[supercat]

Much easier to prove two signatures were written by different people than to prove a pile of signatures were written by the same person.

It might be difficult to distinguish tell the difference between the wroting of one person and of two people, one of whom is trying to imitate the other. If two people were selected at random, however, and asked to write the same text without being given any model to copy from, it is unlikely (though not impossible) that they would look even remotely alike. If ten people are selected at random, the likelihood of all ten having similar handwriting would start to become astronomical unless all samples of handwriting were linked to some logical common model (e.g. if all the people have just finished a penmanship class).

If all samples of handwriting precisely match those of a third-grade cursive text, that would not necessarily imply they were all written by the same person (though it would seem odd). If all samples, however, have an unusual trait, that would be far more incriminating. It would still not prove that they were all written by one person, but it would strongly suggest that they weren't all written by different people.

[steve86]

I don't disagree with any of that. But each signature is such a small sample of writing that it would be hard to draw conclusions. Another thing about signatures is they tend to be so idiosyncratic, with missing letters, flourishes, and style which isn't apparent even in the same individual's normal handwriting. Much easier to compare with a signature on file, especially if the forger hasn't seen it.

Now if the forger had used a quill pen dipped in blood it would be a matter of a simple DNA test..