“One might argue that the consequences of a false negative (a terrorist plan is not detected and many people die) are in some sense much larger than the consequences of a false positive (an innocent person loses privacy or is detained),” the report says. “For this reason, many decision makers assert that it is better to be safe than sorry. But this argument is fallacious. There is no reason to expect that false negatives and false positives trade off against one another in a one-for-one manner.”

The report recommends that the government be particularly careful when using behavioral surveillance to predict dangerous intent. There’s no scientific consensus about whether such technology — brain scanning, for example — actually works, says the report.

“[P]lacing people under suspicion because of their associations and intellectual explorations is a step toward abhorrent government behavior, such as guilt by association and thought crime,” the report says. “This does not mean that government authorities should be categorically proscribed from examining indicators of intent under all circumstances — only that special precautions should be taken when such examination is deemed necessary.”

The report presents two major recommendations. It argues that the U.S. government should follow a framework, such as the one proposed in the report, to evaluate the effectiveness, lawfulness, and consistency with U.S. values of every information-based program for counterterrorism. And it calls for a periodic review of laws and policies related to privacy in light of changing technologies and circumstances.