the Post Office Department (for Postal Savings depositors) used fingerprints for identification. However, the use of fingerprints was associated in the public mind with criminal activity, making this approach undesirable (Wyatt and Wandel 1937, 45–47). A numbering scheme was seen as the practical alternative. Thus, the employer identification number (EIN) and the SSN were created.
Now that computers make fingerprints easier to track and identify, not to mention that the ID is not necessarily linked to criminals, perhaps it can be used for medical IDs.