The Optical Mouse (1981) [pdf]

Skimming the referenced PDF, it discusses the optical mouse peripheral, not the onscreen pointer. (There is one illustration of an onscreen pointer in the classic tilted-arrow form, but as an example of something already extant.)

The alleged "turn it 45 degrees, making it easy to see" rationale is only in surrounding modern text (at these blog posts and the StackExchange answer). It might be right, but if so, it's not supported in the linked 1981 document.

There's also the fact that an upward-pointing (or any other 90-degree direction) pointer quite strongly evokes direction instead of just pointing, while other not-aligned directions evoke pointing more than direction.

Relevant Question regarding this in UX Stackexchange, http://ux.stackexchange.com/questions/52336/why-is-the-mouse...

There has recently been a discussion about this here: https://news.ycombinator.com/item?id=7251987

I thought it was tilted so you could get the maximum "tail" length in a smaller bounding box, because you are using the hypotenuse.

Completely spit balling here but wasn't the abstraction of your cursor originally a finger? If so the 45 degree tilt with the 'finger' pointing to the left makes sense if you just reach out with your right hand and point at something on the screen.

At least for me when I'm pointing at a regular computer monitor my hand is tilted not strait up and down.

This is not an explanation. The only thing you can say is that the first mouse pointer was _also_ tilted. The motivation can be different while result is the same over time. Please don't upvote that superficial answer on SO UX.

Wow, just how many concepts from Xerox PARC have made other companies (MS, Logitech, etc in this case) insane amounts of money.