The Sims series is a powerful world-building toolbox with 14 years of experience. And it's about to get even better.

On September 2, 2014, Electronic Arts' Maxis studio will launch The Sims 4, the new core installment of the hugely successful PC franchise. Like any sequel, the game will feel like an old friend: You're still going to create your own sims, build your own house, determine your own lives, and hopefully not kill or neglect your own avatar. But the technology underneath—the bones and sinews that hold the universe together—is seeing a major upgrade for the fourth generation.

For The Sims 4, the studio built a more powerful character creator, letting players tug, push, pull, and adjust every last detail about a Sim. The game integrates online social aspects, such as sharing house builds, Sims, and room decorations, and streamlined building for players who want to jump into the action.

"In a way, it's sort of like building three or four games at the same time," says The Sims 4 lead producer Lyndsay Pearson, who's been with the franchise since the beginning. "There's always things that we want to do with The Sims that we are never able to realize at various points or things we couldn't yet figure out what to do... Making The Sims to continue to be more believable, more dimensional, and to have more nuance to their behaviors is something we always strive for."

One step forward is tweaking the routing system, or the way Sims navigate. "Ever since The Sims 1 people have complained about the behavior of Sims," says Peter Ingebretson, a software engineer on The Sims 4. After researching human social behavior, the developers made Sims less awkward and more believable. "It's that subtle polish," Pearson says. "Now we can say 'Sims can now walk through doors without getting stuck,' but it's actually extremely difficult to make that happen. "

This image visualizes the potential routing outcomes for a Sim changing posture in The Sims 4

Resting at the heart of every Sims game is its artificial intelligence, the endless web of models and possible outcomes that runs the world behind the scenes. In gaming, AI usually refers to how the environment, and particularly non-player characters, interacts with the world and the player. However, The Sims franchise stands apart.

"In many other games, the AI serves as a foil to the player. The AI is controlling characters to do fundamentally different things than the player characters do….They're really operating in a different world with different possibilities," Ingebretson says. "[In The Sims] if you just sit back and watch your computer for a while, the AI will take over and also control the actions of your sims. That means our AI has to be more honest and has to make more believable decisions."

Listening to Ingebretson describe the ins and outs of The Sims 4 AI feels like a blunt lesson in human psychology. At its most basic, the AI works with the interplay between two mechanics: commodities and utility curves. Commodities represent a Sim's internal state while a utility curve dictates an avatar's desire to fulfill a commodity. Every interaction opens up a series of possible improvements. "For instance, if a Sim drinks a cup of coffee, their energy will go up but their bladder will go down," Ingebretson says. Just as in the real world, everything's a trade-off. So, when making every decision, a Sim considers all possible actions, analyzes their outcomes, references the utility curve, and selects the best one. However, these actions are somewhat randomized by design so the AI doesn't start to feel predictable.

This split-second exchange is what Ingebretson refers to as autonomy—basically, when a Sim is on auto-pilot. In previous Sims games, autonomy was a start-and-stop system. Once a Sim was completely done with an action, it would run autonomously. In The Sims 4, developers have improved the AI's efficiency by creating an autonomy hierarchy. "Instead of considering everything in the entire world every time a Sim is deciding what to do, we first evaluate all of the commodities and figure out what sorts of things are most important to the Sim," Ingebretson says. "That lets us eliminate from consideration a large amount of possibilities." This means Sims become faster and more efficient at making decisions and can multitask rather than following a strict "first this, then that" script.

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To give more insight into the mind of a Sim, Ingebretson describes how they flirt: "So when a Sim is deciding whether to flirt with another Sim, the first thing they've done is scored their desires and found that socialization is important to them. They'll consider a couple possible things. They might decide that socialization is in the top few options for them. They'll score all the things they can do, and we also score Sims in a hierarchical way. They'll pick a Sim that they like, are close to, or are interested in and at that point they'll consider the possible interactions they can perform. One of them might be flirt interaction. That might score highly because the Sim is particularly flirty or they're in a particularly flirty mood. At that point, the Sim will consider a number of different interactions and will choose flirty by possibly telling a joke, possibly telling a story, or possibly getting into a fight."

Sounds like high school.

All these improvements and advancements makes a game feel more natural, but when does an AI system become too efficient? "It's an interesting line we deal with on each of these projects because we can continue to make the Sims smart enough to run their own lives," Pearson says. "What is too smart?"

Left to its own, perfectly optimized devices, the AI could easily take control and play the game for you, letting Sims perform a unnatural number of actions at once or overruling player interaction. The team introduced delay and attention thresholds to artificially limit the system. "It's impossible for a human brain to manage that many streams of input," Ingebretson says. "For The Sims, we're building a model of human life. We need to model the faults of people as well as their efficiencies." What was once a technical limitation, then, has become an artificial one.

To work at on The Sims is to have relentless attention to detail. For the fourth installment, developers tweaked how Sims hold food, stand in a circle while socializing, and even introduced Lamarckian and Mendelian inheritance in the game's gardening system. In The Sims 4, you're in control of your own virtual world—now more than ever.

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