This is the million dollar question, as far as The Thing is concerned. Unfortunately, not only has the answer never been revealed, but the producer and director themselves say they have no idea what the answer is.
There are a number of fan theories - both men are Things; neither man is a Thing; only Childs is a Thing; only Mac is a Thing - but the "evidence" employed to support arguments in favor of one or both men being Things is unconvincing, and it is easy to poke holes in every possibility except "neither of them is a Thing".
Even worse for those who think they have discovered subtle (or not-so-subtle) clues that point to one or both men being Things, the director and producer have both said that they didn't include any such clues, and even they don't know who, if anyone, is a Thing at the end of the film.
Word of God:
Director John Carpenter:
Of course, the man who is best suited to answer the question is the director, John Carpenter. Unfortunately, he is notoriously taciturn and reluctant to disclose too much about his baby (this is his favorite movie). In fact, he can sometimes come across as almost rude, because his answers to these questions are incredibly brusque, curt, and even dismissive. His most common response to this, the mother of all Thing questions, is "I don't know", and I've actually seen interviews where he says "I don't care". His near-silence on this issue seems to be partially motivated by the hurt caused by the movie's utter failure at the box office. That really did crush him, and he refused to talk about the film for quite some time afterwards.
The most significant answer he has ever given is as follows, and was part of a Q and A session on a fan site called Outpost 31:
Q. Was Childs The Thing at the end or not?
A. I don't know. I believe MacReady was a Thing at the end, but Childs...?
- John Carpenter, Outpost #31 Fansite Q&A
As we can see here, he insists on taking the position of an outside observer, rather than the man who made the movie.
This answer is probably disingenuous, as there is an unreleased final scene, in which MacReady is sitting in a medical exam room having just received the results of a blood test, with a very strong implication that he is, indeed, a human. Childs does not appear in this scene, and - had this ending been used in the version of the film that was eventually released - we would be left to wonder what had happened to him and whether he was a human as well.
And if we continue reading the interview linked above, we see Carpenter retract his earlier statement:
Q. You mentioned to us that you believed MacReady was a Thing at the end. Do you have any specific ideas about how and when he was attacked? Some have speculated that he might have cheated the blood test as we didn't see him draw any of his own blood - do you agree?
A. I'm just bullshitting about MacReady being a thing at the end. I have no idea. Nobody does.
- John Carpenter, Outpost #31 Fansite Q&A
Producer Stuart Cohen:
Stuart Cohen, a co-producer on the film, has also done a Q and A session on Outpost 31:
Q: Was there a definite idea whether Childs or Mac was infected at the end?
A: No definite idea.
- Stuart Cohen, Outpost #31 Fansite Q&A
What Cohen is saying here is important, but easy to misinterpret: he isn't simply saying "I don't know", he is actually saying "It was not addressed in the script, and we never decided whether one or both of them were Things."
Cohen's blog supports this statement, albeit in a negative manner: he discusses all sorts of minutiae in exquisite detail, but never suggests that the producers or director ever seriously considered the crucial matter of who was and wasn't infected at the end of the film.
In fact, he goes so far as to say that it doesn't make sense to come up with fan theories about who was or wasn't a Thing in the final scene. Why? Because the initial response to test screenings was highly negative in regards to the ambiguity and hopelessness of the ending. As a result, the production team reworked the final scene over and over. Even the original cut of the ending didn't provide hints as to who was who, but even if it had, the editing and reediting was so chaotic that those hints would have been lost in the mix:
Most of you want to know who was who in the final scene. Unsurprisingly, there is no definitive answer but... what I can say is that given the fact this scene still bears the remnants of this massive re-working with many hands it is counterintuitive for me to think that there exists a hidden level of meaning that offers clues to identity (The J&B bottle being passed back and forth, Childs lack of breath in several cuts, etc.). Believe me, all we were trying to do was keep the scene in the movie...
- Stuart Cohen, The Original Fan: The Ending You Almost Saw
Stuart Cohen did a Reddit AMA in October, 2016, in which he was asked about the theories that Childs is either not breathing normally, or is unwittingly duped into drinking gasoline by MacReady:
Q: Also, do you think there is any reason to believe Childs is drinking gasoline at the end or not breathing regularly?
A: No. There are no specific clues to identity. The film simply wasn't designed to work on that level. THE THING was made for a world in which we hoped to attract people into movie theaters to see it ONCE, and on a large screen - as most now know we failed miserably at that task. Although its flattering we never envisioned the film would be endlessly re-run, rewound, and parsed for meaning...
- Stuart Cohen, Reddit AMA
Screenwriter Bill Lancaster:
Unfortunately, Bill Lancaster died in 1997, when the reevaluation of The Thing (from "absurdly gory, barely watchable flop" to "underappreciated masterpiece") was still in its early stages, and fan Q&As, especially on the internet and at Sci Fi conventions, were far less common. As such, I haven't been able to find any statements about this - or much else related to the film - from Lancaster himself. However, Cohen said, on his blog, that Lancaster subscribed to the belief that both Mac and Childs were human at the end of the movie:
I can tell you what Bill Lancaster thought when he wrote it - he believed both men were human. He felt this was the most poetic way to end things - two individuals moved beyond exhaustion, framed in a tableau of fire and ice, having a last drink in the Antartica night before freezing to death. I subscribe to this view. I can not, however with 100% certainty tell you that John agrees with this.
- Stuart Cohen, The Original Fan: The Ending You Almost Saw
Thus, we have two interesting pieces of information: First, the man who wrote the script thought that Mac and Childs were both human in the final scene. Second, that Cohen - speaking as a fan of the film, not as the producer - likes to think that Mac and Childs remained human throughout the movie. However, he makes it clear that this is not an official statement of fact - merely the personal interpretation he prefers in his role as an audience member.
A search of YouTube will produce several videos of Q and A panels at fan festivals and conventions, but Carpenter himself has participated in relatively few of these; however, the cast members are more than happy to take part in such events, and their insights are both useful and interesting.
Keith David (Childs):
Keith David, the actor who played Childs, is a regular participant and has consistently and emphatically stated that, as far as he is concerned, Childs was not a Thing at the end of the movie. The links in this paragraph are from Sci-Fi/Horror Q&A panels with either Keith David alone, or most of the cast together, and in all of them, David repeats the same basic account:
The scene was shot a couple of times, with different direction from Carpenter each time. In one shoot, Russell and David were told to play the scene as though they knew that one of them was the Thing (actually, in one Q&A panel, he says there was one shoot in which Childs was supposed to know he was the Thing, and one in which Mac was supposed to know he was the Thing). In the shoot that ended up in the film we see today, both men were told to play the scene as though they were not the Thing. And in every interview, David insists "It wasn't me", but this is clearly his opinion, not necessarily the truth.
Kurt Russell (R.J. MacReady):
I'm not aware of any convention Q&A panels involving Russell, or any definitive statement from Russell on this issue, but he did address it once - without answering the question directly. A fan theory suggesting that Childs must be the Thing, because we see MacReady's breath, but not Childs', went viral in 2016. Russell was asked about this suggestion:
When asked about the theory, Russell shook his head before explaining why it may be missing the point.
“John Carpenter and I worked on the ending of that movie together a long time. We were both bringing the audience right back to square one. At the end of the day, that was the position these people were in. They just didn’t know anything,” said Russell.
“They didn’t know if they knew who they were, but had you seen all the things in the movie, you’ve heard MacReady say, ‘I know I’m me,’ Well, you either believe him or you don’t. And Childs — you know, one of my favorite lines in the movie [is], ‘Where were you, Childs?’ And I think that basically says it all,” explained Russell. He continued, “I love that, over the years, that movie has gotten its due because people were able to get past the horrificness of the monster — because it was a horror movie — but to see what the movie was about, which was paranoia.”
- Huffington Post: Kurt Russell Basically Shoots Down That Theory On ‘The Thing’
For a fairly comprehensive review of the important factors, we can turn to the relevant portion of IMDB's FAQ section on The Thing:
Who is infected at the end?
The mother of all questions. Answer: unknown. It is as reasonable to assume that Childs is the Thing as much as it is to assume he is human. There is certainly enough time for him to be assimilated, but he does have an alibi. He claims he saw Blair out in the snow, and since MacReady ordered him to kill Blair, if Blair came back without the others, he was merely following orders. It is not hard to believe Childs' explanation; Garry, Nauls, and Mac had been gone for a good amount of time during their visit to the tool shed to look for Blair, and all of the men were sleep-deprived. Had Childs been a Thing at the end, he could have easily torched Mac with the flamethrower that he was armed with.
(Note: before the others see Childs run out of the building into the storm, we see a quick shot of the coat room where he was stationed and the front door is swinging wide open and the snow had been blowing in for a while, possibly suggesting that Blair/The Thing forced its way in, over-powered Childs, assimilated him, and then ran back out. Which implies the room was empty for a few minutes BEFORE the crew see Childs run away from his posting.)
The theory that you cannot see Childs's breath and he is therefore a Thing is not valid. Firstly, his breath CAN be seen. The reason it is difficult to see is because of the way the lights were set up. Also, we see Bennings earlier in the film after he has been assimilated. If Bennings, a non-perfect imitation, was able to have visible breath, why would Childs, by now a perfect imitation, be unable to do the same? The answer is, he can; it's just the light of the scene.
One case that could be made towards Childs being human involves taking into account the information given in the prequel, where it is revealed that the Thing cannot replicate inorganic material (i.e. piercings, fillings, joint replacements). While this fact is never confirmed in the Carpenter film, there is no evidence denying it apart from the scene where Nauls is killed by a Box-Thing (which was never shot and is therefore not canon). If you look closely in the final scene, Childs has an earring. Taking this logic into account it would be almost definitive proof that he is still human, since if he were a Thing he would have spat out the earring. The only possible explanation for this is that the Thing learned from its previous experiences and so whenever possible would attempt to replace visible inorganic material such as earrings. However, if Childs was infected, and spat out the earring, it could easily have been lost depending on his location (if he were outside at the time, it could have gotten buried in the snow; if Blair had assimilated him inside the coat room, it could have rolled under something). It's also worth mentioning that the men at Outpost 31 were not aware of this fact, so they would not have questioned him suddenly removing it. Also the remaining men were sleep-deprived and probably not thinking clearly, so they probably wouldn't have noticed the missing earring anyway, let alone question it. Therefore, it seems strange that the Childs-Thing would get down on the ground and waste its time looking for an earring for three reasons:
1) As mentioned previously, most likely nobody would have bothered questioning it, since much like how Kate never had any experiences that led her to realize the Thing's cells function independently, Mac never experienced anything leading him to conclude that the Thing could not replicate inorganic material.
2) Getting down and searching for the earring would leave him vulnerable, as someone could theoretically sneak up on him and attack him while he is not looking. Also he would have a hard time explaining why he is searching so desperately when there are more pressing matters at hand.
3) There was really no need to do a perfect impression of anyone anymore. By this point, Mac was the only other survivor, and he was incredibly weak, freezing to death, and unarmed (Kurt Russell said that he had a torch/flamethrower under his coat during that scene), meaning that Childs could have easily assimilated him, and Mac would be powerless to stop him. The fact that he doesn't would strongly suggest that he is still human.
If we wanted to make a case that Childs was the Thing, we could say that when Childs saw Blair out in the storm he was being lured into being assimilated. The Thing got into the camp by getting Childs to open the door, and it got Childs in the process. It then kept Childs away from danger until the situation was over, like protecting the queen in a chess game. When Childs says, "What will we do?" Mac suggests they should maybe do nothing. The way Mac is watching and smiling at Childs suggests that Mac is almost certain Childs is the enemy and has won, but Mac accepts the defeat gracefully, and honorably, and even extends his bottle to Childs, so they can drink together. This could be seen as a big developmental leap for Mac, since the first opponent we saw him lose to was the chess computer, and he immaturely destroyed that opponent when he lost the game by pouring his drink into it and calling it a cheating bitch. Interestingly, in the novelization of the film by Alan Dean Foster, the book ends with Mac initiating a chess game with Childs.
Was Mac infected? There are theories that he was, but the evidence points to 'no'. Mac passes the blood test, and the possibility of him rigging it and managing to cheat his way out are very slim. Also, had he been infected, he would have most likely only taken one man with him to Blair's shed. This way Mac would be able to assimilate another victim. It is also hard to believe that Mac would kill Blair had he been a Thing. Had MacReady been a Thing before the blood test, he could have killed the remaining humans. Instead of tying up Palmer, he could have had Palmer tie everyone down, and while Nauls, Garry, Childs, and Windows were strapped down, could have burned or assimilated all of them. The fact that he does not do this suggests strongly that he is human.
Are both infected? Again, probably not. Some say the dialogue is awkward and cryptic, but there's no strong evidence whatsoever to assume this. The theory of Mac being infected because he gives Childs the bottle and then laughs is not strong; we never see the Thing employ the "single-cell assimilation" technique at all during the film, and we don't even know if it's possible. His actions are not strange; Mac had previously acknowledged that no one was going to survive. He says to Nauls and Garry "We're not getting out of here alive, but neither is that Thing." Two comic book sequels were made in which Childs and Mac survive, pursued by the Thing. At the end of the second comic it is revealed that Childs is the creature. Whether or not these comics are canon is unclear. There is also a PS2 video game which carried on where the original film left off. In the initial scenes of the game Childs' body is found indicating that he was human. His cause of death is not made clear. At the end of the game the main character is rescued by a presumably human Mac flying yet another helicopter. Together they defeat a huge and grotesque incarnation of the Thing.
Conclusion, and My Own Take:
And at last, we have come to the finale of this long winded answer. Unfortunately, we don't know who was infected, if anyone, and we don't know what happened to Mac and Childs after the last shot of them sitting in the snow, waiting to die.
Having watched this movie at least 20 times, and being completely obsessed with it for several years, I will offer my two cents:
Both characters had ample opportunity to be assimilated, as each of them spent a significant amount of time off screen and alone.
However, Mac kills several Things, and little or no time passes between him killing the Blair-Thing and him sitting down in the smoldering ruins of the outpost. As far as we know, the Blair-Thing was the last Thing (unless Childs is infected, but Childs had no opportunity to infect Mac) so it seems that Mac probably isn't infected at the end of the film.
It wouldn't make sense to say that both men are really Things, because they would recognize each other's Thing-ness and drop the pretenses of humanity, pretending to be suspicious of each other, and coming to an uneasy standoff because neither of them has the strength to kill the other. If they were both Things at the end, wouldn't they be more likely to transform into sled dogs and run off to find more people? Or try to build another spaceship or other vehicle to reach a more populated area? Or even just celebrate their victory over the humans, fantasize about what they'll do when they reach a major city, and prepare to go into another cold-induced hibernation until a rescue team shows up? Basically, if both of them were Things, we would expect them to do anything but treat each other with exhausted suspicion, accuse one another of being Things, and share a bottle of whiskey.
The unreleased ending makes it quite clear that Mac is not a Thing. Producer Stuart Cohen describes this version of the ending - which was filmed, but never released - on his blog:
MacReady awaiting a final blood test at McMurdo Station.
This was actually one long shot filmed at Rob's special effects facility, Heartland, on a day sometime after principal photography when Kurt was available (we also did two additional scenes with him that day, but that's another story). I was there when John filmed it, and what I remember was a very deliberate camera dolly down a deserted industrial hallway, slowing panning left and eventually coming to a stop at an open doorway to reveal MacReady alive, alone, and shivering seated on a gurney at the far end of the room. I think he was dressed as he was in the final scene, with the same blanket wrapped around his shoulders. No Childs, and no other actors or extras.
My impression at the time is that this was an honest but half-hearted attempt to provide closure (and clarity) for those who were clamoring for it, and John was never serious about using it. For one thing, the corridor used at Heartland had a sort of harsh tech whiteness to it - we didn't bother to paint it, and after spending the entire film carefully crafting the use of color this didn't look like the sort of image John would want to end on... Never tested or screened, I last saw this piece of film sitting off by itself at the end of an editing room bench...
- Stuart Cohen, The Original Fan: The Ending You Almost Saw
If Childs is a Thing, it is strange that he doesn't try to assimilate Mac, since he is virtually defenseless and almost frozen to death.
On a related note, if Childs is a Thing, and if he had no intention of assimilating Mac, why would he even reveal the fact that he was still alive? It would be at least possible that Mac would try to kill him. Wouldn't it be safer for him to stay away from Mac, if he was a Thing and had no intention of assimilating him?
Considering all of these facts, my impression is that neither Mac nor Childs is a Thing. Some portion of one or another of the Things may have escaped the destruction of the outpost, but chances are the two men we see at the conclusion of the story are both actually men, not Things.
Note 1: Although neither of them are canonical, the original story upon which the film is based, and the short story it inspired, are both relevant here.
The original story is called Who Goes There?, and was written in the 30's by John W. Campbell. In this version of the story, there is no one named Childs, but MacReady survives, uninfected, along with about half of the other 30 people in the camp. In the recent short story The Things, by Peter Watts, Childs is a Thing, but MacReady is not.
Again, neither of these works are canonical, but I thought it was worth mentioning.
Richard Valorum has raised a point that has been made many times before, and while it may, at first glance, appear to offer a conclusive answer, unfortunately, it simply doesn't hold up to scrutiny. People frequently suggest, as did Richard, that in the final scene of the movie, Childs' breath is not visible (at least not as much as Mac's breath is). There are several reasons why this argument doesn't work.
For starters, Childs' breath is visible, although it is less distinct than Mac's. More importantly, there is absolutely no reason to believe that the breath of a Thing is any different from the breath of a human. This scene, showing the death of the Bennings-Thing, clearly shows a cloud of steam coming from his mouth:
And finally, members of the production team, as well as the actor who played Childs, have said that Childs' breath is less visible because of the conditions on set, particularly in regards to the lighting. Mac is resting his head on the brightest source of light in the area, and he is breathing very heavily. This makes his breath quite easy to see. Childs, on the other hand, is sitting in the shadows, and isn't breathing as heavily as Mac. This makes his breath harder to see, but not quite invisible. At the 1:27 mark of the clip Richard posted, you can see a very slight trace of steam coming out of his mouth.
Note 3: I found this reference to Carpenter's idea for a sequel on the Wikipedia page for the movie, which appears to suggest that both Childs and MacReady were still humans at the end of the film:
In 2004, John Carpenter said in an Empire magazine interview that he has a story idea for The Thing II, which centers around the two surviving characters, MacReady and Childs. However, Carpenter felt that due to the higher price associated with his fee, Universal Studios will not pursue his storyline. Carpenter indicated that he would be able to secure both Kurt Russell and Keith David for the sequel. In his story, Carpenter would explain the age difference of the actors between the two installments by having frostbite on their face due to the elements until rescued. The assumption of the sequel would rely on a radio signal being successfully transmitted by Windows before Blair destroyed the communications room. Thus, after the explosion of the base camp, the rescue team would arrive and find MacReady and Childs still alive. Carpenter has not disclosed any other details.