system
Given the following post, respond as if you're commenting on Reddit:
human
How reliable is a handwriting match test?

I was watching the Zodiac movie and, although I'm not sure how truthful the movie is, I'd say it's extremely likely that Arthur Leigh Allen was the Zodiac killer.  
However, throughout the movie they could not pin it on him since the handwriting didn't match. It also mentions that he's ambidextrous, but handwriting samples from both hands did not match.  
  
What I'm asking is, how reliable is the non-match in proving that it was not this man who wrote the letters? And is it possible to forge your handwriting, in a sense, so that they do not match?
gpt
Reliability of handwriting match tests can vary greatly. It depends on :  
- the length and quality of the questioned document : is it a 4 pages long will or 3 words scribbled on a napkin ?  
- the quality of the comparison material : did you collect reference material from the natural writing of the "suspect" or did you make them write something for you ? If so, how long are the dictated samples ?  
- what is the time frame between the creation of the questioned document and the production of the comparison material ? The handwriting changes over time, especially for people writing a lot. Illness (e.g. Alzheimer) can also alter one's hanwriting.  

Finally, it is possible to change you handwriting for a short period of time. So, if the writing was changed for a questioned document, how can you bring the suspect to reproduce it instead of his natural writing. You might get one or two characteristics from the natural writing that are reproduced in the altered writing, but will this be enough to positively identify him ? In some cases yes, in others no.

Handwriting match tests work best for wills that are questioned, because you might show differences between the will and other writing samples from the alleged author of the document. Given you have enough good reference material from this person....

**TL:DR :** quality of the questioned document and quality and availability of comparison material are paramount.