Social Security numbers were never intended to serve as general-purpose national ID numbers. If they were, they'd probably be a lot different from what we have now: nine digits, two hyphens and an invitation to identity thieves. Giving away the last four digits of your Social Security number may not seem like a risk -- there's still five left, right? But if you're worried about someone misusing your number, perhaps you'd best keep all nine digits to yourself.
History
Social Security's numbering scheme dates to 1936. The numbers were designed only for internal use at the Social Security Administration to track people's earnings and benefits. Security wasn't much of a priority, so numbers were assigned according to a simple formula. In the decades that followed, the numbers took on more significance. Because the U.S. lacks a national ID system, Social Security numbers, which just about everybody had to have, became a convenient substitute. Today, your number is the key to a vast trove of information about you and your finances.
Scheme
Social Security numbers take the format XXX-XX-XXXX. The first three digits are the "area number." These are assigned by geographic region. At one point, the area number you got depended on the state where you lived. Since 1972, it's been assigned based on the ZIP code of the mailing address on your application. The second set of digits -- the two digits between the hyphens -- is a "group number." Within each area number, Social Security gives people numbers in "batches," and the group number identifies which batch your number comes out of. The last four digits are the serial number. It's assigned sequentially within each group number -- 0001 is first, then 0002 and so on up to 9999.
Database
Criminals have long used the identities of dead people to commit fraud. To help combat this, the Social Security Administration makes the numbers of dead people available in a public database, along with information about when and where they were born and died. Credit card companies, banks, employers and others can check the database to see whether a number submitted by someone actually belongs to a "ghost." However, the database itself may help crooks steal the identities of the living. In 2009, researchers reported they had used database information to create a computer algorithm to predict the first five digits of a person's Social Security number.
Alessandro Acquisti and Ralph Gross of Carnegie Mellon University reported they were able to identify the first five digits 44 percent of the time for people born after 1989. All they needed to know was when and where they were born. The database helped them pinpoint the area number and group number people were most likely to have been assigned. The year 1989 is important because that's when it became common for infants to receive Social Security numbers at birth. Further research may lead to accurate predictions for people born before 1989.
Considerations
If an identity-stealing cybercriminal with a nasty algorithm can crack the first five digits of your Social Security number, those last four digits might be all that's standing between you and a heap of hassle. You don't need to make the thief's job easier by serving up those precious digits. The Social Security Administration recommends providing your number only when you know exactly who's asking, why they need it and what they plan to do with it.
