The original SRI mouse (Engelbart, et al, mid-1960's) had one button.
By 1968, it had three buttons. Only the left button was used in
conjunction with pointing. The other two buttons were used to confirm
and cancel actions.
In 1974, at Xerox PARC, I implemented a prototype of the Gypsy text
editor that introduced drag-select and needed only one button. (The
final version of Gypsy, 1975, which I developed with Tim Mott, used
all three buttons because they were there.)
In 1981, Xerox shipped the Xerox Star with two buttons.
Apple's mouse first appeared on the Lisa. If we had been designing
the Lisa for power users, we may have provided at least two buttons.
But 99%+ of our target customers were new to mouse use at that
time--new, in fact, to computer use. In our 1980 usability tests, we
observed significant button confusion. "Which button?" "Oops, wrong
button." Or worse, the wrong thing happening without knowing why. We
also observed that users paused before clicking to think about which
button to click. This not only slowed them down, it took their minds
off the task and made them think about the tool.
We also speculated that other pointing devices would emerge in the
future which could have only one button, e.g., a pen whose switch was
built into the tip.
Today, I use a two-button mouse with my employer's PC. I have, in the
past, used a two-button mouse on my Mac, But with Apple's current
"no-button" wired and wireless mouse designs--the entire shell is the
button--I have regained a personal preference for one button. I find
it relaxing to click without thinking, even unconsciously, about
where my right finger is when it clicks. If I have to think about
anything, it is which modifier keys to press with my left hand in an
application like Photoshop.
Manager, Lisa Applications and User Interface, 1980-82