Because I can mirror-write, I had to read the article in The Psychologist (citation below) by McIntosh and Della Sala on the skill. They look at much of the evidence for two theories: mirror-writing has a motor source, and it has a perception source. Of course, I could not help but compare their picture with my own experience. I am left-handed and dyslexic so I have always thought that my mirror-writing skill stems from one or both or these conditions.
This article puts forward the idea that some mirroring is natural for all children learning to write. They have not been required to differentiate mirror images until faced with writing. They learn the shape of letters separately from learning their direction and so there is a shortish time period where mirror-writing is somewhat common.
There is little truth in the idea that mirror-writing is more common in left-handers. Mirror-writing in childhood does of course correlate with age, but the true underlying factor here is the stage of acquisition of writing, with occasional mirror-writing as an intermediate stage between no writing and correct writing …. Specifically, it implies that the general shape of a letter is learned more rapidly than the direction for writing it. The key to understanding this may be to regard mirror-writing not as intrinsically errorful, but as a feat of action generalisation. It is a neat trick for a child to produce a perfect mirrored-form, which they have never been taught, as readily as the correct form that they have been shown repeatedly. For most actions, this mirror-generalisation would be useful, because anything that we do one way may need to be done in reverse at another time. Writing, however, belongs to an unusual, evolutionarily recent, class of actions that have a culturally set directionality, and for which this generalisation is unhelpful. Acquiring the correct direction for writing in ones culture may be a matter of stamping out the unwanted alternative after having learned the general shape of the action.
It seems that mirror-writing has a motor cause. Most mirror-writers cannot read their mirror script, me included. Reading mirror-writing is a separate skill. Much of the discussion in this article has to do with people changing hands from their dominate one or using both hands simultaneously. I had never done any mirror-writing with my (non-dominate) right hand so, of course, I stopped reading and tried. It does not seem to work for me. Of course, I have to take into consideration that I have never done anything with my right voluntarily why start in my 70s. It may be that the ability to mirror-write may actually be quite common but most people have simply never tried to do it.
But I do think that my dyslexia may be involved. I associate my type of dyslexia with a life long problem with identifying whether two images are the same or mirror images from memory (as opposed to have both images in my visual field at the same time or one immediately after the other). Another problem that I have always had is telling whether a turn is clockwise or counter-clockwise without picturing the numbers on a clock and match that image with the motion. Am I going up the numbers or down? Nothing in this article seems to definitely confirm or reject the idea that some types of dyslexia have an affect on mirror perception or motor patterns.
RD McIntosh, & S Della Sala (2012). Mirror-writing The Psychologist, 25 (10), 742-746