The question isn't whether or not you can give away the last four digits of your social security number, it's whether you should. On one hand, they're easy to remember, and only represent a minority of your overall Social Security number. However, they're also the hardest part of the number to guess and, as such, giving them out too readily could leave you open to identity thieves.
History of an SSN
The Social Security number dates back to the 1930's, when it was developed as a tool to track Social Security benefits. It predated computers and advanced record-keeping systems, so it was designed to be easy to assign. Since then, it's morphed into a general purpose identification number for many applications. Unfortunately, it was never designed for this purpose and, as such, isn't particularly secure.
Elements of an SSN
Your social security number has three parts. The first three digits indicate the office that issued your Social Security number. For most people, it's located to where they were born, but if they got their number after birth, it's where they lived at the time. The next two digits are a secondary group number that are less predictable, and the final four digits are assigned in order. 123-45-6789 comes after 123-45-6788 and before 123-45-6790. Once an office hits the 9999 number, it rolls over, so 123-46-0000 would come after 123-45-9999.
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Guessing an SSN
There are one billion potential Social Security numbers – from 000-00-0000 to 999-99-9999. Once an identity thief knows the last four digits of your number, the universe of potential numbers narrows to 100,000 – from 000-00-xxxx to 999-99-xxxx. If he can figure out where you were born and approximately how old you are, he can guess the first three digits, narrowing the number of potential numbers to 100 – from xx-00-xxxx to xx-99-xxxx. The middle two digits don't protect you very much, either, since the Social Security Administration publishes which ones were given out when, making it possible for a smart identity thief to get your Social Security number with just a few guesses.
Sharing Your Last Four
Using the last four digits of your Social Security number is better than using the entire number and having it printed on identification cards or on statements, but it's still inherently unsafe. Many businesses ask for the number of legitimate reasons. You usually can use a different number as an account number or personal identification number, though. All that you have to do is ask. Given that there are other four digit numbers that you should be able to remember, using your Social Security number's last four digits as rarely as possible is the best choice.
Starting on June 25, 2011, the Social Security Administration started randomizing the numbers that it assigns. This makes it much harder to guess a Social Security number from the last four digits since there theoretically isn't any pattern to the first five digits. However, it's still unwise to give out your last four digits if you can avoid it.