expansions of the database are troubling, but at least they are explicit. More worrisome is the effective inclusion of many innocent individuals in the database, via novel and almost completely unregulated search techniques called "partial matching" and "familial searching." By adopting one or both of these search techniques, some states are quietly expanding database coverage to "virtually" include the innocent relatives of profiled offenders—nearly always without any legislative oversight.
Familial searching and partial matching both exploit the same well-known underlying principle: Close relatives are genetically similar. Parents, children, and siblings share, on average, at least half of their DNA. Not surprisingly, similar DNA generates similar DNA profiles, which are the stripped-down numerical records stored in DNA databases. So, even if a crime-scene sample doesn't exactly match any existing offender profile in CODIS, police may still find a partial match—an incomplete DNA match between the forensic evidence and a known offender. If this happens, police know the offender who partially matches the evidence did not himself leave the sample at the crime scene, but—and this is where it gets interesting—it's very possible one of his relatives did. After screening those relatives with follow-up DNA testing, police may have a new lead.
This expanded process fundamentally alters the population that can be searched with DNA databases. Previously, police only matched crime-scene samples against known, profiled offenders—those who had been actually included in CODIS because of a prior arrest or conviction. (So a crime-scene sample left by a first-time offender would turn up no matches in the database.) But with these new techniques, relatives are effectively included in the database through their genetic similarity to a profiled offender. Think of it this way: If you've never been arrested, your DNA profile shouldn't be in CODIS. But if your brother has been arrested and is profiled in CODIS, then whenever these new searches are used, you, too, may be searchable—and targeted for investigation—through the database. These searches render offenders' relatives effectively searchable in CODIS, even though the relatives themselves have never been officially included.
Technically, DNA-database matches that implicate offenders' family members can surface in one of two ways: through either "partial matching" or "familial searching." This distinction is somewhat misleading, since these search techniques differ chiefly in intent. If police deliberately set out to search the database for relatives—usually using specialized software—the technique is called "familial search." If the same partial hits instead arise inadvertently during a normal database search, they are referred to as "partial matches." Of course, the underlying value of both techniques is the same—a partial match between two DNA profiles may suggest a blood relationship.
These leads sometimes bear fruit: In the United Kingdom, for instance, several high-profile cases have been solved this way. But however elegant these techniques may seem, we should think long and hard before allowing the effective expansion of our DNA databases to include the relatives of profiled offenders. Using partial matches—whether deliberately sought or not—is problematic for several reasons.
First, these matches may not be legal. Courts have held that convicts and arrestees have a lower expectation of privacy than ordinary citizens and that the state has a weighty interest in identifying criminals and preventing recidivism. On balance, courts have determined, these factors permit offenders' DNA profiles to be stored in CODIS. But none of these reasons justifies collecting information about offenders' kin. Directly including the DNA of innocent family members in CODIS would be unlawful under existing statutes and likely prohibited under current Fourth Amendment jurisprudence. And if actually including them in the database is prohibited, shouldn't their effective inclusion be so, too?
The police may argue that familial searching and partial matching present no Fourth Amendment problems, since nothing has actually been seized from the relatives and they have not been personally searched. In a sense, they are correct—the family members aren't actually in CODIS—but they are nonetheless "reachable" through a profiled relative. Their inclusion is virtual, a product of biological happenstance. But this end-run shouldn't satisfy lawmakers: If offenders' relatives are to be treated like offenders, there must be a good reason to single them out. There is none.
Second, partial matching and familial searching will greatly aggravate the racial inequality already embedded in offender-based DNA databases. Certain racial and ethnic populations are already overrepresented in CODIS, owing simply to the reality of crime statistics. But implicitly expanding database coverage to include relatives will grossly and unfairly amplify this bias. By applying partial matching and familial searching to a database that includes anyone ever arrested—which seems to be where the system is headed—CODIS could one day approach universal coverage for some races and not for others.