Is there a good idiom that implies fake happiness in the same way 'crocodile tears' imply fake sadness?
Not an idiom but you can say a sinister smile.
Edit: maybe too simple but: a fake smile, or an empty smile
Edit 2: I just found this online at Yahoo! Answers; I quite like this one:
Pan-Am smile > It is named after the airline Pan American World Airways, whose flight attendants would always flash every jet-setter the same, perfunctory smile.
Other alternative terms for a "fake smile" are: Botox smile, a say-cheese smile!
A fake smile only involves the zygomatic major muscles which are forced by a person in order to give a polite smile. The smile is then limited only to the lips and there is no contraction in the eye area.
The study of smiles is a part of gelotology, psychology, and linguistics.
And it may be good to know that scientific word for a "real smile" is "Duchenne Smile" which is named after physician Guillaume Duchenne who first recognized the muscles involved with this smile.
It depends what you're trying to convey. If it's a lack of emotion, or a lack of interest, it could be "a pale smile", "a polite smile", "a perfunctory smile", "a mechanical smile".
To suggest that it's regularly used, "a practised smile", "a receptionist's smile", "a salesman's smile", "a politician's smile".
If there is any negative intent, it could be "an intimidating smile", "an icy smile", "a debt-collector's smile"...
Reading the comments and answers here I was reminded of an excerpt from Ralph Waldo Emerson's Self-Reliance:
There is a mortifying experience in particular which does not fail to wreak itself also in the general history; I mean "the foolish face of praise," the forced smile which we put on in company where we do not feel at ease in answer to conversation which does not interest us. The muscles, not spontaneously moved, but moved by a low usurping willfulness, grow tight about the outline of the face with the most disageeable sensation... [A] man must know how to estimate a sour face... If this aversation had its origin in contempt and resistance like his own, he might well go home with a sad countenance; but the sour faces of the multitude, like their sweet faces, have no deep cause, but are put on and off as the wind blows and a newspaper directs.
Whatever the context, I think forced smile works, because the smile is as disingenuous as the "crocodile tears." The tears aren't coming from real sadness; this smile isn't coming from true happiness. Regardless of motive, the smile is not a reflection of how the smiling person feels. (The references I've found, though, tend to associate "forced smile" with one done from a non-hostile attitude.)
If the context matches Emerson's scenario -- where the smile comes more from polite obligation than how its wearer feels -- I recommend strengthening the reference by using Emersionian, as with:
- The Emersonian "'foolish face of praise'"
- The Emersonian "forced smile"
- The Emersonian "sour faces (of the multitude)"
Depending on the context -- and on whether it's an idiom for a deceptive (i.e. deceitful) smile or a fake smile that you're looking for -- you might want to consider the following phrases:
put-on grin, coated smile, made-up grin, plastered smile, placard smile, frontage smile, facade grin, storefront grin, meet & greet smile, front desk smile, greeter's smile, checkout smile, bogus grin, slick-perfect (or slick cover) grin, schoolmarmish/schoolma'amish grin, straitlaced smile, tight-laced smile, Pecksniffian smile, pharisaical smile, whited sepulcher-ish smile, holier-than-thou smile
There was a lot of anger in the house, but everybody kept it hidden behind a coated smile except Annie. source
Stoop waiting home with frontage smile... source
I can see the sweet sweat on your forehead, and the quivering falter of your facade grin... source
No meet & greet smile and seems the employees are miserable also.source
A front desk smile might have been inviting! source
Florio turned around, his greeter's smile on his face... source
She hands me the bag and we smile a checkout smile. source
Sleek smile, phony smile, humbugging grin, carpetbagging smile, shyster's grin, con artist grin, huckster's grin, two-timing smile, double-crossing smile, two-faced smile, double-dealing smile.
He still can't see the fat face hidden behind his glasses, behind a carpetbagging smile. source http://www.encyclopediaofarkansas.net/encyclopedia/entry-detail.aspx?entryID=1179
Girls manipulate and bully almost silently, often behind a two-timing smile that may show intentions to their classmates. source
Smiling her double-crossing smile, Regina offers to introduce Sally to Aaron but smooches him instead. source
This is more of a cliche than an idiom, but many writers use expressions like:
The smile stopped at the corners of his mouth.
Based on the common observation that the difference between a fake and a genuine smile is in the eyes.
One of my favorite ways of describing a deceptive smile was done by the legend George R.R. Martin himself:
"Littlefinger smiled with his mouth, but not his eyes"
I would say Grinning like Cheshire Cat could be used.
a constantly grinning cat in Lewis Carroll's Alice's Adventures in Wonderland. grin like a Cheshire cat, to smile or grin inscrutably. and its proverbial grin attested from 1770, of unknown origin. A cat with an enormous grin encountered by Alice in Alice's Adventures in Wonderland, by Lewis Carroll.
This would signify someone that is always smiling cheek-to-cheek in a pseudo-deceiving way.
My daughter suggested 'Barbie smile' and 'the smile that comes with the fake laugh you do when someone tells a joke that isn't funny'. I thought I'd share.:)
A tutored smile.
This phrase implies that the subject had to exert some effort and intention to exhibit a smile.
"Opps! My mistake! I thought the topic was 'What is a good Emoticon for a deceptive smile?'".
He smiled with a plastic grin as he quickly exited the Stack Exchange Forum.
From the Bill Murray comedy, "Quick Change," in which Murray's character is dressed as a clown, and in the process of robbing a bank he describes himself as a
smiling on the outside, but crying on the inside kind of clown.
With a slight tweak,
A smiling on the outside, but crying on the inside kind of smile.
Perhaps it's not what you're after, but you could describe a smile without using the word 'smile' at all. Perhaps refer to it as a grimace. Then the reader will catch the sense of off-kilter falseness.
His eyes caught mine and he held my gaze while the corners of his lips pulled up in a kind of grimace, showing his teeth. After a second I realised this was a forced smile...
Overseas, this phenomenon is widely known as the American smile, but plastic smile (suggested in comments) is good too.
I think the word artificial is appropriate here.
An artificial smile.
NB: I think the title here is a bit deceptive because it asks for a deceptive smile, whereas the content of the question clarifies that the OP is looking for a word for:
fake happiness in the same way 'crocodile tears' imply fake sadness
... and clearly the word deceptive has negative connotations unlike fake - which doesn't necessarily imply something negative.
I think maybe the word that seems to be missing from this long list is mischievous.
Saying that someone has a mischievous smile/grin fits the author's exact need.
What I met in a few instances of fiction was
Professional Smile Number Five
This emphasizes the smile is rehearsed, trained, routine, required by code, looks pleasant, and is completely irrelevant to whatever feelings the subject has. It's not so much sinisterly deceptive, as "hide own feelings, appear pleasant and professional".
While it starts to fall under the area of trademarked terms, "Stepford smiler" is used pretty often in my experience, relating back to the novel (and movies), The Stepford Wives. The drawback I see in that is that it carries a connotation that the smile is being enforced, in the case of the source material, through mental reprogramming, but sometimes mandated by parents or employers. I have also seen it in the context of a self-imposed constant smile despite not being happy, but it's rarer.
wolfish grin? though it doesn't encompass every situation you've set out in your question.