I play a smoothtalking scoundrel in a GURPS 4e game. In it, I have the Detect Lies skill. Because of the nature of our campaign, which is a murder mystery, I always suspect whoever I'm talking to is lying to me. Pragmatically speaking, I should be using Detect Lies all the time, but I feel like this would get excessive. I'm hesitant because of a previous campaign in D&D, the version of which I don't remember, in which another player was a paladin who constantly used Detect Evil. I found it annoyingly repetitive, and it also seemed imbalanced. I don't want to be that guy in this game.

Am I just overthinking this?


4 Answers 4


As all things in rpgs, communication is key. I've gm'd a lot of GURPS 4e games, and played in quite a few, and detect lies tends to be one of those skills almost everyone gets. There are a few reasons why it won't be as problematic as "Detect evil" could be in some DnD games. First, uses of detect lies are opposed by the acting skill of the speaker. And even if the speaker doesn't have the skill you still will fail your roll every now and then. Thus, you cannot use it to reliably catch good liars, and might fail even catching bad liars. If your whole party has the skill and you all roll, then success is much more likely, but that's a different problem.

GURPS is a system where degrees of success matter. If you win the quick contest of detect lies by a small margin, most GMs won't tell you exactly what the lie is, just that you are pretty sure that the speaker was not being sincere. I'm not aware of any rule for where the gradient should be, but "knowing exactly what was true and what wasn't" is quite clearly a complete success, that I'd only give on a 5+ roll. So in mystery games it can add a new dimension to the investigation, especially if the rolls are made in secret so you're not aware of any botches you made (or simply manage to willingly incorporate in your investigation information you, as the player, know is false).

Finally, what to me is the main problem in this scenario is responding to every statement by any npc with "I roll detect lies". On one hand it's the right thing for you to do as a player to maximize the use of your character skills. On the other, it can be annoying and slow down the game. I think almost any satisfying solution to this problem has to come from the GM side. Here's what I've done:

  • When a roll to detect lies is likely to succeed, make the lie obvious when roleplaying your npc. Look away, stammer, use "mmm" a lot or whatever mannerism you think fits the npc best. The players tend to catch on rather quickly, and then you don't actually need to roll to make the skill useful.

  • Try not to have too many back-and-forth conversations where the player feels like anything can be a lie. Obviously in an interrogation this won't be possible, but for that:

  • Roll at the beginning and then only for any specific important statement. That's the best balance I found between "roll once and forget", which can be too swingy for my taste, and "roll at every sentence", which can be tedious and unnecessary.

  • Roll in secret at every sentence. This is a solution I don't really like, but in convention games or whenever there are disagreements and your players insist you should roll every time, this is the less obstructive way to do it.

Finally, two things. Even in a murder mystery, you aren't likely to suspect every single word by any npc. This is a bit campaign dependent, but try to only roll when necessary, not just to "make sure" on irrelevant information. And, when in doubt, communicate. As you've seen most solutions to this tend to come from the GM side, so tell them your problem and they'll hopefully take action to accommodate you. And if you suspect your fellow players can be bothered by your use of the skill, ask them for their opinion in a neutral, non-accusatory way. If the end result is "we don't want detect lies in our table", then redistribute those points with your GM's permission and take note of this for the next session 0 you participate in.

  • 1
    \$\begingroup\$ Isn't Detect Lies also one of those things the GM should roll for players, because they won't generally know whether they've succeeded or failed? \$\endgroup\$
    – Zeiss Ikon
    Feb 6, 2020 at 17:15
  • \$\begingroup\$ @Zeiss I think that's mostly a style preference. I usually roll in secret in big games or when playing with people I don't know and let everyone roll as they wish on my usual table. I don't know of any rule specifying which rolls should be secret and which shouldn't. \$\endgroup\$
    – LordHieros
    Feb 6, 2020 at 17:50
  • \$\begingroup\$ Yep, the Basic Set couches it as a suggestion (however, as I recall, using "should" rather than "may want to") in the "how to GM" section, not as a rule, but I've always thought it made the game better. Just makes it easier for even experienced players to avoid metagaming. \$\endgroup\$
    – Zeiss Ikon
    Feb 6, 2020 at 18:12
  • \$\begingroup\$ @ZeissIkon Where/how do you see the Basic Set showing the GM rolling Detect Lies as merely a suggestion? I see "the GM rolls". Also, I think it needs to be this way, or else the player could see from the roll whether he succeeded or not, which would be illogical and vastly increase the accuracy of the information. \$\endgroup\$
    – Dronz
    Feb 6, 2020 at 23:13
  • \$\begingroup\$ @Dronz I was going from memory, and might well have conflated versions going all the way back to 1986. Or I might just have remembered it wrong. \$\endgroup\$
    – Zeiss Ikon
    Feb 7, 2020 at 12:21

It's called Detect Lies, not Solve Mystery.

Obviously the final arbiter here is your GM, but a good way to approach this problem is to treat it as a mirror, because GURPS is often a pretty symmetric game. When your scoundrel tried to lie to somebody, how would your GM rule that you should roll? Every sentence, every essential falsehood, or just one roll to cover the whole conversation? Asking for the same consideration when it's your turn to be lied to would be a good place to start.

But "only the murderer lies" is for logic puzzles. People can lie about all kinds of things and for all kinds of reasons, so it's not guaranteed that your GM has made a murder mystery that you can solve only and exclusively by working out who's lying to you. Here are some common scenarios:

  • The likely suspects all suspect each other, and will lie to deflect suspicion. No one suspects the murderer, who has fled the scene under some pretext.
  • The murderer has duped some gullible patsy into believing they did the deed instead. The patsy completely believes they're the killer and will lie (but not well) to protect themselves.
  • Johnny Law isn't about to haul some upstanding citizen into court for murder on some known scoundrel's say-so. Where's your evidence?

Even in the unlikely scenario that you ace every roll and know who's lying about what, all that it gives you is an advantage in the investigation.

If your GM hasn't taken your abilities into account and their scenario would be ruined by a successful Detect Lies roll at the right time, you'd probably find out about this when you approach them to talk about when it's a good time for you to use Detect Lies. All the more reason to have that conversation!


Watch Lie to Me, Read Mysteries, and Don't Worry

These two sources should be enough to show how to avoid most of the lie-detection problems you seem to be worried about.

Lie to Me is very useful because it demonstrates mysteries where the whole party has near-impeccable lie detection ability (which is better than most RPG parties can boast). Sure, sure, a non-interactive medium is easier, I know. But many of the lessons still apply. Some of the most important ones:

  • A lie detection need not be an end of a quest, it can be a starter.
  • You know an NPC lied, you may even know which part of a statement is a lie (depending on detection method used), but you don't know the why, which can be very important.
  • Finding what the truth is not necessarily easy even if you spot a lie.
  • Diverting an outrageous question ('Did you kill the Mayor?') is not too hard, and pressing on or repeating it will have social repercussions, especially against powerful people.
  • Speaking of which, gaining audiences with some suspects or witnesses in general can be a challenge worthy of the adventure.
  • Suspects and witnesses can be wrong or ignorant and this can still necessitate additional investigation or occasionally cause wrong conclusions if one doesn't have the full picture.

Compared to that, Mysteries is more of a general advice, but it does show many sides of investigation other than pure lie detection. But of course a whole book will not fit in an answer. The lack of modern and/or futuristic forensic techniques can be compensated for with magical substitutes (if your campaign has those). Be warned however, that some of the crunch published in Mysteries doesn't have the polish that later books have (e.g. Social Engineering has a better update on nuanced Influence rolls, and Enhanced Senses has some better info on hearing and vision).

Bonus advice: since you want to use Detect Lies a lot, I suggest you take the Standard Operating Procedure Perk (from Power-Ups 2: Perks, of course) and talk to your GM about minimising unnecessary rolling by entrusting the decision about the importance to the GM. Unless you don't have a sufficient level of mutual trust and understanding, of course.


Imagine how you would detect a lie in real life. Asking somebody who doesn't feel any pressure an answer which can be answered in one word without knowing any details will not succeed. That's why real life investigators don't ask for the big picture.

If players outright ask "Are you the killer?" lie detection should fail. They have to figure out details first.

The first question might be "Where were you when the victim was killed?". The answer might be detected as a lie. This wouldn't be the end of the mystery, it would be the start of the subquest of figuring out where the person really was. Maybe he is the killer? Maybe he commited adultery?

Another question could be about the way the victim died. Was it poison? And if so which reason could the alchemist have to lie about the origin of the poison? Does he want to cover up his criminal connection to the black market? Or did he sell the poison to the killer?

And who found the victim? And where? Maybe it was moved? Who might have a reason to do and lie about it? Who might have seen it?

If the players have figured out enough details they can confront the killer and he will have a way harder time to patch up a story that matches all the details the players now know.

Also keep in mind that lie detection only works as long as the person you are talking to knows that he is not telling the truth. What if he has been fouled, bewitched or lied to himself?

Of course it is up to the GM to create a mystery that not only accounts for the players being able to detect lies, but to incorporate this skill as an entertaining way to solve parts of the puzzle.

  • \$\begingroup\$ This might be improved by pointing out the player's previous experience is not really the same - a D&D Paladin's Detect Evil ability is magic, and doesn't need to follow any realistic principles. \$\endgroup\$
    – T.J.L.
    Feb 7, 2020 at 13:57

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