Government enforces it. I don't know how exactly it is in the USA, but in my country there is "land registry", which is maintained and updated by legal courts.

Government enforces it. I don't know how exactly it is in the USA, but in my country there is "land registry", which is maintained and updated by legal courts. Within the registry, every land/house/flat ownership is noted and this registry is also the final instance whenever there are legal battles regarding ownership. Whenever you take a loan for your house, a bank will want to add a note to the registry that it has first claim to your property in case of not paying, premature death, and so on. So one cannot just give his house to his relatives, as the court will block the transfer when it will see in the registry that the bank has primary claim to it. To do something like that, one must make a deal with the bank that new owner will either pay up the loan or take it on his name.