This is complicated but I will try to explain simply:
In western culture, (english speaking countries) and I am guessing that most of the people on this board are from them, we are very 'mind' orientated. For hundreds of years we have considered most emotions (especially for men) to be a weakness. The same is in eastern asian culture.
In all humans you have emotions urging you to do one thing, and your mind telling you to do another thing. Sometimes they want the same thing. But most of the time they are in battle, consciously, and sub-consciously.
For most people, when you are straight your mind has control, and your emotions sit in the background, (unless you are schizerphrenic or manic-depressed).
For most people, when you are on drugs, especially 'feeling' drugs like MDMA or LSD, your emotions are in control of your actions, and your mind sits in the background.
Of course, on coke and amphetamines your brain is still working alot, but your emotions are enhanced as well.
When you are child your emotions rule, but as you get older and your mind expands, society and family change you so that your mind gains control of your emotions. You learn to control yourself.
In most societies, we spent most of our growth on mind (logical growth) like school, and spend very little time on emotional growth (especially men) so that is why we have a society full of emotionally retarded people, but many of them have great minds. (eg most scientists or computer programers are a good example of this).
If you want to read more about this, read the book, "the Art of Happiness" by the Dali Lama and some american psychiatrist I cannot remember the name of.
And about MDMA being a truth drug - I think in small doses in a controlled enviroment (not a rave) it is a very truthfull drug (example: psychotherapy - read "Ecstasy: the complete guide" by Julie Holland M.D. to learn more about this)
But obviously if you take lots of pills in a crazy enviroment like a rave, and you mix other drugs (like weed), and your brain becomes scrabbled eggs, then obviously you are going to start saying crazy things, that may not be truthful. I do not think it is lying, I think it is just a mis-wired brain out putting garbage.