The neocortex is the part of our brain that is responsible for rational thought, and the limbic system is responsible for emotions. The limbic system reacts to the world instantaneously, without us consciously thinking about how we react. Since these reactions happen without conscious thought, the limbic system is often considered the most “honest” part of the brain. It is also the source of “non-verbal” communication.
We can lie with our words (the neocortex), but we can’t fake a reaction caused by a reflex of the limbic system. Body language is based off the changes this section of the brain causes to us externally. It is hard to fake and most people don’t realize that their non-verbal communication speaks volumes.
So how trustworthy is non-verbal communication? The basic message of body language is “comfort”. It is easy to tell with great certainty when someone feels uncomfortable, but hard to judge the exact cause of that discomfort. It may come from the guilt of telling a lie, but it could also come from a fear of being falsely accused.