In an account jointly owned by father and son, there was one security and one money market account. The father was the primary owner of the account. The father died in 2012 and son became the surviving owner of the account. Son moved the security to another account and subsequently sold it in 2012. Is the cost basis of that security the father's death date? Or because the son jointly owned the account before his father's passing, is the cost basis linked to the actual date that the father purchased the security--a date that not known without research by the surviving son.

“ Is the cost basis of that security the father's death date? Or because the son jointly owned the account before his father's passing, is the cost basis linked to the actual date that the father purchased the security--a date that not known without research by the surviving son”=========Correct; where it involves an investment account with joint tenants ownership, the surviving joint owner receive a stepped up cost basis for the deceased joint owner's 50% ownership of the account; the basis for your share is the stepped up. Then when the stock is sold you have your original basis for 50% and the new stepped up basis for 50%.