The Genius AI Behind The Sims

The Sims is a digital dollhouse, 
filled with little computer people.

And it's your job to keep 
them alive, keep them happy,

and keep their house furnished 
with ever more expensive trinkets.

But micromanaging the needs and desires of 
an entire family can get pretty tedious.

So the developers at Maxis soon realised that

they'd need to give these guys 
a certain amount of autonomy.

Free will.

The power to think for themselves, if 
the player stops giving direct commands.

Which is actually...

a pretty complicated task! These characters 
need to read as believable humans.

And they need to be able to adapt to any 
possible house you build around them,

or any social situation they find themselves in.

And - crucially - they can't be so smart that 
the player can just sit back and do nothing.

So how do you make a video game 
AI that can do all of that?

Well, that's what I want to explore in this video.

I'm going to break down the clever ways that Maxis 
designed autonomy in this four-game franchise -

though with a focus on the original game,

and - let's be honest - the 
best one, which is The Sims 3.

It's not going to get too technical,

but there is a metric buttload of resources 
in the description if you want to dive deeper.

With that out of the way, I'm Mark Brown,

this is Game Maker's Toolkit, 
and here's how The Sims... think.

Okay - decision making in The Sims 
is all driven by a set of mental

and physical needs - dubbed "motives", by Maxis.

In The Sims 1 that's hunger, 
hygiene, fun, energy,

bladder, social, comfort, and room, or tidiness.

They all go from negative 100 to positive 100,

and if you combine them all, you get 
the Sim's overall level of happiness.

These meters are constantly ticking down - 
each decaying at slightly different rates,

and faster if the Sim is 
performing a related action.

The bladder meter drops more quickly 
when the Sim is eating, for instance.

Maxis carefully tuned all of these 
rates to hopefully match a normal

human schedule - like needing eight 
hours of sleep, and three meals a day.

So. If the player doesn't give the Sim 
a command, it will decide what to do by

itself - and that decision is driven 
by a desire to fulfil these motives.

Now you might assume that a Sim 
knows how to meet its own needs:

that a toilet will solve bladder, 
that fridges and stoves solve hunger,

and books, TVs, and pinball tables solve fun.

But it doesn't work like that - 
it's actually, the other way around.

Instead, all of the objects in 
the Sim's house contain this data,

and will broadcast what they can offer.

A bed will say "sleep on me to get 10 energy",

a toilet offers plus 20 to bladder if you 
use it, or plus 5 to room if you clean it.

And other Sims will offer themselves 
as a way to top up social points.

Maxis calls these "advertisements".

So. Whenever the Sim decides what to do,

it quickly makes a list of every object 
in the house - and what they can offer.

The Sim can then take that advertised number 
and weigh it based on its current needs.

It does this by applying a 
multiplier to the promised score,

based on the Sim's current motive levels.

For example, if the Sim is well-rested, 
the bed's energy-boosting score is damped

down - but if the Sim is knackered, then 
that same score is boosted dramatically.

Now, the Sim can take the list of interactions, 
and their weighted scores, and rank them.

At the top of the list is the interaction that 
will give the most benefit to the Sim right now...

and so the digital dude should do just that.

This clever system was actually inspired by one 
of designer Will Wright's previous games: SimAnt -

a virtual ant colony where the critters would 
be tempted to move by attractive pheromones.

It's basically the same idea 
- but instead of pheromones,

its TVs, fridges, pinball machines, and bath tubs.

However - people aren't ants.

And so if this was implemented 
exactly as I just described,

it would lead to weird behaviours that make 
Sims seem either robotic or irrational.

So, many careful tweaks are 
needed to make it work properly.

For instance, not all needs are equal.

If a Sim is both starving 
to death and utterly bored,

they should probably eat a 
sandwich rather than binge Netflix.

But if the fridge and TV are 
both offering the same reward...

which should The Sim pick?

Well, more rational behaviour can be achieved 
by defining unique curves for each motive.

For hunger, we want a curve like this: 
now, the fridge's score will be reduced

to almost zero when the Sim is full, but will 
be extremely high when the Sim is starving -

making that more important than any other motive.

Similar, smaller curves are used for the 
other physiological needs like bladder,

hygiene, and energy.

However, less important needs like fun, social,

and comfort actually increase 
as the Sim becomes more happy.

This was inspired by Maslow's hierarchy 
of needs, which says humans will focus

on cognitive and social tasks only when their 
more basic lizard-brain needs are met.

Plus - while peeing will satiate your 
bladder, you can never have too much fun.

We can also weigh the scores 
based on other factors, too.

For instance, each Sim has its own 
personality - based on their score

in areas like niceness, neatness, and playfulness.

So while a pinball machine and a bookcase 
might advertise the same boost to fun,

a Sim will weigh this based 
on their level of playfulness.

Therefore, a more serious Sim 
will choose to pick up a book.

Plus, we can weigh the interaction based on its

distance from the Sim - which will 
make nearby objects more tempting.

Finally, some interactions can override everything 
- like a telephone call or a school bus.

And other interactions can 
be locked entirely - so,

certain objects won't advertise themselves 
to kids, or adults, or a visiting guest.

All of these factors combined 
give us a much more useful score,

which has been adjusted to the different 
needs, personality, and proximity of the Sim.

And it's from these scores that 
the Sim's next move is picked.

But - perhaps most importantly of all - the

Sim doesn't actually choose 
the best option every time.

Instead - the Sim picks one of the 
top scoring interactions... at random.

This stops the Sims from feeling 
robotic and overly predictable,

and it also means they're not able 
to fulfil all their needs perfectly.

Which, you know, actually gives 
the player something to do.

What I've just described is a special type 
of AI called utility AI, or needs-based AI.

This is a system where a character 
has a predefined set of needs,

then looks around at its current options, and 
picks the one that will fulfil its needs best -

the one with the most utility.

And in The Sims, it means a little person can be

dropped into any house and will 
then act in a lifelike manner:

carefully attending to its 
hunger and energy when necessary,

but socialising and having fun when not.

Plus, by putting the information 
on the objects, instead of the Sim,

it's really easy for Maxis to add 
hundreds of objects to the game -

or make endless expansion packs - 
without touching the rest of the code.

And so for these reasons, this basic system has 
been used in all four games in the franchise.

But, Maxis wanted to take things even further when 
making the ambitious third game in the series.

Okay, so, in The Sims 1, we saw how 
each Sim's personality - like their

neatness and playfulness - would 
impact their decision making.

This would make each Sim feel somewhat distinct.

But for The Sims 3 this idea 
was expanded dramatically by

swapping the personality meters with traits.

Things like neat, neurotic, heavy 
sleeper, and commitment issues.

With five trait slots, and 60-odd 
traits in the base game alone,

that's about 5 million possible Sims - 
effectively making every character unique.

Now these traits have many impacts on 
the Sim - like, how a clumsy Sim will

trip themselves up while walking, and 
a slob will fart and burp while idle.

Sims also get bonus moodlets 
for acting to their traits - and

there are certain interactions that are 
exclusive to Sims with certain traits:

like how a computer whizz can 
make cash through hacking.

But these traits also impact 
on the Sim's decision making.

This time, however, instead of weighing 
the scores against the Sim's personality,

the Sims 3 actually just adds 
more motives to the pile.

So, in the code, every Sim has their 
usual basic needs, like hunger, energy,

and bladder - but also five more 
motives related to their traits.

Therefore a couch potato needs 
to sit on the sofa and watch TV,

just like how they need to pee, eat, and sleep.

And then, in the same way that a fridge 
advertises itself as a fix for hunger,

objects can advertise themselves as a 
fix for the Sim's trait-related motives.

For instance, interactions like scare a Sim, 
steal candy from a child, smash a dollhouse,

and write a trolling comment on the computer 
are all advertised to Sims with the evil trait.

This means that Sims are encouraged 
to enact their unique personalities

autonomously - while also juggling 
their standard, everyday needs.

Now this idea - of adding additional motives to

a Sim - can also be used to 
simulate social situations.

For instance, if a Sim enters a gym they 
will temporarily be given the additional

motive "be in gym" - which is satisfied by 
objects like a workout bench or treadmill.

When they leave, the motive is removed.

Back at home, a Sim will be motivated to act as

a hospitable host for as long 
as they have guests around -

meanwhile, the guests will be motivated to 
act in an way that's socially acceptable.

And in The Sims Medieval, a Sim 
is given work-related motives

while they're on the clock, and 
then loses them during breaks.

However: these motives can 
also be affected by traits.

For example, if a Sim sits down to have a picnic,

then the Sim's friends and family are motivated 
to join them - while strangers are discouraged.

But a Sim with the "inappropriate" 
trait will override this - and

they will in fact be motivated to 
sit with people they don't know.

Just to make them feel uncomfortable.

All of this means that Sims will act naturally and 
believably in whatever location they end up in -

but with a few Sims standing out as acting 
unusually, because of their unique personality.

And this is all done by simply tweaking the Sim's

motivations - rather than hand-scripting a 
bunch of specific rules and interactions.

Though, that being said, sometimes 
you do need more hard-coded rules.

Take that house visit I mentioned.

A visiting sim is indeed motivated 
to act in an appropriate way.

But what happens if they stay for too long? Or if

the player steps in and makes 
the Sim act inappropriately?

Well, to make this work,

the house visit is overseen by a set 
of rules that dictate social norms.

So if a Sim sleeps in the host's 
bed or uses their computer, the ruleset

will instruct the host to warn the Sim, and 
then kick them out if they go even further.

Likewise, conversations need 
more hands-on authoring to

reflect extremely specific pair-ups of 
people, or super distinct social cues.

For example, how should a Sim respond to 
a joke if they are steaming mad at the

joke teller? Or if the other Sim has 
said the same joke 5 times in a row?

This is done through a series of rules - with 
an input, a set of conditions, and the output.

So for that joke I just mentioned...

if the listener has the 'good sense 
of humour' trait, they'll laugh.

If their long-term relationship 
is sour, they'll be insulted.

And if the Sim repeatedly tells 
jokes, the listener will be bored.

These so-called "production rules" are ranked in 
order of specificity, as defined by the designer,

and the top-scoring outcome 
is used to pick the response.

Maxis ended up writing 1000s of hand-crafted rules

to determine the outcome of 100s 
of possible conversation topics.

But, because the rules just go in a big 
stack and the most specific one is picked,

they can't clash, break the game, cause an 
infinite loop, or lead to any other issue.

Just like with the objects, designers can 
simply keep adding more and more to the pile.

So far, I've been talking about individual Sims,

or small social situations 
like a conversation or a party.

But what about simulating an entire 
town's worth of little people?

You see, in The Sims 3, your house 
exists in an open world neighbourhood

with dozens of other Sims, and features 
homes, restaurants, a park, and so on.

Maxis needed to invent new tools to 
make all of this work - but, in truth,

the neighbourhood actually operates in 
a similar way to the Sims themselves.

That's because the town 
also has a bunch of motives,

and will make choices that 
will help it fulfil its needs.

For instance - it ideally wants to 
maintain a 50/50 gender balance.

So when a new Sim is added to the neighbourhood,

the probability of it being male or female 
is weighted by the current needs of the city.

It also has a desired employment 
rate of about 80% - so it can force

background Sims to get hired or fired as seen fit.

Basically - once a day, at midnight, the 
town will check its own happiness level,

and take a few actions in 
order to satisfy its needs.

The individual lots have 
motives too - for instance,

the restaurant may wish to have roughly 8 people 
eating outside, during lunch and dinner hours.

To achieve this, the lot will temporarily give

the "eat outside" motive to a bunch of 
Sims to tempt them to the restaurant.

It can even narrow the focus to 
Sims with certain traits - like

culinary - and discourage those 
with other traits - like frugal.

Now, these Sims that appear at the restaurant 
seem like fully autonomous characters.

They have needs, they have families,

they may even have had a baby and 
changed job since you last saw them.

So are they are actually living out their 
own little simulated lives, just off camera?

Well, not quite.

The background Sims are actually 
simulated at a very low level of detail.

Each day, the system looks at each Sim,

and scores possible big life 
changes that could happen to them -

like getting a job, falling in love 
with someone, or getting married.

These are, of course, weighted using things like 
traits, existing relationships, and career paths.

Also, the designers created 
charts for what a Sim's needs

will likely look like throughout the day.

If it's just after lunch, for instance, 
they're probably not going to be very hungry.

And so when a background Sim is 
promoted into being a foreground Sim,

because they're in proximity to the player,

the system checks the time of day and 
snaps all their motives to the chart.

Clever stuff.

So we've got tools to simulate characters,

conversations, house visits, 
restaurants and entire towns.

And when you have such clever tech, it's 
tempting to simulate, well, everything.

Take, for instance, the urinal rule.

If you pee while standing up, you know the 
deal: when picking a place to take a leak,

you should always try to maintain a 
one urinal buffer from other people.

It's like an unwritten social rule.

So Maxis added this rule to the game, as well...

but, before the game launched,
they took it out.

That's because when the Sim followed that 
rule it was just kind of predictable and dull.

But if Sims picked toilets at random, it would

often lead to funny and memorable 
moments of Sims acting awkwardly.

You see, while The Sims might 
have started life as a rather

nerdy simulation game - essentially 
SimCity, but at a much smaller scale -

it quickly grew into something very different.

In playtests for the original 
game, Will Wright was surprised

that players would tell elaborate 
stories about their Sim families -

embellishing details, reading into their 
decisions, and playing up random moments.

"It was fascinating to me how readily people 
would build a story around this," Wright says.

To support this, Maxis added a 
last-minute feature to the game:

the ability to take a screenshot, write a caption,

and create a simple comic book which could 
be uploaded to EA's website with a click.

The feature was extremely popular, 
proving that for many people...

The Sims was an avenue for storytelling.

And that's still true today - just search 
YouTube for The Sims to see what I mean.

But supporting storytelling is a delicate 
balance: the game needs to be specific enough

to create stories, but not so specific that 
there's no room for the player's imagination.

So the final piece of this AI puzzle...

is knowing when to hold back.

Knowing what NOT to simulate.

Knowing when to make urinal choice 
random, rather than scripted.

One way to do this is to 
carefully employ ambiguity.

Basically, to leave some things unsaid,

so players can make assumptions, and project 
their own head-canon on the events on screen.

A good example of this is the Sim's 
iconic babbling dialect, known as Simlish.

In early design documents we can see 
that Maxis experimented with having

the Sims speak real world languages like 
English, Navajo, Estonian, and Ukranian -

but due to technical and logistical reasons,

they ended up hiring two improv comics 
to just make up absolute nonsense.

Sim: Droba droba droba.
Seeka neeba huh.

But this actually works wonders - when the 
Sims babble gobbledygook at each other,

the player can assume, fill in gaps,

project their own imagination on the 
Sims, and make the game their own.

"If we used actual language, the game 
would flatten and shrink, and everyone

would be having the same experience", 
says EA's Matt Brown - no relation.

Ambiguity can also make Sims feels smarter than

they actually are, and hide cases where 
the simulation is doing something wrong.

Another avenue is to always follow 
the player's lead - and never use

autonomy to go against the story 
the player is trying to tell.

That's why Sims will use free will to 
relieve their bladder and fill their stomach,

but won't autonomously quit their 
job or try romancing a random Sim.

Just like with the Nemesis 
System in Shadow of War,

Maxis took inspiration from the concept 
"yes, and", from improvisational comedy -

so the Sims try to build on the player's 
actions, and try not to negate them.

For instance, if a player makes two 
Sims fall in love - the autonomous

system shouldn't make them break up again.

Likewise, user-created Sims enter 
the world with no sexual preference.

But if you instruct a male Sim 
to, say, flirt with other dudes,

you're suggesting that he should be bi 
or gay and the game will run with that.

The game should always try to maintain 
the consistency of the player's story.

So, The Sims has a fascinating AI system - 
which allows for realistic human simulations,

and creative player-led storytelling.

And most of it is driven by a very simple concept.

Characters, locations, and entire 
neighbourhoods are given a bunch of

needs - and then try to make choices 
that will best fulfil those needs.

With a little clever tuning, this leads to 
characters who feel believable, social situations

where Sims act appropriately, and entire 
neighbourhoods that stay consistent and balanced.

But this is not just useful for 
running quirky goal-free life

simulations - and we can see utility AI 
being used in other types of games, too.

For instance, in XCOM, an enemy unit will consider 
every tile it can move to and score them based on

factors like distance, flanking opportunities, 
angle, cover bonus, visibility, and proximity -

before moving to the tile with the best score.

It can also be used in procedural generation, to 
build worlds that fit predetermined requirements.

Utility AI is a fantastic addition 
to a game maker's toolkit, and its

implementation in The Sims should serve as 
wonderful inspiration to game designers.

And now, just to melt your brain, 
here's Katy Perry singing in Simlish.

Katy Perry: [Absolute nonsense]

Katy Perry: [Just complete gobbledegook]

Katy Perry: [I think I'm having a stroke]