The December 2015 Uniform Assessment Instrument of Virginia's Department for Aging and Rehabilitative Services (an instruction manual for administration of certain public social services) contains the following astonishing assertion about Social Security Numbers (SSNs) in the USA (emphasis mine):
13.12.1 Name and vital information
Record the full name of the individual....
The SSN is a nine-digit number, which will be used to track information on all individuals who are assessed. It is important that every person have a unique number. Most individuals should have a SSN, but you will find that some female individual’s use their Medicare number as their SSN and/or their husband's SSN as their own....
On occasion, the assessor will need to generate a dummy social security number for an individual. This will happen when a female individual only has her husband's number and he is still alive, or when there is no number to be found. In the cases where the wife is using the husband's SSN, generate a dummy number for the wife in the following way....
I can believe that there was a time in US history when men were more likely to have Social Security numbers than women because they were the ones more likely to be going out and doing things that required an SSN, but SSN's were created in 1936 and it has long been nearly impossible to function in the US without a number and it is difficult to believe that a practice of women just using their husbands' numbers was common as late as 2015. It's in fact difficult to believe this was ever common - presumably, if a woman in the late 1930's needed to do something that required an SSN (e.g. get a Federal job), she would just go and get one herself rather than just putting her husband's number down.
Reading this document was the first time I've ever heard of this practice. Even after working several years in healthcare administration (where we tracked SSN's for everyone), I have to admit that this was never a thing I encountered or even heard of ever happening. None of our training covered this scenario - it was assumed that anyone using a SSN other than their own was per se committing fraud and deserved to be punished by catapult or at the very minimum reported for investigation.
When my niece was born, my brother applied for an SSN for her that very month. The idea that they could simply wait 18 years to see if she could just get by with a future husband's number wasn't on the table at all.
Is it common today in the USA for women to use their husband's SSN as their own, or was this ever a common practice?
One possibility that I thought of was that this is an accommodation for women who are present in the USA unlawfully (not eligible for an SSN) but lawfully married to someone with an SSN, but that doesn't seem satisfactory because their is no similar accommodation for men who are in the US unlawfully but using their "legal" spouse's SSN. I also note that if this were the case, I would have expected the document to just come out and say it, e.g. "If the patient is not eligible for a SSN for reasons, generate a dummy number in the following manner....".
Another possibility that I considered was that this is related to Social Security survivorship benefits, i.e. where a woman is receiving benefits under her husband's Social Security account (e.g. as a widow). That doesn't make sense either - the systems I've used have always been smart enough to handle that. If a patient was in that kind of scenario, we would put their actual SSN on their biography tab and the funding source's SSN on the insurance tab (e.g. "This is Mary Smith, SSN 123-45-6789, she is receiving benefits under her husband William Smith's SS account, SSN 987-65-4321").
If this was never actually a thing that people actually did, but there is some state or Federal regulation that requires agencies to theoretically permit this (e.g. "Notwithstanding any other provision of law, a woman lawfully married to a Social Security Number holder may lawfully use such number for all purposes as if it were her own and no agency may require her to apply for her own number as a condition of receiving any services whatsoever as may otherwise be authorized under law...."), that's an answer.