Example: heavy breathing and moaning, despite being as easy to fake as words are easy to speak, are seen as “truer” messages than words. If your lover says “do so and so” in bed he might be lying. If he whispers and moans while you do so and so, he might just as well be lying, and for the same reasons.
Nevertheless, “discovering” his intentions through his non-verbal behavior is seen as more reliable, more spontaneous, than being told so. Reading his body language is not seen as reading at all. The artificiality of reading seems to not be present in the body language.
I believe this shows, more than anything else, a prejudice against intentionality.
If you say something you are believed to have thought about it, and to have chosen by will to say so. Saying is supposed to always be an act of calculation, of interest.
I fail to see how body language is less rational than that. Almost all of the signs of body language are possible to be consciously performed. Seemingly unconscious ones, like pupil dilatation or blood-pressure variations almost never can be perceived with only the bare senses.
To give precedence to non-verbal behavior in reading someone’s actions makes an implicit gamble: that you are more able to read the other and his unconscious telltales than he is. But there are plenty of reasons to believe just the opposite!