well first off the body language needs to be interpreted, there may be some people who can tell lies and never be caught by body language and there might be some people who will always be caught by body language.
But perhaps the people who will always be caught by body language have issues that cause them not just to be reliably caught when lying but to mislead the interpreters of their body language into thinking they're lying when not.
Given that part of the task of interpreting someone as lying by body language also involves understanding cultural differences, and as a human activity is prone to human bias, it might be beneficial for rational people to discount the use of body language interpretation in determining lies completely as an unmeasurable process.
In short body language may not be completely meaningless, but it is probably best to treat it as if it were.
Unless you know someone very well, you'll be missing context when interpreting body language. Even then, sociopaths and con artists can seem extremely well known to you and you won't have a clue that everything you think you know is a lie.
Actors train to control this. You have to wonder if so many Hollywood marriages fail because neither partner can trust that the other isn't just really good at putting on a persona.
If you are having difficulty understanding a problem, try drawing a picture.
the method of drawing a picture is pretty well understood, thus understanding the limits of the method also are.
However understanding the limits of using body language as a lie detector in any instance where used requires understanding the biases of the person doing the detecting (which may be yourself, and knowing one's self is traditionally a tricky thing), what their current mental state is in, the culture that the person who may be lying is from, do they have any ongoing problems or conditions that might make them appear to be lying when not, is the potential liar able to mask their lying better than most people, and probably a few more things that are hard to pin down that I haven't thought of here.
Getting the heuristic to work in reasonable, measurable way so that one can decide is this giving a good approximation of a correct answer, am I making progress using this, is an important part of having an heuristic. As an example, common heuristics used in construction or software project management.
So I would think heuristics that are well understood and that do not depend on imponderable questions that philosophers have been arguing about for millennia should be used, but ones that are not free of these problems should not.
Not everything is political BS. Some things are just common sense. People can read body language and sometimes they can tell if someone is lying, especially if they know the liar well.
I can usually tell when my 3 year old daughter is lying because either her speech is quieter, or she acts excessively generous. It also helps that she always blames one of the dogs, typically a stuffed one.
It's funny the stuff she thinks she needs to sneak. A couple weeks ago I stumbled upon her in the back bedroom, shamefully eating a graham cracker. We celebrate her willingly eating anything that isn't pure sucrose...
“Common sense” is just a modern positive term for folk mythology; and particularly when it comes to reading “body language” to detect lies, much of that popular wisdom is based on fabrications deliberately popularized by the law enforcement community (not unlike polygraphy, and, until the same fell apart, the FBI’s “fiber science”.)
Relying on body language (not independent knowledge of the facts of the claim) to detect lies is at best barely better than a coin flip and tends to rely (like polygraphy, which is also not reliable) on indicators of arousal and the known false assumption that arousal = deception, but with less accuracy in detecting arousal than a polygraph.
