Questions relating to sticking out one’s tongue can turn out to be surprisingly complicated. Its meaning in babies and children may be quite different from adults who do the same thing. Nuances abound: Is the tongue sticking straight out? To the left? Right? Hanging down? Or might it actually be curled? What’s the accompanying facial expression and context in which it occurs? To add to the question’s complexity, tongue protrusion can signify one thing in one culture and the opposite in another.
This body language quirk is fascinating because of how variable its connotations can be. Exploring the subject on the Internet, I could find no definitive explanation of it. That is, no scholar appears to have covered tongue protrusion systematically, comprehensively, and cross-culturally. So at this point it’s safe to say that no ultimate, authoritative source yet exists to account for the phenomenon. And frankly, to do so would be no easy task, for—despite its universality—it’s extremely relative.