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As I mentioned in a previous question, I am writing an alternate history story with many different countries by the time 2023 rolls around. Vitruvia is one such country that runs many experiments and creates devices that would be considered wacky and even unethical by modern EU standards. They created a memory destroyer, love potion, and now they invented injectable truth serum.

Causing amnesia or destroying memories can be somewhat explained by neurology (certain areas of the brain are stimulated or suppressed). For a semi-realistic truth serum, the issue is more complex. Lying is an action instead of an emotion or a recording. If I wanted a somewhat hard science explanation for a truth serum, how can a person be neurologically changed so that lying/dishonesty is impossible (while under the drug's effects)?

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    $\begingroup$ Are you looking for generalized structure names? Serums hit everything at once, so you would be affecting a specific set of receptors, not actual physical structures. If you are doing near-future fiction, you could have your bad guys scan the person's brain, then use electromagnets to activate a chemical in a targeted area, but you can't directly target physical locations with just a serum. $\endgroup$ Nov 29, 2023 at 22:01
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    $\begingroup$ There's an interesting and somewhat related article on method actors getting a brain scan you may find informative/interesting and the implications - possibly confounding. $\endgroup$ Nov 29, 2023 at 22:07
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    $\begingroup$ I'm not entirely sure we understand the brain well enough to answer this question to the expectation of science-based. Lying is a choice. For those low on the sociopathic scale, the effort of lying changes body dynamics (useful for polygraph operators). For those high on the sociopathic scale, the concept of lying doesn't really exist, there's only the momentary truth that achieves a desired goal. Interesting question, though. I have a thought, but it's a science-fiction level rationalization rather than a science-based explanation. Is that acceptable? $\endgroup$
    – JBH
    Nov 30, 2023 at 0:48
  • $\begingroup$ @JBH, Lying isn't a choice. It's a skill. This particular skill relies upon our future-simulation centers and imagination capacity. You can't lie if you can't make things up. You can't lie convincingly if you can't compare your fabrications to what might actually exist. You won't lie unless you're motivated to do so, and motivations are heavily hormonal. Try this on for size: If you're delusional and you describe reality as you perceive it, are you lying? $\endgroup$ Nov 30, 2023 at 4:58
  • $\begingroup$ @RobertRapplean Semantics. Using a hammer is also a skill... and you need to choose to use the skill. The only things in life that aren't skills are human autonomic functions... and there are a fair number of people in the world who can demonstrate that even those can be developed as skills. I don't address the use of imagination in my answer because I'd already loaded it with three options. Frankly, every aspect of the brain could be used in a story to rationalize a truth serum. It's just a matter of connecting dots in a cool way. $\endgroup$
    – JBH
    Nov 30, 2023 at 6:55

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I'm going to shift from to . This will make sense as you read the answer.

Lying is a choice

Current thinking about the psychological processes involved in deception holds that people typically tell the truth more easily than they tell a lie and that lying requires far more cognitive resources. First, we must become aware of the truth; then we have to invent a plausible scenario that is consistent and does not contradict the observable facts. At the same time, we must suppress the truth so that we do not spill the beans—that is, we must engage in response inhibition. What is more, we must be able to assess accurately the reactions of the listener so that, if necessary, we can deftly produce adaptations to our original story line. And there is the ethical dimension, whereby we have to make a conscious decision to transgress a social norm. All this deciding and self-control implies that lying is managed by the prefrontal cortex—the region at the front of the brain responsible for executive control, which includes such processes as planning and regulating emotions and behavior. (The Art of Lying)

That paragraph tells us some useful things:

  1. We don't actually know what part of the brain encompasses "lying." This will make more sense momentarily.

  2. The part of the brain that might control lying is the prefrontal cortex.

However...

  1. Lying appears to require more cognitive resources. In other words, we need to think about lying while we don't need to think about the truth. Why?
  • We must know there is a truth before we can invent a lie. That's important. Theoretically, a "truth serum" would have no effect on someone who believes they know the truth.

  • A lie must be invented. It may be a preconditioned response (i.e., "practiced"), but it must nevertheless be constructed after the truth is known.

  • Finally, when a lie is told, the truth is suppressed. We can assume a practiced liar is someone who's good at suppressing the truth, but I can't find any science to back that idea up. In other words, it's like lifting a 4kg weight. We can practice to the point that it's "easy" to lift the weight, but we must lift the weight, nonetheless, and we know we're lifting it regardless the ease.

But the choice requires a reference

Beliefs are not just cold mental premises, but are ‘hot stuff’ intertwined with emotions (conscious or unconscious). Perhaps, that is why we feel threatened or react with sometimes uncalled for aggression, when we believe our beliefs are being challenged! Research findings have repeatedly pointed out that the emotional brain is no longer confined to the classical locales of the hippocampus, amygdala and hypothalamus. The sensory inputs we receive from the environment undergo a filtering process as they travel across one or more synapses, ultimately reaching the area of higher processing, like the frontal lobes. There, the sensory information enters our conscious awareness. What portion of this sensory information enters is determined by our beliefs. Fortunately for us, receptors on the cell membranes are flexible, which can alter in sensitivity and conformation. In other words, even when we feel stuck ‘emotionally’, there is always a biochemical potential for change and possible growth. When we choose to change our thoughts (bursts of neurochemicals!), we become open and receptive to other pieces of sensory information hitherto blocked by our beliefs! When we change our thinking, we change our beliefs. When we change our beliefs, we change our behavior. ... Total brain function is required in stabilizing the belief and in responding to environmental system. Some of the brain regions and the neural circuits are very important in establishing beliefs and executing emotions. Frontal lobes play a major role in beliefs. Mental representations of the world are integrated with sub-cortical information by prefrontal cortex. Amygdala and Hippocampus are involved in the process of thinking and thus help in execution of beliefs. NMDA receptor is involved in thinking and in the development of beliefs. (The biochemistry of belief)

There isn't a specific part of the brain that regulates "lying." There are parts of the brain that are involved with the choice to lie. As previously mentioned, the ability to lie begins with knowledge of the truth — but what, really, is truth? It's one thing to stick our finger into a light socket and thereby learn the "truth" about electricity. It's another to be told the location of a pot of gold... and believe it's the truth.

So, while the prefrontal cortex could be used to simplistically rationalize a chemically-induced inhibition to lie, a more complex solution is a chemically-induced change to the frontal lobe that strengthens beliefs. The stronger the belief, the harder it is to lie about it because denial of the belief is emotionally connected (and this is my opinion) to your personal sense of self-worth.

But it could also be said that, since many aspects of the brain serve to filter incoming sensory data through our systems of belief, that the chemical reduces the strength of those filters, resulting in a higher likelihood that what is said is "the unvarnished truth" (as the police would say, "an excited utterance") and not a fabrication, consciously or unconsciously. However, since this isn't addressing just one aspect of the brain, the onus would be on you to explain the more complex consequence of the drug since the affected area of the brain is no longer carrying a significant burden toward suspension of disbelief.

But there's a third choice...

Finally, we could attack the area of our brain that governs, somewhat generally, that we care about things. The amydgala is an area of your brain responsible for emotional processing, especially fear and anxiety. When you really, really, really simplify things, we lie because we're afraid or anxious. We want a better deal than we deserve, or we don't want to get caught, or we're passionate about keeping a secret... If your chemical stunts the effects of the amygdala, then what you've removed is the subjects ability to care about keeping the secret.

Conclusion

Because lying is a choice and not an autonomic response of the body, it is unlikely that there is just one part of the brain that governs lying. In a complex story explanation, your "truth serum" could (and likely would) be a concoction of chemicals that affect different parts of the brain in different ways. One that inhibits the subject's ability to think about altering the truth; another that strengthens the value of the truth to the subject, making it harder to lie about without causing emotional turmoil, and a third that interferes with that emotional control to reduce the subject's ability to care about the truth as something valuable at all.

Or any other combination of effects. Lowering inhibition, increasing anxiety (the "excited utterance"). Perhaps my main point is that lying is a complex behavior and a suitable world explanation would require an equally complex series of effects that would rationalize the idea of "I can't lie about it" to the reader.

Can you see why I asked if I could answer as ? We really don't know what, specifically, the brain does to affect a lie. We can measure what parts of the brain light up when someone does lie, but we really don't have a clue about how humans do it. That's why I had to step away from .

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If I understand the way that real-world 'truth drugs' work correctly, they appear to work by impairing higher brain cognitive functions, that reduce the subject's ability to consider the consequences of their actions or understand the position that they are in.

Under the effects of such drugs, the interrogator can, as appropriate to the circumstances, either trick or threaten the subject into revealing the desired information, and the subject's impaired cognition makes them less able to resist.

There are quite a number of anaesthetic drugs (including alcohol: ever heard the saying, "In vino veritas?") that have this effect to varying degrees. It is a little known fact that in some countries, when personnel with knowledge of particularly sensitive state secrets must be administered an anaesthetic for medical procedures, they are required to be accompanied by a person with the same clearances who can supervise them when going into and coming out of anaesthesia, in order to distract them if they begin to talk about things they shouldn't.

However, such drugs are not foolproof, as some people have proven resistant to them, or have developed methods to counter their effects, Such as 'only tell secrets when somebody says the magic word' self-hypnosis, in which the magic word isn't "please", or espionage tradecraft that means that spies and couriers don't know each-other, because they don't ever meet.

However, there are other methods that can be used to interrogate people, such as the 'Aha!' response detectable in brain-wave analysis, in which interrogators can show the subject evidence of a crime (or whatever), and see if the subject recognises it. It is a technique subject to false positives, but the right calibration positives and experimental negatives can rule a subject out with a high degree of certainty. While this method doesn't give an interrogator anything they don't already know or suspect, it can at least point them in the right direction, and help them determine who to drug in order to try to get more information.

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