I think its use has been replaced by something else by now in most parts of the world, but why do they choose the drugs they choose for “truth serum” purposes?
(The results of a narco test aren’t admissible in court)
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There is no such thing as a truth serum. You can’t force people to tell the truth. There’s no “lying center” in the brain that you can selectively switch off.
Lying is harder than telling the truth. The truth doesn’t require you to make stuff up. Making stuff up isn’t so hard, but making it believable and coherent is. The truth is always self-consistent. You don’t have to make all the pieces fit when telling the truth – they already fit by default. But when constructing a lie, you have to think hard to make sure you don’t contradict yourself, or any facts that the other person already knows. And with every question you are asked, you have to consult your memory about the lies you’ve told so far. Also, you have to suppress the urge to simply answer the question truthfully, which is what people are naturally inclined to do.
In short, lying requires all this hard thinking and self-control. What putative “truth serums” often try to do is shut down the parts of the brain that are necessary to do that. Lower inhibitions and suppress higher cognitive functions. In theory, that could cause a person to tell the truth simply because they lack the necessary functions for lying. However, when you shut down all these things, it does a lot more than just make it harder for the person to lie. By suppressing the person’s self-control, they can’t inhibit themselves from responding to any impulse that presents itself. They will likely find it very hard to focus on your questions and be very distractible. As in, if they see you wearing a hat, they might comment on that with the first thing that pops into their head, rather than answer your question. This impulsiveness may also interfere with their truth-telling. Without proper inhibition, thinking and speech can become very associative. Meaning they might start off answering your question, but then what they’re saying triggers all kinds of associations and that derails their story in a completely irrelevant direction. Like “Johnny? Yeah I saw him at the bar. I like bars, they serve nice drinks. I drank a whisky once that had a snake in it. I hate snakes….” and so on.
Worse, when you remove a person’s inhibitions, they become very suggestible and their memories become very open to fabrication. So even if they seem to be confessing to something or giving you relevant information, they might simply be spinning a completely made-up story in response to what you prompted them with. In other words, they will still be lying – just not deliberately.
At best, these kinds of interrogation techniques can be used as leads for further investigation. For instance, if a person says they buried the bodies in a specific location, you can go search for them there. But you can’t take the statement itself as any sort of reliable confession. Besides which of course it is ethically questionable to be subjecting people to this sort of treatment – especially suspects who have not been proven guilty of any crime.
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