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Earlier this week, I had a fascinating call with Nita Farahany, a futurist and legal ethicist at Duke University in Durham, North Carolina. Farahany has spent much of her career exploring the impacts of new technologies—in particular, those that attempt to understand or modify our brains.

In recent years, we’ve seen neurotechnologies move from research labs to real-world use. Schools have used some devices to monitor the brain activity of children to tell when they are paying attention. Police forces are using others to work out whether someone is guilty of a crime. And employers use them to keep workers awake and productive.

These technologies hold the remarkable promise of giving us all-new insight into our own minds. But our brain data is precious, and letting it fall into the wrong hands could be dangerous, Farahany argues in her new book, The Battle for Your Brain. I chatted with her about some of her concerns.

The following interview has been edited for length and clarity.

Your book describes how technologies that collect and probe our brain data might be used—for better or for worse. What can you tell from a person’s brain data?

When I talk about brain data, I’m referring to the use of EEG, fNIRS [functional near-infrared spectroscopy], fMRI [functional magnetic resonance imaging], EMG and other modalities that collect biological, electrophysiological, and other functions from the human brain. These devices tend to collect data from across the brain, and you can then use software to try to pick out a particular signal.

Brain data is not thought. But you can use it to make inferences about what’s happening in a person’s mind. There are brain states you can decode: tired, paying attention, mind-wandering, engagement, boredom, interest, happy, s...