>> Why not just use the SSN?
What if the person does not have a SSN? For example, a foreign person.
In general, having the primary key a non-user dependable value aliviates a lot of problems. It is usually not a good idea to depend on a business rule (like, SSN must be unique) to design your database. Business rules change, and you don't want your database integrity to depend on those changing rules.
Hector Correa