Brian,
    I think that is undoubtedly its source. Like any passive, the first
auxiliary (am, are,was, were) would carry the tense and "supposed"
would be past participle.  But think about how awkward it sounds to
say "the government supposes me to pay my taxes." It's not so awkward
to say "the government requires me to pay me taxes" or "obligates me
to pay my taxes." The alternative possibility is that it has become a
three word construction that acts like a modal auxiliary. "I am
supposed to pay my taxes." "I should pay my taxes." "I must pay my
taxes."
   A close parallel would be "am going to," which started out as meaning
movement toward a goal (I am going to the park), broadened out as an
expression of intention (I am going to vote in the next election), and
now can be used as modal predictor, as parallel to "will" ("It is going
to rain").
    Other similar constructs would be "ought to" and "have to" and "be
able to."
   This seems to me another good example of grammaticalization at work.
Words or phrases can change their function over time, and sometimes
they will seem to be part way there.

Craig


> Isn't it passive voice? If "we are supposed to x," someone (or everyone)
> supposes that we should and will x, but the identity of the supposer isn't
> really relevant, so we leave it out by using passive voice (in which case
> we use "-ed" even in the present tense).
>
> ________________________________
> From: Assembly for the Teaching of English Grammar
> [[log in to unmask]] on behalf of Linda Comerford
> [[log in to unmask]]
> Sent: Saturday, February 19, 2011 6:07 PM
> To: [log in to unmask]
> Subject: Supposed versus Suppose
>
> Help!
>
> During an oral grammar workshop, somehow the class got into a discussion
> about the difference between "supposed" and "suppose."  The participants
> didn't pronounce "supposed" with the "d" and had assumed the word was
> "suppose."  We discussed how past tense verbs have the "ed" at the end,
> whether we enunciate it or not, and thought that would suffice.  It didn't
> because someone pointed out that "we are supposed to" is an an example of
> a present tense verb that still needed the "d" at the end.  Okay, I must
> admit that stumped me.
>
> Further confusion arose when someone contrasted "supposed" with "suppose"
> like, "Do you suppose we will ever resolve these questions?"  At that
> point, I wasn't sure we ever would and called a break hoping I could find
> a dictionary to differentiate those words and how they worked.  The
> dictionary was no help at all; the explanations were contradictory instead
> of enlightening.
>
> Can any of you help me with this?  I'd appreciate whatever you can send
> either through the listserv or directly to me.  Since I'm "supposed" to
> follow up with the class, I "suppose" I should have a clear explanation
> for the class.  Thanks so much.
>
> Linda
>
>
> Linda Comerford
> 317.786.6404
> [log in to unmask]
> www.comerfordconsulting.com<https://webmail.smcm.edu/owa/UrlBlockedError.aspx>
>
>
> ________________________________
> From: Assembly for the Teaching of English Grammar
> [mailto:[log in to unmask]] On Behalf Of STAHLKE, HERBERT F
> Sent: Tuesday, February 15, 2011 10:55 PM
> To: [log in to unmask]
> Subject: "thats" for "whose"
>
> We’ve had considerable discussion of relative “that” from time to time,
> and I thought the following exchange from ADS-L might be of interest.
>
> Herbert F. W. Stahlke, Ph.D.
> Emeritus Professor of English
> Ball State University
> Muncie, IN  47306
> [log in to unmask]
>
> ---------------------- Information from the mail header
> -----------------------
> Sender:       American Dialect Society
> <[log in to unmask]<mailto:[log in to unmask]>>
> Poster:       Jonathan Lighter
> <[log in to unmask]<mailto:[log in to unmask]>>
> Subject:      Re: "I've a 24" 2.4Ghz iMac _that's_ hard drive recently
> packed
>              in."
> -------------------------------------------------------------------------------
> I mentioned this some years ago. I had a freshman in the early '80s who
> insisted that "that's" was correct because "whose" referred to people.
>
> When I surveyed English Department graduate students with a
> fill-in-the-blank quiz, a fair number filled in the blanks with "that's"
> instead of "whose."
>
> God knows what they wrote in their own papers. They were mainly working on
> masters' rather than doctoral degrees, if that makes anyone feel better.
> And
> did I mention that the degrees would be in English?  Yeah, I guess I did.
>
> JL
>
>
>>
>> On Tue, Feb 15, 2011 at 4:30 PM, Laurence Horn
>> <[log in to unmask]<mailto:[log in to unmask]>>
>> wrote:
>>
>> > it's an instance of "that" (reanalyzed from complementizer to
>> > relative pronoun) in the genitive, as noted.
>> >
>
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