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

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

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and keep their house furnished 
with ever more expensive trinkets.

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

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So the developers at Maxis soon realised that

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

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Free will.

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

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Which is actually...

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

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

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or any social situation they find themselves in.

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

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

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

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

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though with a focus on the original game,

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

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It's not going to get too technical,

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

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

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

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

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

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

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

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They all go from negative 100 to positive 100,

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

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

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and faster if the Sim is 
performing a related action.

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

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

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

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

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

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

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

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and books, TVs, and pinball tables solve fun.

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

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

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and will broadcast what they can offer.

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

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

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

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Maxis calls these "advertisements".

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So. Whenever the Sim decides what to do,

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

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

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

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based on the Sim's current motive levels.

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

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

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

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

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and so the digital dude should do just that.

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

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

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

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

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However - people aren't ants.

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

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

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

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For instance, not all needs are equal.

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

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they should probably eat a 
sandwich rather than binge Netflix.

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

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which should The Sim pick?

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

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

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

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making that more important than any other motive.

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

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hygiene, and energy.

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However, less important needs like fun, social,

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and comfort actually increase 
as the Sim becomes more happy.

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

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

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

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

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

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in areas like niceness, neatness, and playfulness.

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

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

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

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Plus, we can weigh the interaction based on its

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

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

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And other interactions can 
be locked entirely - so,

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

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

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

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

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But - perhaps most importantly of all - the

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

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

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This stops the Sims from feeling 
robotic and overly predictable,

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

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

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

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

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

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the one with the most utility.

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

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

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carefully attending to its 
hunger and energy when necessary,

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but socialising and having fun when not.

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

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

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

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

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

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

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neatness and playfulness - would 
impact their decision making.

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This would make each Sim feel somewhat distinct.

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But for The Sims 3 this idea 
was expanded dramatically by

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swapping the personality meters with traits.

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

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

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

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

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

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

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

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like how a computer whizz can 
make cash through hacking.

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

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

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

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

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

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

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

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

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

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

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

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This means that Sims are encouraged 
to enact their unique personalities

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autonomously - while also juggling 
their standard, everyday needs.

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Now this idea - of adding additional motives to

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

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

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

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When they leave, the motive is removed.

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

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

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

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

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

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However: these motives can 
also be affected by traits.

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

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

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

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

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Just to make them feel uncomfortable.

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

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

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

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

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

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Take that house visit I mentioned.

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

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

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the player steps in and makes 
the Sim act inappropriately?

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Well, to make this work,

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

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

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

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Likewise, conversations need 
more hands-on authoring to

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

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

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

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

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So for that joke I just mentioned...

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

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

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

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

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

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Maxis ended up writing 1000s of hand-crafted rules

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

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

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

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

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

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or small social situations 
like a conversation or a party.

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

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

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

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

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

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

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

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

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So when a new Sim is added to the neighbourhood,

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

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

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background Sims to get hired or fired as seen fit.

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

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

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The individual lots have 
motives too - for instance,

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

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To achieve this, the lot will temporarily give

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

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

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

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

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They have needs, they have families,

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

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

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Well, not quite.

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

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Each day, the system looks at each Sim,

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

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

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

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

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will likely look like throughout the day.

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

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

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because they're in proximity to the player,

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

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Clever stuff.

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So we've got tools to simulate characters,

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conversations, house visits, 
restaurants and entire towns.

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

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Take, for instance, the urinal rule.

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

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

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It's like an unwritten social rule.

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

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

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

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But if Sims picked toilets at random, it would

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

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

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

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it quickly grew into something very different.

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In playtests for the original 
game, Will Wright was surprised

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that players would tell elaborate 
stories about their Sim families -

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

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

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

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the ability to take a screenshot, write a caption,

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

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

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The Sims was an avenue for storytelling.

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

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

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

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So the final piece of this AI puzzle...

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is knowing when to hold back.

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Knowing what NOT to simulate.

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

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One way to do this is to 
carefully employ ambiguity.

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Basically, to leave some things unsaid,

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

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

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

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

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but due to technical and logistical reasons,

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

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Sim: Droba droba droba.
Seeka neeba huh.

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

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the player can assume, fill in gaps,

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

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

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

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Ambiguity can also make Sims feels smarter than

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

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

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

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

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

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Just like with the Nemesis 
System in Shadow of War,

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

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

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00:19:26,100 --> 00:19:30,300
For instance, if a player makes two 
Sims fall in love - the autonomous

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00:19:30,300 --> 00:19:32,220
system shouldn't make them break up again.

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00:19:32,220 --> 00:19:37,140
Likewise, user-created Sims enter 
the world with no sexual preference.

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00:19:37,140 --> 00:19:41,640
But if you instruct a male Sim 
to, say, flirt with other dudes,

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

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00:19:46,020 --> 00:19:50,760
The game should always try to maintain 
the consistency of the player's story.

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00:19:51,660 --> 00:19:57,540
So, The Sims has a fascinating AI system - 
which allows for realistic human simulations,

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00:19:57,540 --> 00:20:00,720
and creative player-led storytelling.

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00:20:00,720 --> 00:20:04,320
And most of it is driven by a very simple concept.

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00:20:04,320 --> 00:20:09,600
Characters, locations, and entire 
neighbourhoods are given a bunch of

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00:20:09,600 --> 00:20:13,980
needs - and then try to make choices 
that will best fulfil those needs.

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00:20:13,980 --> 00:20:20,400
With a little clever tuning, this leads to 
characters who feel believable, social situations

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00:20:20,400 --> 00:20:26,520
where Sims act appropriately, and entire 
neighbourhoods that stay consistent and balanced.

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00:20:26,520 --> 00:20:31,080
But this is not just useful for 
running quirky goal-free life

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

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00:20:36,600 --> 00:20:43,380
For instance, in XCOM, an enemy unit will consider 
every tile it can move to and score them based on

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00:20:43,380 --> 00:20:50,580
factors like distance, flanking opportunities, 
angle, cover bonus, visibility, and proximity -

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00:20:50,580 --> 00:20:53,220
before moving to the tile with the best score.

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

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00:21:00,180 --> 00:21:05,820
Utility AI is a fantastic addition 
to a game maker's toolkit, and its

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00:21:05,820 --> 00:21:10,840
implementation in The Sims should serve as 
wonderful inspiration to game designers.

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00:21:10,860 --> 00:21:16,320
And now, just to melt your brain, 
here's Katy Perry singing in Simlish.

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00:21:16,320 --> 00:21:24,396
Katy Perry: [Absolute nonsense]

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00:21:24,396 --> 00:21:32,101
Katy Perry: [Just complete gobbledegook]

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00:21:32,101 --> 00:21:37,900
Katy Perry: [I think I'm having a stroke]