i.e left-to-right, right-to-left, top-to-bottom etc.
My guesses so far :
- righty/lefty person,
- material on which writing is done,
- tools by which writing is done etc.
But I'm not able to convince myself on any particular point.
It's important to note that the incidence of left-handedness in middle-eastern countries is roughly the same as those in western countries, and if anything, left-handedness is even more discouraged in those countries because of a cultural belief that left hands are less clean (or intended for less clean work) than right hands. From that perspective, the incidence of left or right handedness doesn't seem to be a compelling consideration for the direction of text.
That said, there is an argument that can be found on the internet that states that when Hebrew was first written, it was written in stone and clay, by right handed people. The left hand was used to hold the chisel, the right hand the hammer, so writing right to left allowed right handed people to see into the space they were writing. The argument goes further to say that most middle eastern countries also write right to left because of the influence of the Hebrews in their early histories, but I've never found concrete evidence of this. That said, it does go to medium and 'handedness' being considered in concert as an influence on script direction.
The outlier here is Asian writing styles, that were traditionally vertical. This may have more to do with the design of the script itself. The design of the characters (especially the compound characters) lend themselves to vertical interpretation, so character design may well be a consideration in the orientation of the text as if the 'pen' tends to end on a downward stroke instead of a lateral one, then the text is probably more easily written vertically as it reduces the distance the hand must travel between characters.
The outlier of the outliers is of course Hangul, which is the Korean form of writing and was designed by a select body of linguists appointed by the ruler of the Korean peninsula around the 14th century. This really is a nerd's writing system as it was designed from the ground up by their own (I say this as a computational linguist by trade). While it was traditionally (and is still for artistic purposes) written vertically in similar form to Chinese, today it's largely written horizontally in Western form and a close examination of the characters will tell you that such a change in orientation isn't that difficult for the characters as they lend themselves to being written quickly in either orientation.
Hangul Rocks (but I digress).
The key point here is that left or right handedness is ONLY a consideration in conjunction with the medium and writing tools as a whole; what lends itself to convenient writing with hammer and chisel is very different to what lends itself to convenient writing with ink. Also, in terms of horizontal or vertical orientation, the style of the characters will be highly influential.
Of course, the one consideration which we've overlooked in this dissertation, and arguably it's the most important, is convention.
If the people around you are writing left to right, top to bottom, you're more likely to write that way yourself, even if your script is different, and especially if it's the same, just in a different language. Why? Because writing is only done by people who want to be understood by others. There's little point having a writing system if people can't understand it, and even less point if you're not leaving words for others of different languages to one day interpret into their own. Writing is about the efficient transfer of knowledge between individuals, cultures, and eons. In other words, people WANT their writings to be read.
Just like it's believed that many Arabic countries write from right to left because the Hebrews did it first, and that Hangul started out vertical (like China) but is now often horizontal (like Europe, USA, etc.) one can expect new scripts will follow whatever is the dominant convention surrounding them.
If on the other hand writing is new to your world, then pick whatever is the most convenient;
Horizontal orientation will be decided by a combination of which hand is used most commonly and what media, tools and techniques are used. The choice to go vertical will be decided more by the nature of the characters used.
The important thing to remember out of all of this is that people who write want their work to be read. As such, they'll pick the orientation that is most likely to ensure that objective is met.
I am a left-handed person; so although I learned to write from left-to-right and top-to-bottom, that is a terrible way to write if you use ink that can be smudged; because my hand travels over just-written text before the ink has dried.
As a child, I tried various correctives. For a few years I rotated my paper 90 degrees clockwise (so the top pointed to my right) and I could write my lines vertically and progress to the left (down the page). I taught myself to write the characters sideways, obviously, so when the page was read normally the characters appeared normally. Now I still write similar to that, with the page upright, my hand curled over above the text. Also, I need to be slightly careful writing on the board in class, to not smudge the text to the left of what I am writing.
In grade school (ages 6-12) my teacher tried to force me to write right-handed. That didn't last a day, my father (also a lefty) took care of that (I don't know what he told them).
In general, for a left-handed inventor using any smudgeable medium (ink, chalk, paint), the most natural directions would be right-to-left, bottom to top, so the hand was always in an unmarked part of the page.
Writing direction was not always carved in stone, so to say. For example:
Ancient Egyptian inscriptions are written indifferently left-to-right or right-to-left. To know which is which, look for the hieroglyphs in the shape of animals, and note which way they face. Ancient Egyptians even liked symmetry, so that inscriptions on the left and right sides of a monument, for example, are written in opposing directions.
The oldest Greek inscriptions were written boustrophedon "like the oxen when ploughing", one line left-to-right then the next right-to-left and so on.
In the ancient Near and Middle East, cuneiform script (used to write Sumerian, Hittite and a host of Semitic languages, notably Akkadian) was initially written top-to-bottom right-to-left, but switched to left-to-right top-to-bottom (rotating all characters 90° counter-clockwise!) around the middle of the 3rd millennium BCE.
Phoenician script (from where the Hebrew, Arabic, Greek, Latin and Cyrillic alphabets come) was written right-to-left but also sometimes boustrophedon (see above).
Etruscan script, the immediate predecessor of the Latin alphabet we all love, was written left-to-right, or right-to-left, or boustrophedon.
The oldest Latin inscription ever found, the Praenestine fibula, is written right-to-left...
The Praeneste fibula, carrying oldest Latin inscription ever found (7th century BCE): MANIOS MED FHEFHAKED NVMASIOI (Manius made me for Numerius), written right-to-left. Photograph by Pax:Vobiscum, available on Wikimedia under the CC BY-SA 3.0 license.