Well, you'd need to reward the player for deciding to search for the truth even if it's uncomfortable, i.e. the player would have the choice between consequence X and the truth (and consequence Y), where consequence X is somehow more attractive.
For example, X could be an immediate, straightforward reward (gold, XP, completing your quest, finishing the level), and Y could be a rare achievement ("truth seeker!"), or Y could only become apparent in the long term (giving you access to the Temple of Truth later in the game), or it could open up some options ("unlocked character class: Monk of Truth").
You could also make it known to the player that in the past, he did not always find the truth ("Oh, and that swamp monster you killed last week? When the head inquisitor examined the remains, he saw it was actually the farmer's daughter under a curse. Don't worry, we won't tell her father, he's just some redneck, who cares? Now your next mission ..."), which could encourage him to think and investigate a bit more next time.
Example: a cooperative multiplayer game, the players are exploring/colonizing a dangerous planet; each is the head of a probe dropped at one point of the planet, and uses robots to explore the environment, dig for resources, experiment, etc. The only communication between players is by broadcasting research notes about the environment: "red trees are very dangerous", "areas containing yellow bulbous crawlers tend to be good mining sites for zirconium", "sector H41 is a safe place to build a processing plant", etc. Each player has a reputation score, and you win reputation by broadcasting notes, and lose some by changing your mind. A mothership orbiting the planet beams down a fixed amount of resources and energy each day, and players with a higher reputation get a bigger share (but it stays zero sum). The environment should be hostile enough that the whole expedition has a chance of being wiped out if the players make bad choices. So the players have an incentive to tell the truth to maximize the efficiency of the expedition as a whole, but each player has an incentive to make claims too early and not revise them to get a bigger share of the daily resource beam (this could be improved a number of ways!)
Other example: standard fantasy RPG, complete quests, kill orcs, collect gold and XP, have a "reputation" with various factions. But sometimes the NPC giving you a quest is lying a bit - the old woman looking for her cat is actually a necromancer, the farmer complaining about the orcs actually settled in the middle of their territory, etc. - this sometimes won't matter, but may sometimes have nasty consequences if you're too gullible. You can complete the quest as usual (and get the normal reward). You may get increases to your intelligence stat if you see through lies (just as you'd get an increase in strength for hacking orcs).
I'm sorry, I can't quite put my finger on it, but "I notice that I am confused"... that is, tere's something about this proposals that doesn't quite satisfy me... perhaps we're in a Double Illusion of Transparency?
This article aims to prove that Ace Attorney is possibly the first rationalist game in the lesswrongian sense, or at least a remarkable proto-example, and that it subliminally works to raise the sanity waterline in the general population, and might provide a template on which to base future works that aim to achieve a similar effect.
The Ace Attorney series of games for the Nintendo DS console puts you in the shoes of Phoenix Wright, an attorney who, in the vein of Perry Mason, takes on difficult cases to defend his clients from a judicial system that is heavily inspired by that of Japan, in which the odds are so stacked against the defense it's practically a Kangaroo Court where your clients are guilty until proven innocent.
For those unfamiliar with the game, and those who want to explore the "social criticism" aspect of the game, I wholeheartedly recommend this most excellent article from The Escapist. Now that that's out of the way, we can move on to what makes this relevant for Less Wrong. What makes this game uniquely interesting from a Rationalist POV is that the entire game mechanics are based on
That the judicial system is Japanese-inspired also means the legal system is inquisitorial: the court has an active role in the case (whereas the adversarial system in the West reduces the role of the court to a form of referee) and its (alleged) mission is to dig out the truth. That and the lack of in dubio pro reo mean you can't just be content with putting your client's guilt in reasonable doubt, you have to thoroughly prove their innocence and find the true culprit and get them imprisoned. That means you have to find out the entire story and you can't leave any threads hanging.
Additionally, the fact that you are a lame attorney facing an unsympathetic judge and egomaniacal, dirty-playing, high-status prosecutors who *have led the police investigation and only prosecute when they think they have all the cards in their hand* means you will. not. catch. a break. Every single move you make will be scrutinized, you will face constant sarcasm, dismissal, condescending and ridicule, and sometimes a single mistake on your part (presenting the wrong piece of evidence) can cost you the entire case. This game forces you to take an unflinching stand for the truth in the face of every social sanction imaginable (including, obviously, attempts at your own life). Of course, the plot goes out of its way to make things difficult for you: everyone is as unhelpful as possible, and even your clients need to have the truth pried out of their mouths with the determination of a dentist. Other witnesses can cast remarkably subtle webs of lies that really force you to think out of the box in order to find their weak point. And, since the cases are Always Murder, your client's life is always on the line, and that's when you don't have another person in grave distress. This serves to motivate you and draw you into the story, but it also adds to the constant pressure you are in to find the truth.
But that's not all. In the latest sequel, Ace Attorney Investigation, you take the role of Miles Edgeworth, a prosecutor who Defected From Decadence and restricts himself to ethical methods in crime-solving, eschewing the questionable methods he used in the past, and which most of his colleagues still practice with abandon. The battle doesn't take place in court (which, unless Phoenix or his successor Apollo are defending, is but a formality) but during investigation, which is where the case is won for a prosecutor (if they aren't certain they have enough evidence to get a conviction, prosecutors just don't... er... prosecute). This means you have to investigate the crime scenes, interrogate the suspects, and find the connections between the clues in order to reconstruct what happened. This is represented in the game by an entire gameplay mechanic for logical deductions (and a fair bit of Will Mass Guessing) that are hilariously over-the-top, concluding with a literal "Eureka!". The interrogations are no piece of cake either: oftentimes, (and, surprisingly, realistically enough in a police investigation) you have to take your suspects through excruciating logical baby steps to break their lies, since they can rely on something as cheap as semantics. Actual Eureka Moments, that is, sudden piecing of mental puzzles in a moment where deductive thinking is stalled, thanks to someone saying something unrelated that just happens to trigger the right association, is also a common phenomenon during investigation: composing a good hypothesis with nowhere near enough evidence is, of course, another rationalist skill, one that is underrated by modern Science as it is now.2
So, to sum it up, what virtues does these games teach?
I rest my case.
1. Sorry, couldn't resist the reference: I'm just that geeky. Sue me.
2. That, and, honestly, who could resist a game that names one of it's tracks "Logic, The Way To The Truth" and, when winning a case, "Solution! Splendid deduction." "Cornered", which plays when you are punching a hole in a witness's declaration that is so huge it could swallow galaxies, leaving them no room whatsoever to continue with their lies and often leading to spectacular villainous breakdowns(MASSIVE SPOILER ALERT), remains an all-time classic. (One clip is even peppered with quite interesting quotes on Truth.)