[sjwright]

The up–down orientation unquestionably comes from the near-universality of light sources being above you. You'll notice that intense up-lighting is almost never found in the natural or artificial world.

(It's true that upwards-firing lights are often used in interior decoration, but these are almost always to illuminate walls and ceilings above us which then bounce light down to the contents of a room. Also, some techniques in photographic and studio lighting do use some up-firing lights, but these act as shadow fill, never as the primary light source—except when an unusual look is intended.)

To the extent that we expect a certain left–right orientation, this will almost certainly be predominantly a matter of consistency with prior graphical user interfaces. The near-universal standard of a top/left lighting metaphor goes at least as far back as the original Apple Lisa/Macintosh which cast its 1-bit, 1 pixel shadows to the bottom/right.

The fact that people of near-identical cultures can natively integrate either left-hand driving or right-hand driving suggests to me that there's no inherent reason why it needed to be one way or the other. Had the first interfaces begun with a top/right lighting metaphor, we'd probably be all as native to that as British people are with right-hand-drive vehicles.

[Moru]

Upward-firing lights are aimed at the roof to spread the light and create more ambient light. So is primarily used as a down-light by reflection anyway.

I'm not sure the left/right shadows are a learned thing from user interfaces. As a kid, long before I got my first computer I was drawing 3D forms. Shadows always was drawn as light came from the upper left.

Maybe this is because I draw with right hand so prefer light to come from the left so I don't shadow my hand?

[hunter2_]

Now you've got me wondering if the left/right shadow preference might actually be rooted in other natural phenomena besides human dominant handedness, like something with the rotation of the earth a.k.a. sun setting in the west.

[hnlmorg]

You're over thinking things there. More likely it's just an artefact of those interfaces being designed in the countries who read left to right.

Off topic but I like how you've referenced that infamous IRC meme in your HN handle.

[mPReDiToR]

What? The username just shows as *_ for me?