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Inspired by the other currently trending signature question.

The world over signatures are used to tie identities and consent to documents. Signing something means you accept it and it becomes legally binding. However if signatures are to have more than a symbolic meaning, they must be verifiable. Otherwise I can always claim that I didn't really sign that document when it becomes convenient for me.

But how can this be done with any degree of confidence? It feels to me like signatures could be faked too easily, and the process for verifying them is more art than science (as far as I understand it, it just involves a supposed "expert" looking at them and judging how "similar" it is to some other example).

So, why are they used everywhere?

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    You can claim whatever you want; doesn't mean the court will believe you. The relevant question would be not specifically whether you signed the document but whether you consented to it. The court would consider all relevant evidence in deciding whether you consented to it, including the circumstances. The signature itself is only one part of that and is mainly symbolic. If the counterparty is really worried about fraud they will require additional evidence of consent and identity, such as an ID check. Jan 15 at 17:29
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    I believe they are not used everywhere - Google "hanko" for what they do in Japan. Jan 16 at 4:58
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    There are even completely legitimate signature-cloning devices used by many celebrities and even presidents for decades.
    – pipe
    Jan 16 at 12:05
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    Tradition. Forging a signature is easy today, but it wasn't always. When penmanship was a practised art and when people invested effort into producing a unique and reproducible legal signature it was most typically quite easy for experts to distinguish a forgery from a person's true hand. For its time it was an effective way to produce a distinctive mark that was difficult to copy but reasonably easy to verify.
    – J...
    Jan 16 at 12:10
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    Don't mistake graphology (a pseudoscience) and the forensic analysis of a handwritting (pretty much an established scientific practice with reproducible results).
    – fraxinus
    Jan 16 at 16:17

5 Answers 5

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TL;DR: You have to do something to accept a document; a signature is often used because it is simple and permanent; signature alone is often not enough (e.g. wills); claiming a false signature annuls the contract for both parties and you cannot keep the accrued benefits.

Signing something means you accept it and it becomes legally binding.

Many things can become binding without signature (including your usage of this website) and signatures alone are not considered trustworthy in many circumstances.

Important documents (e.g. wills, marriages, real estate transactions) in many jurisdictions require a public official (notaries, marriage commissioners etc.) or a trustworthy person (lawyers, doctors, professional engineers, etc.), who will often require rigourous identification documents, and/or witnesses to be involved for the documents to be legally effective.

Some high-value commercial transactions will also involve witnesses for signature, or require witnesses for executions of certain articles.

Otherwise I can always claim that I didn't really sign that document when it becomes convenient for me.

You can and people do. Then the parties go to court or other dispute resolution mechanism, and the judge will consider all relevant evidences to decide (often on a balance of probabilities, i.e. more likely than not) if you consented to a document. Particularly, "when it becomes convenient for me" is often after some elements of the contract having been executed, which is evidence in favour of the existence of the contract and nonexistence of a contract removes obligations and rights for both supposed parties, as such the executed part could be undone. If for a sales contract the other party has sent you a computer, you cannot claim that you did not sign that contract and keep the computer they sent. Without the contract, the computer is not rightfully yours.

Also, claiming false statements for benefit or under oath is fraud/perjury and can be criminally prosecuted.

So, why are they used everywhere?

You have to do something to affirm your consent to a document. It is symbolic but symbolic does not mean meaningless and a symbol of your consent is often desirable.

A signature is simple and:

  • affirmative of your intention, unlike a simple visual inspection of document (perhaps eventually someone will argue your eye movement can be used, e.g. for VR)
  • permanent, unlike oral declarations (which can still be legally valid, even if hard to prove)1

which is enough for most purposes. As a bonus, it is also somewhat unique and can be compared to certain extent.

For purposes demanding higher level of confidence, ID documents can be demanded and more complex procedures (e.g. with notaries and witnesses) can be undertaken.


  1. Of course, you could make an audio recording for oral declarations, but audio recorders were not commonly available and it would be too complicated if the entire contract is not read aloud in that recording.
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    I’ve assumed there’s a lot of jurisprudence related to ink signatures, that has not yet been matched by things like digital signatures, so counsel for landlords and similar businesses prefer ink because they have a lot of experience litigating with ink. I wonder if that’s another possible reason for ink’s endurance. Jan 16 at 18:32
  • "audio recorders are not commonly available" - that used to be true until relatively recently, but these days many people are more likely to be carrying an audio recorder (in the form of a phone) than a pen :)
    – psmears
    Jan 17 at 15:34
  • @psmears Oops, there was a typo; but yeah, in jurisdictions that allow it audio recordings are sometimes used, though the initial recording is usually done covertly and lack of definite context remains problematic.
    – xngtng
    Jan 17 at 15:41
  • @psmears Portable audio recording has been affordable for decades - I had pocket cassette recorders in the 90s and they weren't new. What's more recent is ubiquitous portable recording ability; if you're carrying around a pocket computer with a microphone, audio recording is simple software. Of course audio forgery is plausible at a similar level to signature forgery. I could have decent shot at faking something to pass casual inspection, but wouldn't stand a chance against a forensic examination if suspicions were raised
    – Chris H
    Jan 18 at 10:57
  • @ChrisH: Affordable, yes, but not always on hand when you needed it :)
    – psmears
    Jan 18 at 11:04
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Contracts don't need to be written; oral contracts are perfectly valid. However, when it comes to enforcing a contract, it's much easier to provide evidence if the contract is written and signed.

From a practical viewpoint, written signatures aren't a very strong form of authentication. Financial institutions have figured this out: modern credit card transactions use a PIN or biometrics on your phone for authentication rather than a signature.

What makes ink signatures useful, though, is that forgery is considered a crime. Therefore, if someone forges a signature to falsify a written contract, then there is a risk of criminal punishment in addition to the civil claim of misrepresentational fraud, which serves as a deterrent.

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    Oral contracts are valid in most jurisdictions, but have limitations which vary based on jurisdiction. Some possible limitations include: maximum permitted value of the contract, the types of things being contracted (e.g. real estate, wills, etc.), and/or any other characteristic of the contract which that particular jurisdiction has decided requires the contract to be in writing. While I understand what you're trying to say wrt. oral contracts being an option, it's not something where a blanket statement is appropriate or accurate.
    – Makyen
    Jan 16 at 16:58
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    Forging a signature is a crime, but is denying you have signed a document that you have signed a crime in many documents? (Of course in both cases it could be e.g. obtaining a pecuniary advantage by deception, and possibly perjury if the evidence is given in court, but it's interesting that forging appears to be an "extra" crime to denial)
    – abligh
    Jan 16 at 17:01
  • How is a PIN more secure than a signature? Jan 18 at 15:17
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    @SolomonUcko In the context of credit/debit card transactions, it's difficult for retail staff with limited training to recognise a forged signature (so in most cases they don't even bother to check the signature). A PIN known only to the cardholder is likely to be more secure in this context. That's the basis of how payment cards work in many countries now.
    – Daveoc64
    Jan 18 at 17:01
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Turning more to your question about confidence...

Written signatures have a few attributes that make them more secure than you'd think.

If you think about a signature, it is probably one of the most fluid pieces of writing you ever do, in your life. Even if the signature isn't identical each time, the individual loops and sections of it are practiced for your entire lifetime, they have a fluidity and flow to them that's incredibly hard to replicate to a forensic level. Even if your name changes. The way your muscles work on a tiny scale, which is very individual and learned. how you hold, move and use a pen are different from everyone else on the planet.

Then think about the material. Pen and paper don't just take ink on the surface. The pen indents the paper, the pen speeds up, slows down, loops and curves, the tip changes its angle on the page.....

In short, yes you can easily scan or copy the ink shape with any good copier. Yes you can fake a signature by practice and repetition to copy it well enough for casual and ordinary inspection.

But copying it well enough for forensic checking is incredibly hard. You have to capture not just the look of it, but all the things that show up to a forensic graphologist. Whether the hand holding it paused or was fluid, how the pen was held, where it dig in or skimmed the surface, if it has the same feel of fluidity as the hundred other samples done by someone who's used that signature all their life and its tied into the handwriting they've developed since childhood.....

Its really hard to fake handwriting well, under a microscope.....

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    The thing is, even I make my own signature... differently every time. Sometimes it's fluid and easy; other times it's more jarring, etc. I'm not really sure if I could pass the check against my own signatures...
    – Vilx-
    Jan 16 at 20:59
  • Doesn't actually matter, at least I don't think. Its like voiceprints - a voice actor can put on many different voices and they sound totally different, but the voice prints still show the same features. As far as I am aware, your handwriting probably will too, there's too much that's "just you" about it.
    – Stilez
    Jan 16 at 21:16
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    @Stilez: But a signature isn't handwriting, at least not past the first few letters. It's just a scribble.
    – Vikki
    Jan 17 at 6:07
  • Totally irrelevant, as far as I know. It uses the skills learned from handwriting, same motor skills, same writing habits, and same long term use.
    – Stilez
    Jan 17 at 9:24
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    Most of my "signatures" are an absolute scrawl that's barely more than a wavy line... and yet I've been told by someone who claimed to have some training in handwriting recognition that there are detectable features shared with my (much more legible) "official" signatures (e.g. on my driver's license).
    – Matthew
    Jan 17 at 17:37
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Because it's always been this way and because it's convenient.

yes, by modern standards a signature is absolutely a trash-tier proof of identity/intent. But a few years ago, it was almost the only way we had that was acceptably convenient. That's why signature forging is quite a serious crime in a lot of countries: in an attempt to make this method somewhat more "secure".

Alternative everyday solutions exist, but only for some context. You have an authenticator app for thee couple of websites, in Germany your ID card can have a digital proof of identity function, but the first comes with the website host having to set it all up, and the latter takes the user to have the dedicated hardware AND the host to fiddle with government interfaces.

And neither really provides a universal "Yeah it's me and I agree with this document" function. That's why we still use signatures, it's not a good, but its the least bad currently available method (if you weigh in convenience and ease of use)

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tl;dr:

  • A signature is good enough.
  • A signature reverses the onus of proof.
  • A signature certifies that this specific agreement was entered into.

A signature was and is good enough for not-too-important documents. (As others have pointed out, some documents of great importance require notarization or some other additional validation.) Keep in mind that there simply is (correct me) no entirely fake-proof evidence in any case: A witness can err, can be malicious or can be bought, a photograph or video can be doctored or can be misleading, by perspective, light or omission.

What we have are degrees of confidence, and for everyday business a signature was and is good enough: It is not that easy to fake convincingly and it is readily verifiable with a model signature (something my bank (Postbank) did until about 1990: They kept a physical signature file in the branch where I could withdraw money).

Perhaps the question is based on a misunderstanding of the function of evidence: It is almost never a 100% proof. Instead, it moves the burden to provide evidence for their claim to the other party.

If the contract bears my recognizable signature and I deny ever having signed it it is up to me to produce evidence for my claim. If I can't, the contract stands.

That said, in the digital age where a digital signature requires knowledge of a well-kept secret whose compromising would be catastrophic an ink signature seems indeed terribly unsafe and trivial to fake. One has to wonder how the business and legal world could ever be built on such a flimsy base.

One answer is, of course, that forgery of documents was always a serious criminal act. Even before most people could write their name contracts were formed, promises made and loyalties sworn; those promises were enforced by an honor system whose violation also bore serious consequences. A signature is simply a written record of a mutual promise (and as such typically legally redundant). In business the signature is not so much a proof that both parties entered into an agreement; instead, it is proof that both parties entered into this specific contract with these payments and dates and side conditions and mutual obligations and exit clauses — lest one of them "forgets" one tiny detail. That the contract was entered would usually be hard to deny; the signature says that you have read the fine print.

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