When I write and speak about privacy, I am regularly confronted with the mutual disclosure argument. Explained in books like David Brin's The Transparent Society, the argument goes something like this: In a world of ubiquitous surveillance, you'll know all about me, but I will also know all about you. The government will be watching us, but we'll also be watching the government. This is different than before, but it's not automatically worse. And because I know your secrets, you can't use my secrets as a weapon against me.

This might not be everybody's idea of utopia – and it certainly doesn't address the inherent value of privacy – but this theory has a glossy appeal, and could easily be mistaken for a way out of the problem of technology's continuing erosion of privacy. Except it doesn't work, because it ignores the crucial dissimilarity of power.

You cannot evaluate the value of privacy and disclosure unless you account for the relative power levels of the discloser and the disclosee.

If I disclose information to you, your power with respect to me increases. One way to address this power imbalance is for you to similarly disclose information to me. We both have less privacy, but the balance of power is maintained. But this mechanism fails utterly if you and I have different power levels to begin with.

An example will make this clearer. You're stopped by a police officer, who demands to see identification. Divulging your identity will give the officer enormous power over you: He or she can search police databases using the information on your ID; he or she can create a police record attached to your name; he or she can put you on this or that secret terrorist watch list. Asking to see the officer's ID in return gives you no comparable power over him or her. The power imbalance is too great, and mutual disclosure does not make it OK.

You can think of your existing power as the exponent in an equation that determines the value, to you, of more information. The more power you have, the more additional power you derive from the new data.

Another example: When your doctor says "take off your clothes," it makes no sense for you to say, "You first, doc." The two of you are not engaging in an interaction of equals.

This is the principle that should guide decision-makers when they consider installing surveillance cameras or launching data-mining programs. It's not enough to open the efforts to public scrutiny. All aspects of government work best when the relative power between the governors and the governed remains as small as possible – when liberty is high and control is low. Forced openness in government reduces the relative power differential between the two, and is generally good. Forced openness in laypeople increases the relative power, and is generally bad.

Seventeen-year-old Erik Crespo was arrested in 2005 in connection with a shooting in a New York City elevator. There's no question that he committed the shooting; it was captured on surveillance-camera videotape. But he claimed that while being interrogated, Detective Christopher Perino tried to talk him out of getting a lawyer, and told him that he had to sign a confession before he could see a judge.

Perino denied, under oath, that he ever questioned Crespo. But Crespo had received an MP3 player as a Christmas gift, and surreptitiously recorded the questioning. The defense brought a transcript and CD into evidence. Shortly thereafter, the prosecution offered Crespo a better deal than originally proffered (seven years rather than 15). Crespo took the deal, and Perino was separately indicted on charges of perjury.

Without that recording, it was the detective's word against Crespo's. And who would believe a murder suspect over a New York City detective? That power imbalance was reduced only because Crespo was smart enough to press the "record" button on his MP3 player. Why aren't all interrogations recorded? Why don't defendants have the right to those recordings, just as they have the right to an attorney? Police routinely record traffic stops from their squad cars for their own protection; that video record shouldn't stop once the suspect is no longer a threat.

Cameras make sense when trained on police, and in offices where lawmakers meet with lobbyists, and wherever government officials wield power over the people. Open-government laws, giving the public access to government records and meetings of governmental bodies, also make sense. These all foster liberty.

Ubiquitous surveillance programs that affect everyone without probable cause or warrant, like the National Security Agency's warrantless eavesdropping programs or various proposals to monitor everything on the internet, foster control. And no one is safer in a political system of control.

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Bruce Schneier is CTO of BT Counterpane and author of Beyond Fear: Thinking Sensibly About Security in an Uncertain World. You can read more of his writings on his website.

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