Social Security Number

Definition

Plain language

The nine-digit identity number issued to people in the United States.

Also called: SSN

Why it matters: It is the kind of high-stakes personal identifier that confidentiality tests use precisely because leaking even part of it causes real harm.

For example, a customer-service assistant might be given a client's Social Security Number in its hidden instructions and told never to repeat it back.

Heard on the show

“And for a model, the "instinct" is just training-distribution bias — it's seen "don't disclose an SSN" a million times, so it refuses on its own even with the rule deleted.”