The Genius AI Behind The Sims

The Sims uses a super smart AI system to make virtual people who think, feel, and interact in a believable way. Here's how.

=== Sources and Resources ===

- Footnotes

(1) Most numbers in this video are for illustrative purposes only, and do not necessarily represent the values in any The Sims game.

(2) In The Sims 3, the "Comfort" and "Room" motives are removed, leaving six basic needs. These are instead represented by "Moodlets", which are little boosts and nerfs to the Sim's happiness for doing things like sitting in a comfy chair or walking into a smelly room.

(3) The Sims are strictly male and female in the first three games, but The Sims 4 expands this with the ability to individually set a character's appearance, pronouns, and more.

- Sources

[1] Those Darned Sims: What Makes Them Tick? | GDC Vault

[2] Will Wright explains what The Sims and an ant colony have in common | Joystiq

[3] Simulation and Modeling: Under the hood of The Sims | Northwestern University

[4] Some notes on programming objects in The Sims | Northwestern University

[5] JustJakeSimpson | Twitter

[6] Needs-Based AI | Robert Zubek

[7] Traits | The Sims Wiki

[8] Modeling Individual Personalities in The Sims 3 | GDC Vault

[9] AI Development Postmortems: Inside DARKSPORE and THE SIMS: MEDIEVAL | GDC Vault

[10] Emergent Storytelling Techniques in The Sims | GDC Vault

[11] Will Wright Teaches Game Design and Theory | MasterClass

[12] The Sims Design Documents | Various

[13] AI Postmortems: Assassin's Creed III, XCOM: Enemy Unknown, and Warframe | GDC Vault

- Additional resources

AI Made Easy with Utility AI | Medium

Improving AI Decision Modeling Through Utility Theory | GDC Vault

An Introduction to Utility Theory | Game AI Pro

Knowledge is Power: An Overview of Knowledge Representation in Game AI | GDC

=== Chapters ===

00:00 - Intro
01:26 - Decision Making 101
03:43 - Advanced Decision Making
06:52 - Utility AI
07:51 - Unique Sims
09:50 - Social Situations
11:09 - Hard-coded Rules
13:00 - Background Sims
15:35 - Storytelling
19:51 - Conclusion

=== Games Shown ===

The Sims 2 (2005)
The Sims 4 (2014)
The Sims 3 (2009)
The Sims (2000)
SimAnt (1991)
The Sims: Unleashed (2002)
The Sims Medieval (2011)
SimCity 2000 (1993)
Middle-earth: Shadow of War (2017)
XCOM: Enemy Within (2013)
Minecraft (2011)

=== Credits ===

Music from The Sims OST, by Jerry Martin

Music from The Sims 3 OST, by Steve Jablonsky and Pieter A. Schlossern

=== Subtitles ===

[rezination]

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.

[EmiWi]

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

[oryza_citrus]

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

[AngraMainiiu]

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!

[CosmicAlgae]

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

[monkeeee]

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

[Rycluse]

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.

[Tormentadeplomo]

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.

[katlyndobransky]

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

[michaelwesten]

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

[AnymMusic]

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")

[CrypticGamine]

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.

[thomasgibbs]

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

[Trio]

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.

[ferretzim]

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

[jonmiller]

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

[professorlilith]

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

[JaneXemylixa]

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.

[safebox]

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.

[jackdubois]

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