How do we decide which is the correct suffix of agency?

For example, we have 'teacher,' but 'educator.'
'Writer,' but 'editor.'

And I've seen 'advisor' both ways.

I know I could LIU, but as WW said, it'd be fun to see the different ideas here. If this thread expires, I'll go to google .

Parbly whether the root is from Latin or English has a lot to do with it. Your teacher/editor and writer/editor certainly fit this hypothesis. Adviser just looks wrong to me. I have seen adapter and adaptor on mini power convertors for computers and peripherals but they tend to come from countries in which English is not the primary language so I claim they don't count.

Looking over my text here I see computer doesn't follow my hypothesis.

This also brings to mind a question I've been ruminating lately as to why the seeming demise of the -ess in words that take the -or suffix. Especially, now it seems that actress is getting jettisoned in favor of actor.
In fact, the new AFI Awards renamed the category "Best Female Actor in A Leading Role" in lieu of "Best Actress." This befuddles me, because I've never heard actress deemed denigrating to any degree from the feminist viewpoint. In fact, to me actress always seemed relegated to a higher pedestal than actor, thus "Best Actress" being traditionally saved as the final, and most important, individual category at the Academy Awards. On the other hand, why do we say doctor and never doctress?
But I've also heard the -ess suffix applied to certain -er words as in hunter/huntress (especially when talking about the animal world, as in lioness...hmm, well there you are...female lion?)
Which brings us back to the determination of the -er,-or in the first place...perhaps an "adviser" is more generic, and "doctor" and "actor" is more specifically descriptive of a professional person. However, I'm sure there's plenty exceptions that can be found to all of these examples. I like Faldage's very credible proposal...but, there, too he found an exception almost immediately. Although it seems like there must be a common root to all this somewhere.

Crossthreading: here's two more contributions from Dr. Bill's new post...monitor and invigilator.

he found an exception almost immediately

I have fine tuned my hypothesis (scientific method in action). -er for English roots and -er or -or for Latin depending...

Haven't decided yet ...on what

there was a woman on a UK chat show a few years ago (might have been Joan Plowright) who upon being called an actress said " I am an actor darling , have you ever heard of a doctress?"

the Duncster

I am an actor darling , have you ever heard of a doctress?

I think by the time women gained any sort of noticeable presence as doctors, the acceptance of the humanity of women by the patriarchal establishment had advanced far enough to accord them no special designation. A doctor is a doctor regardless of sex. Female actors were common well before this stage of evolution had been reached.

stage of evolution

That's a pun, right?

stage of evolution

That's a pun, right?

You always recognize my puns before I do.

And I guess prince and princess will never become a common word (no pun intended, but we'll take 'em where we can get 'em! ). Unless, of course, we start saying male prince and female prince.

stage of evolution

Now you could say a theatrical production of "Inherit the Wind" (the Scopes monkey trial) is truly a stage of evolution!...Couldn't you?

Actually, the meanings of "Prince" and "Princess" don't exactly correlate around gender.

The term "Prince" quite often means someone of power, a ruler. Me old mate Niccolo Machiavelli called his little book on the use and abuse of power "The Prince", and "Prince" in this case really meant "tyrant" rather than someone of undisputed royalty. Well, they were all tyrants in those days, weren't they? Nowadays we call them "president".

And Niccolo meant what he said, because he was tortured at least once by one of 'em, yet remained a fan of absolutist rule.

And "prince" can also be used in a genderless sense, too.
Queen Elizabeth I (Lizzy One) often referred to herself as a "prince" in the sense of "ruler".

"Princess", on the other hand, simply doesn't have the same connotations. Unless, of course, you refer to the current Princess Royal, who is recognised as something of a power in English royalty terms, but that's solely on the basis of who she is and her actions, not what her rank is. Sorry, girls!