SSNs are fairly predictable- if you know region of birth and DOB you can get awfully close, for a wide range of the population.

https://www.pnas.org/doi/10.1073/pnas.0904891106

Konerding's 12th law, amended: "There is no bit of pseudonymized data which cannot be de-anonymized by a sufficiently motivated MIT grad student" (not entirely joking; see https://archive.nytimes.com/bits.blogs.nytimes.com/2015/01/2...)