The Genius AI Behind The Sims 

Want more deep-dive explanations into video game AI? Check out this video about how Shadow of Mordor's Nemesis system works - ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-Lm_AzK27mZY.html

I love when the ground-breaking AI makes my sims wash their dishes in the bathroom sink.

Fun fact: Every Sim is actually controlled by a minimum wage EA employee

I was an AI programmer on The Sims Medieval (which was based on The Sims 3) and the lead AI programmer on The Sims 4. This is pretty spot-on! There's more to it of course, but the core of the AI system is just as described here. Things like visiting other sims were called Situations which helped define some of those rules. They weren't hard-coded, we managed it all with buffs, mostly created by designers. Buffs can be thought of as tags and you could have tests on interactions that said "don't allow someone with this buff to do this thing". So if you visited another sim, you got a visiting buff which prevented you from doing things that were inappropriate. You could also be invited to stay over, which was a different buff that relaxed many of those rules. Beds were a special case; they were owned by different sims so you would never autonomously use someone else's bed unless you were very close (like romantically involved). Socials on The Sims 4 were governed by a special autonomy mode called subaction autonomy, which was a much simpler weighted random compared to the complex utility system of full autonomy. Designers would tune each social and weights for things like reactions and which social was chosen. Weights were adjusted by traits, buffs, and the short-term context (the short-term companion to the long-term relationship score). Short-term context models things like "I love my wife, but I'm pissed at her right now", though in practice it was more like "this conversation is tanking" or "this is a really funny conversation". For example, when you're laughing and joking around with your friends, you're more likely to laugh at jokes than if you're just told a joke out of nowhere. This is even more true if everyone is laughing (that's why sitcoms have a laugh track). For things like venues (restaurants and so on), we used the same core system. It's a Situation under the covers. Meta autonomy pulls in sims to fulfill different roles based on filters. It tries to use existing sims but if it can't, it creates a new sim. Townies are sims that don't live in any specific lot in the world. Any sim you can interact with are fully simulated. The auto-satisfy curves are used when you, the player, enter a lot with a bunch of sims that are already there. We use that to determine starting motives. The low LOD simulation is very, very light. It's mostly just story progression (getting a new job, promotion, etc.) One thing the video talks about at the end is the ambiguity of the AI. This was very intentionally designed. Sims need to be reactive and always living in the moment. So while you might have a sim with an ambition to be a movie star, very little actually causes them to do that. They might enjoy movies more, for example, but not much else. Sims don't plan in any way. They run an AI tick and choose something to do based on motives, traits, mood, and so on. We've experimented with planning, but it caused confusion because players didn't know why their sim was doing what they were doing. Playtesters felt like they didn't have as much control. Simlish was created by Stephen Kearin and Gerri Lawlor, two incredibly talented improvisers. I recommend looking them up, they're both awesome! (I've been doing improv for many, many years and I've had the pleasure of working with Stephen on several occasions.) Sims being gay or bi vs straight is not entirely for storytelling, it's because of other countries. The design is that if you never ever initiate any gay content, you will never see gay content. This helped us get around laws in countries like Russia. That said, the video is correct that your player-controlled sim will never romance another sim, though non-player controlled sims can get married through story progression. Still, you won't see two men or two women get married unless you directly make it happen because of this issue. There is a sort-of yes, and idea in the sims, but it's less designed than you might think. Basically, we have a set of interactions that can never be done autonomously, like quitting your job, getting married, and so on. We have the player make the big life decisions while the AI takes care of the rest. If anyone has any questions, I'm happy to answer them.

My favourite part of the genius Sims AI is when visitors just go play games on your Sim's laptop

My favorite thing about sims AI is the way they choose the furthest sink they can get to wash dishes rather than using the one that is literally right next to them in the kitchen where they eat

When The Sims 2 came out, the Grim Reaper was programmed to perform an action at random after reaping the last sim on a lot. While the reaper performed that action, the timer would expire and the game-over screen would at last pop up. Somewhere on a hard drive from a computer I replaced in 2006, there's a screenshot I took of the Grim Reaper taking a dump in my sim's toilet after I'd killed off said sim in a fit of divine wrath.

The insane trait is the best one because of the fact that it overrides ALL parameters! Leading to crazy situations like romancing the Grim Reaper or fishing in swimming pools!

This is the kind of stuff I love in game design. Behind the scenes, nothing but math. But with clever tweaking it comes together into an incredibly lifelike system.

I always really liked TS3’s AI. When a sim was hungry, they’d go eat on their own. If they were tired, they’d go to bed. If they were married, they’d have romantic interactions with their spouse. If a sim was a book worm, they’d always be reading. I remember my Sim staying over with a guy who was living with his two siblings, and on the first night he read one his siblings to bed automatically. I just love it when sims automatically do cute little things like that on their own. They still did stupid things of course, like always wanting to watch TV or go outside when it was raining, but if they didn’t then what’s the point of playing? And I adored the open world because even though I wasn’t playing with the other sims, they’d still be living their own lives. Some families would separate, the mother being single with two kids while the dad had to move in somewhere else. Or some sims would have a child while the father was living in another home. Some sims would adopt dogs or their kids would move out and live with someone else. I was so invested in what my townies were doing, I’d spend hours in edit town

It is very true that ambiguity and randomness allows people to fill the gaps with their imagination and it makes game characters look smarter or more complex than they really are. Many years ago I made a simple 2D action game in which the enemies could do a number of things, like run towards the player, escape from the player, take cover behind an object, shoot, jump, etc... and they just pick one at random. It wasn't deeper than that. Some tester was very excited at the "complexity" of those enemies and shouted: "they are learning from my behaviour!". What was totally not true, but made me very happy. I think the secret of game AI is not to make them actually intelligent, but to make the player think that they are.

The Sims team is a good example of how a good team can exist inside of a terrible company.

This really goes to show how complicated game development really is. Smth so seemingly simple and potentially randomized is in actuality dozens and dozens of systems working in tandem with eachother, crossing over, all in order to try and predict the unpredictable a.k.a the player's decisions (like the classic "removing the pool ladder")

speaking of maxis, you should take a peep at spore 2008 and explain how the heck they managed to make all the creature parts move, dance, animate no mater what kinda monstrocity you make or how you can import other people's creations just by dragging and dropping in a .jpeg file into your game folder

I love the fact that one of the most human things is to create stories about everything that happens, trying to explain, find patterns and reasons in everything. Even when we know it's pure math and events randomly generated by a computer.

When I played sims 3 I somehow managed to make the entire household into "alcoholics", they would stop in the middle of tasks I set to go back to the juice bar. Once I decided one of the adults to take the kid camping in the mountains, when I looked back at the house he was already there at the juice bar and the kid was cycling home on his own.. great game

The one thing I love about the Sims AI that you didn't pick up on, which is particularly clear in the Sims 4, is that while Sims can survive without you to achieve any sort of fulfilling growth they need your intervention. So bored Sims won't take up a hobby but will just sit and watch TV or game, it take player involvement to develop them into better human beings and ultimately 'win' the simulation

18:35 I remember when I was younger and playing Sims 2 Pets, whenever the speech bubble showed what I thought looked like a spinning top (but was probably a radio dish), I always pretended that my sim was telling other sims his spinning top was better than theirs

Honestly, I would love it if I could sit back and do nothing while playing the Sims. I want them to be able to build relationships, develop hobbies, and take care of themselves without me. Then I could just swoop in occasionally as a benevolent (or hostile) deity

It is always so cool to learn how smart programmers solve these creative problems in games. I feel they don`t get nearly as much credit as they deserve.

I remember seeing a blog that went into the full detail of how systems like The Sims worked. The whole "objects send out requests, NPCs just wait for an object to tell them it's free" approach; it's really interesting. But the URL lost ownership, and the blog was never archived that I can find. Which is a shame.

The funny thing with the way it's coded, with the understanding of it (made mods for Sims 3), you could make a TV that fills the hunger meter (was just a fun thing I did to understand how the meters worked)

The way simulation game players pick up on EVERYTHING in order to tell a story is so cool. Spore, for all its shallowness, builds on the same ideas of ambiguity and emergence to create a universe in the player's mind. A genius at that sort of thing was Parkaboy, an amazing creator who even implemented the game's constant malfunctions and crashes into his most prolific species' backstory as a cycle of devastating economical crises.

Genius? You could've fooled me. They can't even climb out of a pool without a ladder.

"Little computer people" - I see, and appreciate, what you did there. (For those unaware - Little Computer People was the name of a 1985 doll house style life sim for various 8 bit microcomputers such as the c64. A cute reference to what is probably the earliest example of The Sims' genre)

The way Mark says urinal is the oddest, most British way of saying urinal I've ever heard. 😂

Name something that you took away from this video: “People aren’t ants.”

The Sims for me will never exceed the feeling of going forward in time and seeing my descendants in a Sims 3 DLC. Peak

I like what's said about how ambiguity in a game leaves room for the player to let their imagination fill in the blanks. One place I see this in is the older Pokemon games, very specifically the ones in a pixelated style. During battles, any attack a pokemon does is very simple on screen: the game says "This pokemon used this move", and then it plays an extremely simple animation with extra effects to get the general idea of what the move was across to the player, and then you see how it effected the pokemon. You could look at this as underwhelming, but I actually really appreciate the lack of flare there because as a kid, it always resulted in me imagining in my head a much more complicated fight. It's almost like how when you read a book and hear something like "Dory fumbled-ly kicked off the wall and landed above the monster", its not showing you a specific explicit scene like an animation might show, but instead offers an interesting idea that prompts your imagination to think up something unique to your head and lets everyone share their different interpretations and thoughts.

My, the doll house part. I remember when the game came out and my brothers and friends were raging fanboys for this game and I could never get them to admit it was at its heart a doll house game. Good times.

In all my years playing 3, I never knew the traits themselves acted as needs.

In the other examples at the end, you forgot the most important factor that XCOM's AI judges: optimal player suffering. Seriously though, really value these videos as a programmer myself. The "advertisement" idea was super eye-opening, because I would also be stuck trying to make the characters "know" everything they need to do and what's around them, and that's a better system on just about every level.

I never noticed how smart the AI was, like how they make a grilled cheese at 2 in the morning or walk across the street to talk everyone or die in all swimmable water known to man.

I used to love the Sims 1 & 2. Incredible soundtracks when you’re building or buying things in game. The Sims 4 has a very cool adaptive soundtrack that introduces or removes music stems while you do different things. I hope the Sims 5 does something fun with the soundtrack but I’m also very likely to skip it if it’s full of microtransactions

Great video! The systems behind the Sims and other simulation-heavy games are truly fascinating. As an aside, it seems the current tech hype cycle completely poisoned the word “ai”. Makes it hard to have conversations about the “classic” video game ai.

Love this kinda stuff, just shows how video games are both art and engineering at the same time.

feel like Sims 2 fans are going to give you some heat for that opening statement

This just reminded me of playing The Sims 2 on a Ps2 with my brother on 2-player mode and whenever one of our characters went to sleep, the other had to go on auto while we fast fowarded until the other woke up so we both could play as long as possible. We mostly hated the AI, specially for going for social interactions with characters we didn't care about(we usually only picked one girl to date and only talked to her lol) when we mostly just liked grinding stats for promotions at our jobs.

I just did a bunch of sims research for a documentary and read a lot of interesting things (especially the items broadcasting their needs), I'm so glad you made a whole video about this stuff

Finally some Sims 3 appreciation. If that game wasn't such a technical mess (especially when you throw in all the expansions, some of which are clearly not designed to mesh) it would remain the unchallenged peak of the franchise.

My favorite part of the genius AI is when sims wash the dishes in the bathroom sink.. even if it's on another floor, and there's a dishwasher available

My favourite sim game was probably castaway. There was something about that survival experience that felt really engaging long before the many early access survival games you see today. I kinda wish a similar game was made today, with less of a focus on playing 1 person surviving and more about keeping a group alive and happy.

Never really got into SIMS but you explaining it makes it 100% more interesting.

I'd love to see a follow up video to this which explores why the AI in Sims 4 is SO BAD in comparison - with Sims 3 it was possible to watch a sim for a bit and get a sense of their traits whereas in 4 every sim feels a lot more generic.

I love having the sims 3 up and running by itself while I work. I look over to see where they end up with bits of inputs here and there. The oddest thing that happened was one sim quit their part-time job while on a trip to Egypt. I dont know how or why so I'm still confused there.

This was really interesting, the whole system of having the different needs of a Sim be weighted differently, and then later adding in traits to further modify a Sim's needs hierarchy, is so smart. I also love that objects broadcast themselves, rather than a Sim simply looking for X-Object to fulfill X-Need. That makes SO much sense (and makes it way easier for them to make tons of new items for expansions lol). Speaking of Expansions, if they, say, add in a new trait in an expansion, would they then have to go back and modify old items and let some of those older items be able to advertise themselves as fulfilling X-points for Y-New-Trait, or would they just have only the items/activities in the new expansion fulfill those needs?

If you haven't seen Tynan Sylvester's 2017 GDC talk, "RimWorld: Contrarian, Ridiculous, and Impossible Game Design Methods" you might really enjoy it. He refers to the "filling in" that players do (esp. wrt causal information) as apophenia, and as in The Sims, it was a core part of RimWorld's design.

Oh yea. The genius AI that cancels my actions, gets stuck, goes through objects, keepings randomly getting up and sitting down... love it.

This is a great example of the power of data-driven programming. Even the production rules still act as data, it's just more structured than "quirkiness = 10". As you said, making these things data allows for emergent behaviors and extensibility. More programmers, especially game programmers, need to familiarize themselves with this paradigm.

My girlfriend does exactly as they intended. She plays for hours creating and making these elaborate stories. It truly is impressive the level of simulation achieved and really makes the "Are WE a simulation?" All the more interesting.

so according to the sims 3, sitting at the sofa to watch tv is at the very top of maslow's hierarchy of needs

One of my favorite gaming stories is from the original Sims. My girlfriend at the time was playing it, and she downloaded all sorts of custom skins. She also, like many, meticulously murdered so, so many of her Sims' neighbors and specifically didn't mourn their graves to create ghosts. So this household contained Trent Reznor (Nine Inch Nails), Shirley Manson (Garbage), and The Crow. Trent and Shirley were in a relationship, but Trent would flirt with The Crow once in a while. Shirley hated this, and would beat The Crow up. He would retreat to his castle-dungeon-themed second story room and go out on the balcony and just start crying, overlooking the random available ghosts haunting the 100+ person graveyard. So he's crying to Charles Manson and some of The Spive Girls, and the weirdest thing would happen. His social meter would *go up*. I have no idea what mechanically was allowing that to happen, for it to be The Crow of all people just made it so poetically perfect. I couldn't believe what I was seeing. What a fantastic game.

Have 40h+ in last two weeks in the Sims 3 and this video drops, what a coincidence. Been playing this series on and off since around 2004. 3rd hands down the best, wish 5th will have open world as well

This is the most well put-together video essay on a certain topic. You linking the resources and the little edits in the video too bring the value of the video really high

I only wish you had spent a bit of time talking about the HUGE leaps made from sims 1 to sims 2, but ig it's really 1/2 to 3 that had the biggest jump re personality ai and then the sims 4 happened. (would LOVE if you did a follow up re how the ai falls apart in sims 4)

when i tried out the sims back in the day, i somehow binged it for 8h straight and after that i had one of the most horrible stress dreams: i was an employee somewhere and people delivered empty status bars to my desk. i had to stare at them real hard to make them fill up again, which was very straining, because when i stopped concentrating for a second, they emptied out again. and there were so SO many of those bars and people dumped more and more of them onto my desk, completely ignorant of the fact that i couldnt keep up... i never played the sims ever again after that xD

The annoying thing that happens though is that the sims are prone to take naps and have snacks all the time. Rather than waiting for when the Energy bar is lower and have a proper night's sleep, they'll have random naps or go to sleep in the middle of the afternoon. I've always found the AI very frustrating so I never have autonomy on; I have to do a lot more work interrupting what the AI is telling my sims to do.

The restaurant discouraging frugal sims is hysterical😂

This video was really cool and informative! I’d love it if you did one explaining the action queue system, especially why the hell do the Sims in 4 get to just decide to drop all their actions sometimes. I don’t recall them ever doing that in previous games. Before, if an action was on the queue, they would only not do it if it was literally impossible for them, now it seems they can wipe the queue for no apparent reason.

the classic sims music is just... so soothing for the soul

This is a great video! The best simulation games are the ones that allow the player to create their own elaborate stores / head cannon. Rimworld does this perfectly.

Your explanation was so cool and interesting! It made me love the Sims 3 even more (and it's still my favorite of the franchise), and it really provided context for the silly behavior sims can have.

About the Decision Making 101 section, the devs of Mark of the Ninja did some similar work on noise sources and explained it very well. It's a common pattern in game to build relationship in reverse like described here so it's good to see how and why by different professionals

This was such a fun video to watch, I could hear you talk about these things for hours!

"And, let's be honest - the best one, which is the Sims 3." Them's fightin' words, my friend

I LOVE THE SIMS, FINALLY AN EPISODE FROM GMTK!

This was so interesting! I've loved the sims for years and never knew that's how their motivations worked, thanks for making this video!!

it seems there's a precursor version of the "Different personality traits unlock different unique interactions" thing in even the earlier games--for example, in Sims 2, only a Sim with high Playfulness would autonomously juggle bottles from the fridge or play "pirate" in the bathtub. It's interesting to trace things back through the series that way.

It's funny because some of my sims are based on characters I've written, and for the most part the things they do autonomously are oddly in-character for them, even with only the basic 3 personality traits. The last part about ambiguity really shed a light on how I've been filling in the gaps and I have to say, very well-put sir

This was a fantastic overview, ended up getting some ideas on how to implement some of this into a wee turn based strategy game I've been working on

The AI logic really sounds like fuzzy logic in action.

This was a fantastic episode. Somehow very different from your typical videos yet very on brand.

Motive level-of-detail is an especially neat little trick

That was super interesting, as always! I noticed you talked a lot about code, a video where you talk about how Developing changed your approach to making GMTK videos would be fascinating! ❤

I absolutely love all your vids and their what got me into game dev!

This is honestly the best video I've seen for a long time. I am developing a management simulation game, and we were struggling with making the AI feel realistic and complex, without have a pile of convoluted rule sets. All I can say is, we are gonna lean fully into this utility AI stuff. Not only will it make sre NPCs cact more believable, but its actually going to cut down development time a whole lot. Thank you sir!

I did not expect that deep of an analysis and I am still amazed of how great the Sims AI works. Thanks for the amazing video.

Interesting. Rimworld has a very similar set of meters controlling its pawns. Food, rest, recreation, etc. And they have traits. I guess I see where Tynan's inspiration came from. It's quite an elegant system

If possible, I think a video about how Dwarf Fortress works could be incredibly interesting.

I'm actually building a somewhat similar game, and I came to the same conclusion to have the items be the driving factor as being the best way to do this as well(after encountering difficulties primarily in how manageable the system was to design when it was character centric). Realizing this also allowed me to more easily add more items in my game, and have them be instantly useable by players(as mentioned about The Sims packs and I can understand why/how they are so easily able to drop a couple dozen items in a month). Many people talking about the silly things that Sims sometimes do in the game is a testament to how unnoticeably fantastic the AI is for MOST of the time. It's so good that people think it's funny when something happens slightly strange from a human perspective. For the Sims this is a triumph that I don't believe any other game can boast as other games' AI are comparatively so non-human that nobody would point out something so trivial. Too bad I didn't have this video as a blueprint before I started. 😆

This video was really interesting. Great to see another gmtk vid.

Awesome, thanks for the summary of the AI behind the sims! If possible, I think it'd be really cool if you could show us if there are other games that have played off this formula or tweaked it, would be super cool to see what impacts this kind of design has had on the game industry :).

I love deep dives into complicated subjects like AI! If you'd want to dive deeper into how the XCOM AI thinks for example, I'd be all for that. The little clip at the end has already got me plenty excited!

so informative! and well paced/sourced too. great job! thank you for sharing :)

I love GMT I love Sims my life is complete with this video

18:53 this part in particular was very interesting because it nails exactly what makes the Sims 4 so frustrating at the moment. I haven't played in a month, but last time I did Sims would always start random fights even with their lovers and close ones, would wish to break up out of the blue, would get intruding and annoying fears popping up all of the time... And NPCs now have a set sexuality too

The worse thing ever is food/energy depletion which I run into a lot with pregnancy in Sims 2. And while preggers and post-preggers it completely makes sense the body is essentially thrown out of wack but since you only get a few days with a very needy baby sim who can’t take care of themselves getting sims out of this loop is sometimes frustrating. Getting them to eat a little before they pass out and get enough sleep to get up and eat a little food is frustrating.

Thank you for this, I've always been curious about the ai behind the Sims, this was fascinating!

Thank for such interesting video, always wondered how this all simulation was working.

I love when sims pick up the dirty dishes to go washing them in the bathroom - even if it's on another floor.

This explains what I'd already guessed, the occults like aliens and vampires will (mostly) leave your Sim alone entirely if you never interact with them. I'd guess that talking to them once or researching vampires/sending out the single on the satellite gives your Sim a hidden buff letting the AI know you're interested in either an abduction or a break-in (if the vampire, like Vlad for example, can do that.) I assume also that being an A student (perhaps also parenting) gives a buff/trait that encourages a child or teen Sim to do their homework without player input, which is cool because it inspires the interpretation of habits or "learning" over time what the player wants akin to how the game notices who your Sim is romancing. I'd be amused to know if the hidden fruitcake trait is only rolled after they eat it for the first time, so they'd never find out whether they like it or not until they actually try the dish. It doesn't really matter when it happens but it makes me wonder if cooking skill and the quality of the fruitcake has any impact on more Sims liking it (otherwise it has whatever weight they assigned to the odds.)

I’ve always been curious of the game series operated. Been playing The Sims since 1. I hope you do more on The Sims. :)

great video as always. I remember being obsessed with the sims as a kid.

I admittedly haven't played any of the Sims games, but I _have_ watched people play them on RU-vid, and I genuinely never thought about how their AI worked. It just felt so normal for them to do things on their own that I never questioned it.

Sims 4 AI left the chat

The testers story and katy part was so adorable!!!!

Incredible episode and resources. I feel like this might come in handy in a future game

It is interesting but I saw in retrospectives from the first game that it was not so much 'a little tuning' the AI was a nightmare tune up, and capable of very easily badly breaking the game, with each new addition or change potentially getting Sims stuck in infinite loops or dying in very stupid ways. Though I expect this got a lot easier with experience and the ability to build upon it, probably with some of the systems mentioned in this video. Still, I ironically found the story of the Sims AI to be more of a cautionary tale, that sometimes what appears to be a simple but powerful system can easily become unmanageable when scaled up. (Though this was something Maxis themselves didnt actually learn if you also watch retrospectives on the Sim City reboot, again, really interesting, something I think could have been developed in to something great just like the Sims system if the series hadnt pretty much crashed out in one iteration, but... hoo boy, what a hilariously unhinged way of going about a city simulation.)

This video reticulates my splines. (yes i know that's more of a SimCity thing) We need more games that do this whole "Simprov" thing. 'cause while there are plenty of life sims out there, I don't know of many that channel that same kind of emergent interaction that The Sims does. Though IIRC Paradox is doing their own spin on The Sims, called Life by You, directed by Sims alumnus Rod Humble. Hopefully it's able to steal some of the thunder of the upcoming Sims 5, and not fail terribly like The Callisto Protocol did relative to Dead Space Remake.

Sims: Do stupid push-ups in absolutely every situation. This guy: Genius!