I live in a very safe neighborhood surrounded by wonderful neighbors, but I always lock my house and car because I know it’s the smart thing to do. A locked door is not much of a deterrent to a truly determined thief, but an unlocked one is just an invitation.

In the same way, I keep my personal identifying information private. My home wireless network is password protected, and I never use an unsecured network to transmit any information about myself. I use a Post Office box as my business address, I don’t advertise my birthday on Facebook, and I scrutinize every request for my Social Security Number.

I take these measures because I live in fear that someone will access my personal information in order to obtain credit fraudulently or drain my financial accounts. Clearing these charges and restoring any accounts that have been tapped is laborious and leads to difficulties and endless frustration whenever the victim seeks to open or close an account, tries to board an airplane, or attempts to change jobs—identity theft can destroy a person’s good name, credit report and financial life.

When I opened my first bank account, I obtained the number that has identified me since, my Social Security Number. Those nine digits were prominently listed on my first driver’s license, and I never hesitated to place that same number on my checks when I shopped at the mall in the days before the Internet.

Those days are history. Today, I am astounded by the volume of requests for this particular number despite the identity theft epidemic that plagues this country. While federal law bans the use of Social Security Numbers on drivers’ licenses, state ID cards, or motor vehicle registration forms, many people still have older state documents listing this particularly sensitive piece of information. As a holdover from prior times, many insurance companies, including Medicare and Medicaid, often use the Social Security Number as a part or whole of their identifying number, as does the Veterans’ Administration and some colleges and universities, although this culture is changing.

Originally, your Social Security Number was not intended to be an identifier; however, it has come to be just that. Accordingly, there are times when you must use it (and oftentimes your date of birth) to identify yourself; fortunately, as more organizations become cognizant of the threat of identity theft, other methods of identification are replacing it. Still, you must provide it whenever you participate in any financial transaction that will be reported to the IRS, or whenever you wish to give someone access to your credit reports, such as a bank, credit card company, or potential landlord or employer.

But not everyone who asks is entitled to receive this information. Most medical offices, for example, still request this information on their new patient data sheets, and I recently heard of a veterinary clinic that insisted on a Social Security Number before treatment was provided. Be aware that, while current laws allow the doctor, the vet, the attorney, or the merchant to request my Social Security Number, they do not mandate that I furnish it. And any provider who insists that this information is necessary to complete my transaction has just lost my business.

It’s not always easy to tell when you should divulge this information, and when you should not, but here are a few general rules to follow:

1) If there is a governmental requirement, the person asking for the information will be able to provide you with the precise language in the federal law that permits this. Be aware that all governmental agencies can ask for the information, but you are not required to give it to most of them;

2) Never reply to unsolicited e-mails, phone calls, or even regular mail offers asking for personal information. A form that miraculously appears in your in-box without you having first initiated the contact may be legitimate, but chances are good that it is not;

3) Never give out your information to an IRS or state tax agent who calls you on the phone or arrives at your home unannounced. No tax agency works like that. The “agent” standing on your doorstep, threatening you with tax liens, wage garnishment, or worse if you do not provide the information, is a fraud; and

4) Never provide any level of personal information if you cannot immediately understand why it is necessary to do so. Do not allow yourself to be bullied into giving information about yourself that it is not reasonable for that particular person or company to know. If they insist, take your discussion to the manager; if the manager insists, take your business elsewhere.

Unfortunately, preventing identity theft is not as easy as locking your doors; you have no control over your personal information that is already out in the world, lurking in unseen and unregulated spaces, just waiting for the person who has the time, the will, and the technology to access it. But by preventing further unnecessary dissemination of this information, you may succeed in limiting that thief’s opportunity.