Brain–computer interfaces, once used exclusively for clinical research, are now under development at several wealthy startups and a major tech company, and rudimentary versions are already popping up in online stores.

Why it matters: If users unlock the information inside their heads and give companies and governments access, they’re inviting privacy risks far greater than today’s worries over social media data, experts say — and raising the specter of discrimination based on what goes on inside a person’s head.