Is it legal to send a document with your signature claiming to be somebody else?
Is it legal to allow somebody to sign a document "for you" (using your name)?
I'll answer these two questions together.
It depends on what laws apply to the specific document.
For example:
- For a contract for the sale of land, the Statute of Frauds requires a signature. However, the Statute provides that the signature may be made by a person authorised by you.
- In Australia, a Commonwealth income tax return must be signed by the taxpayer. The taxpayer must sign it personally. If you get your assistant to sign it then you haven't filled it in properly and you are liable to be fined for failure to lodge a valid tax return. There are rules in the legislation for corporations, e.g. the public officer signs it. Sometimes the law allows a tax agent/accountant to sign a form. The theme is that the law provides specific rules.
- In Australia, banknotes must bear the signatures of two senior officials such as the Secretary of the Treasury and the Governor of the Reserve Bank. However, the law imposing this requirement allows a great deal of flexibility about how these signatures are applied, freeing these officials from the tedium of signing millions of banknotes: Reserve Bank Act 1959, s 37.
In the case of a letter, there are no specific laws about who can sign it and how. You could very well get an employee to sign it for you.
The other issue to be aware of is deception. If I write you a letter signed "Kim Kardashian", but she has no idea I've done this and has not given me permission, but I lead you to believe that Kim Kardashian just wrote you a letter saying you're great, and then you go and embarrass yourself posting that letter on Facebook, then potentially I am in a bit of trouble. (A more serious example would be where I impersonate your bank and purport to waive the repayments on your mortgage, resulting in you defaulting.) In such a case, the relevant laws are the various laws to do with deception, not laws specifically about who can sign a document.
Is it legal for a single individual to use multiple signatures when signing legal documents (ie, is there a requirement your signature be identifiable or consistent)?
This is fine. A real signature is not like a cryptographic signature. Its function is not to, by itself, prove the authenticity of a document. The act of signing something is significant because it shows commitment, etc, and for very important documents (e.g. deeds) people (or the law) will ask for the signature to be witnessed. The verification in that case comes from the witness being able to testify as to who they saw sign the document, not the appearance of the signature itself.
Keep in mind that the law related to signatures evolved at a time when not everybody could write their name and continues to accomodate people who cannot today.
There is no reason or principle requiring an individual to use a consistent signature.
There is a practical exception in relation to banks. Banks keep a specimen signature on file for each customer and use this to decide whether to accept withdrawal slips, cheques etc. It is not an ideal system (it is not hard to forge a signature well enough to get past a teller) but in conjunction with the banks' other fraud prevention methods it is the best they can do. However, this is a matter of their own risk management not a legal requirement. If you were transacting in person at the branch and the teller knew you and the bank manager was happy, legally you could sign withdrawal slips with an "X" or a different signature every time.
