0:00:00.180,0:00:05.580
The Sims is a digital dollhouse, [br]filled with little computer people.

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

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and keep their house furnished [br]with ever more expensive trinkets.

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But micromanaging the needs and desires of [br]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 [br]a certain amount of autonomy.

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Free will.

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

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Which is actually...

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

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And they need to be able to adapt to any [br]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 [br]the player can just sit back and do nothing.

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So how do you make a video game [br]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 [br]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 [br]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 [br]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, [br]and here's how The Sims... think.

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Okay - decision making in The Sims [br]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, [br]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 [br]the Sim's overall level of happiness.

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

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and faster if the Sim is [br]performing a related action.

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

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

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

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

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

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

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that a toilet will solve bladder, [br]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 - [br]it's actually, the other way around.

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Instead, all of the objects in [br]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 [br]use it, or plus 5 to room if you clean it.

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And other Sims will offer themselves [br]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 [br]in the house - and what they can offer.

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

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It does this by applying a [br]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, [br]the bed's energy-boosting score is damped

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

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

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At the top of the list is the interaction that [br]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 [br]of designer Will Wright's previous games: SimAnt -

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

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It's basically the same idea [br]- 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 [br]exactly as I just described,

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

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So, many careful tweaks are [br]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 [br]to death and utterly bored,

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they should probably eat a [br]sandwich rather than binge Netflix.

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But if the fridge and TV are [br]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 [br]by defining unique curves for each motive.

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

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to almost zero when the Sim is full, but will [br]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 [br]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 [br]as the Sim becomes more happy.

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

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

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

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

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For instance, each Sim has its own [br]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 [br]might advertise the same boost to fun,

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

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Therefore, a more serious Sim [br]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 [br]make nearby objects more tempting.

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

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And other interactions can [br]be locked entirely - so,

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

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

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

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And it's from these scores that [br]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 [br]the best option every time.

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

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This stops the Sims from feeling [br]robotic and overly predictable,

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

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

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

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

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then looks around at its current options, and [br]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 [br]then act in a lifelike manner:

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carefully attending to its [br]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 [br]on the objects, instead of the Sim,

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

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

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

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

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

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neatness and playfulness - would [br]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 [br]was expanded dramatically by

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swapping the personality meters with traits.

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Things like neat, neurotic, heavy [br]sleeper, and commitment issues.

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

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

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

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

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

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there are certain interactions that are [br]exclusive to Sims with certain traits:

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like how a computer whizz can [br]make cash through hacking.

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

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

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the Sims 3 actually just adds [br]more motives to the pile.

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

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

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Therefore a couch potato needs [br]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 [br]advertises itself as a fix for hunger,

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

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

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

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This means that Sims are encouraged [br]to enact their unique personalities

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autonomously - while also juggling [br]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 [br]simulate social situations.

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

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motive "be in gym" - which is satisfied by [br]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 [br]as they have guests around -

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

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

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

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However: these motives can [br]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 [br]to join them - while strangers are discouraged.

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

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they will in fact be motivated to [br]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 [br]believably in whatever location they end up in -

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but with a few Sims standing out as acting [br]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 [br]bunch of specific rules and interactions.

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Though, that being said, sometimes [br]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 [br]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 [br]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 [br]of rules that dictate social norms.

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

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

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Likewise, conversations need [br]more hands-on authoring to

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

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

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

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This is done through a series of rules - with [br]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 [br]of humour' trait, they'll laugh.

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

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

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

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and the top-scoring outcome [br]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 [br]of possible conversation topics.

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

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

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Just like with the objects, designers can [br]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 [br]like a conversation or a party.

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

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

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

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

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

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

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and will make choices that [br]will help it fulfil its needs.

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For instance - it ideally wants to [br]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 [br]is weighted by the current needs of the city.

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It also has a desired employment [br]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 [br]town will check its own happiness level,

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

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The individual lots have [br]motives too - for instance,

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the restaurant may wish to have roughly 8 people [br]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 [br]Sims to tempt them to the restaurant.

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

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culinary - and discourage those [br]with other traits - like frugal.

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Now, these Sims that appear at the restaurant [br]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 [br]changed job since you last saw them.

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

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Well, not quite.

0:14:51.120,0:14:55.740
The background Sims are actually [br]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 [br]changes that could happen to them -

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

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

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Also, the designers created [br]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, [br]they're probably not going to be very hungry.

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

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because they're in proximity to the player,

0:15:28.680,0:15:33.420
the system checks the time of day and [br]snaps all their motives to the chart.

0:15:33.420,0:15:34.440
Clever stuff.

0:15:35.580,0:15:38.280
So we've got tools to simulate characters,

0:15:38.280,0:15:42.840
conversations, house visits, [br]restaurants and entire towns.

0:15:42.840,0:15:47.880
And when you have such clever tech, it's [br]tempting to simulate, well, everything.

0:15:47.880,0:15:51.120
Take, for instance, the urinal rule.

0:15:51.120,0:15:56.160
If you pee while standing up, you know the [br]deal: when picking a place to take a leak,

0:15:56.160,0:15:59.700
you should always try to maintain a [br]one urinal buffer from other people.

0:15:59.700,0:16:02.400
It's like an unwritten social rule.

0:16:02.400,0:16:05.580
So Maxis added this rule to the game, as well...

0:16:05.580,0:16:08.760
but, before the game launched, [br]they took it out.

0:16:08.760,0:16:14.040
That's because when the Sim followed that [br]rule it was just kind of predictable and dull.

0:16:14.040,0:16:17.100
But if Sims picked toilets at random, it would

0:16:17.100,0:16:21.780
often lead to funny and memorable [br]moments of Sims acting awkwardly.

0:16:21.780,0:16:25.800
You see, while The Sims might [br]have started life as a rather

0:16:25.800,0:16:30.630
nerdy simulation game - essentially [br]SimCity, but at a much smaller scale -

0:16:30.630,0:16:32.700
it quickly grew into something very different.

0:16:32.700,0:16:36.720
In playtests for the original [br]game, Will Wright was surprised

0:16:36.720,0:16:40.650
that players would tell elaborate [br]stories about their Sim families -

0:16:40.650,0:16:45.360
embellishing details, reading into their [br]decisions, and playing up random moments.

0:16:45.360,0:16:50.400
"It was fascinating to me how readily people [br]would build a story around this," Wright says.

0:16:50.400,0:16:54.180
To support this, Maxis added a [br]last-minute feature to the game:

0:16:54.180,0:16:57.000
the ability to take a screenshot, write a caption,

0:16:57.000,0:17:02.880
and create a simple comic book which could [br]be uploaded to EA's website with a click.

0:17:02.880,0:17:07.500
The feature was extremely popular, [br]proving that for many people...

0:17:07.500,0:17:10.740
The Sims was an avenue for storytelling.

0:17:10.740,0:17:16.140
And that's still true today - just search [br]YouTube for The Sims to see what I mean.

0:17:16.740,0:17:22.260
But supporting storytelling is a delicate [br]balance: the game needs to be specific enough

0:17:22.260,0:17:27.780
to create stories, but not so specific that [br]there's no room for the player's imagination.

0:17:27.780,0:17:31.260
So the final piece of this AI puzzle...

0:17:31.260,0:17:33.420
is knowing when to hold back.

0:17:33.420,0:17:36.420
Knowing what NOT to simulate.

0:17:36.420,0:17:40.860
Knowing when to make urinal choice [br]random, rather than scripted.

0:17:41.400,0:17:45.480
One way to do this is to [br]carefully employ ambiguity.

0:17:45.480,0:17:48.420
Basically, to leave some things unsaid,

0:17:48.420,0:17:53.700
so players can make assumptions, and project [br]their own head-canon on the events on screen.

0:17:53.700,0:17:59.520
A good example of this is the Sim's [br]iconic babbling dialect, known as Simlish.

0:17:59.520,0:18:03.240
In early design documents we can see [br]that Maxis experimented with having

0:18:03.240,0:18:09.720
the Sims speak real world languages like [br]English, Navajo, Estonian, and Ukranian -

0:18:09.720,0:18:12.480
but due to technical and logistical reasons,

0:18:12.480,0:18:17.400
they ended up hiring two improv comics [br]to just make up absolute nonsense.

0:18:17.400,0:18:22.980
Sim: Droba droba droba.[br]Seeka neeba huh.

0:18:22.980,0:18:28.440
But this actually works wonders - when the [br]Sims babble gobbledygook at each other,

0:18:28.440,0:18:31.680
the player can assume, fill in gaps,

0:18:31.680,0:18:36.060
project their own imagination on the [br]Sims, and make the game their own.

0:18:36.060,0:18:41.640
"If we used actual language, the game [br]would flatten and shrink, and everyone

0:18:41.640,0:18:46.440
would be having the same experience", [br]says EA's Matt Brown - no relation.

0:18:46.440,0:18:49.800
Ambiguity can also make Sims feels smarter than

0:18:49.800,0:18:53.580
they actually are, and hide cases where [br]the simulation is doing something wrong.

0:18:53.580,0:18:57.900
Another avenue is to always follow [br]the player's lead - and never use

0:18:57.900,0:19:01.680
autonomy to go against the story [br]the player is trying to tell.

0:19:01.680,0:19:06.420
That's why Sims will use free will to [br]relieve their bladder and fill their stomach,

0:19:06.420,0:19:10.920
but won't autonomously quit their [br]job or try romancing a random Sim.

0:19:10.920,0:19:14.520
Just like with the Nemesis [br]System in Shadow of War,

0:19:14.520,0:19:20.520
Maxis took inspiration from the concept [br]"yes, and", from improvisational comedy -

0:19:20.520,0:19:26.100
so the Sims try to build on the player's [br]actions, and try not to negate them.

0:19:26.100,0:19:30.300
For instance, if a player makes two [br]Sims fall in love - the autonomous

0:19:30.300,0:19:32.220
system shouldn't make them break up again.

0:19:32.220,0:19:37.140
Likewise, user-created Sims enter [br]the world with no sexual preference.

0:19:37.140,0:19:41.640
But if you instruct a male Sim [br]to, say, flirt with other dudes,

0:19:41.640,0:19:46.020
you're suggesting that he should be bi [br]or gay and the game will run with that.

0:19:46.020,0:19:50.760
The game should always try to maintain [br]the consistency of the player's story.

0:19:51.660,0:19:57.540
So, The Sims has a fascinating AI system - [br]which allows for realistic human simulations,

0:19:57.540,0:20:00.720
and creative player-led storytelling.

0:20:00.720,0:20:04.320
And most of it is driven by a very simple concept.

0:20:04.320,0:20:09.600
Characters, locations, and entire [br]neighbourhoods are given a bunch of

0:20:09.600,0:20:13.980
needs - and then try to make choices [br]that will best fulfil those needs.

0:20:13.980,0:20:20.400
With a little clever tuning, this leads to [br]characters who feel believable, social situations

0:20:20.400,0:20:26.520
where Sims act appropriately, and entire [br]neighbourhoods that stay consistent and balanced.

0:20:26.520,0:20:31.080
But this is not just useful for [br]running quirky goal-free life

0:20:31.080,0:20:36.600
simulations - and we can see utility AI [br]being used in other types of games, too.

0:20:36.600,0:20:43.380
For instance, in XCOM, an enemy unit will consider [br]every tile it can move to and score them based on

0:20:43.380,0:20:50.580
factors like distance, flanking opportunities, [br]angle, cover bonus, visibility, and proximity -

0:20:50.580,0:20:53.220
before moving to the tile with the best score.

0:20:53.940,0:21:00.180
It can also be used in procedural generation, to [br]build worlds that fit predetermined requirements.

0:21:00.180,0:21:05.820
Utility AI is a fantastic addition [br]to a game maker's toolkit, and its

0:21:05.820,0:21:10.840
implementation in The Sims should serve as [br]wonderful inspiration to game designers.

0:21:10.860,0:21:16.320
And now, just to melt your brain, [br]here's Katy Perry singing in Simlish.

0:21:16.320,0:21:24.396
Katy Perry: [Absolute nonsense]

0:21:24.396,0:21:32.101
Katy Perry: [Just complete gobbledegook]

0:21:32.101,0:21:37.900
Katy Perry: [I think I'm having a stroke]