Maxis

Maxis added autonomy because micromanaging a family's needs was tedious. Sims needed enough free will to look after themselves, but not so much intelligence that the player could sit back and do nothing.

Autonomy had to meet three demands at once: Sims read as believable humans, adapt to any house or social situation, and still leave the player work to do.

Objects advertise what they offer, and each Sim scores offers against its needs. A bed advertises "sleep on me to get 10 energy" and a toilet offers +20 bladder. An exhausted Sim boosts the bed's score dramatically, a well-rested one damps it, and the Sim takes the top option.

In The Sims 1 there are eight motives (hunger, hygiene, fun, energy, bladder, social, comfort, room), each from -100 to +100, decaying at rates tuned to roughly eight hours of sleep and three meals a day. Will Wright took the idea from SimAnt, where ants follow pheromones and Sims follow fridges and TVs.

Tuning curves and randomness make scores feel human instead of robotic. Each motive has its own curve modelled on Maslow's hierarchy, so physical needs dominate when low while fun, social and comfort grow more attractive as the Sim gets happier. The Sim then picks at random among the top-scoring options, so it never meets its needs perfectly.

Personality and distance also change scores: a serious Sim picks the bookcase over a pinball machine offering the same fun, and nearby objects score higher. Some interactions, like a phone call or the school bus, override everything; the imperfection gives the player something to do.

Putting the data on objects let Maxis scale content without new code. Because behaviour lives in object advertisements, Maxis can add hundreds of objects and endless expansion packs without touching the rest of the code, and Sims act lifelike in any house.

This needs-based or utility AI, where the option with the most utility wins, runs in all four games in the franchise.

Traits are extra motives, so one system makes every Sim unique. Five trait slots and about 60 traits give roughly 5 million possible Sims in The Sims 3. Objects advertise to trait motives the way fridges advertise to hunger, so a couch potato needs the sofa and TV as much as it needs to pee.

Evil Sims get advertisements for scaring Sims, stealing candy from a child or writing trolling comments. Traits also add idle behaviour (a clumsy Sim trips, a slob burps), bonus moodlets and exclusive interactions, replacing the Sims 1 personality meters.

Temporary context motives make Sims behave properly in any setting. Motives like "be in gym" or "be hospitable" are added and removed as situations change, so hosts and guests act fittingly without hand-scripting specific rules.

Traits can override the norm: at a picnic strangers are discouraged from joining, but an "inappropriate" Sim sits with them to make them uncomfortable. In The Sims Medieval, work motives apply only while a Sim is on the clock.

Ranked production rules cover social cases autonomy can't handle. Conversations run on input, conditions and output rules where the most specific match wins: a joke gets a laugh from a Sim with good humour, an insult if the relationship is sour, boredom if repeated. Maxis wrote thousands of rules for hundreds of conversation topics.

Because rules sit in a stack ranked by specificity, they can't clash or loop and designers can keep adding them. House visits use a similar ruleset: a guest who sleeps in the host's bed gets a warning, then gets kicked out if they go further.

Towns and lots run on the same motive idea as Sims. A restaurant wanting about 8 diners at meal times hands Sims an "eat outside" motive, favouring culinary Sims and discouraging frugal ones. The town aims for a 50/50 gender balance and about 80% employment, checking itself at midnight.

Off-screen Sims are only faked at low detail. Background Sims are scored once a day for big life changes like getting a job, falling in love or marrying, weighted by traits, relationships and careers. When one comes on screen, its motives snap to a designer-made chart for the time of day.

The charts encode likely needs, such as low hunger just after lunch. Neighbours look like they live full lives off camera, but they don't.

Maxis cut a realistic rule because random behaviour was funnier. A urinal one-gap rule was added to The Sims and removed before launch, because random toilet choice produced more memorable moments.

"When the Sim followed that rule it was just kind of predictable and dull."

Simlish keeps speech ambiguous so players fill in meaning. Maxis tried English, Navajo, Estonian and Ukrainian before having two improv comics make up Simlish.

EA's Matt Brown: "If we used actual language, the game would flatten and shrink, and everyone would be having the same experience." Ambiguity also hides moments where the simulation goes wrong.

A last-minute comic maker let players share the stories they invented. Playtests showed players building elaborate stories around their Sim families, so Maxis added a tool to screenshot, caption and publish simple comics to EA's website. It was extremely popular.

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

Autonomy follows the player's lead and never negates it. Sims eat and relieve themselves on their own, but won't quit a job or romance a random Sim. The rule is improv's "yes, and".

A male Sim the player tells to flirt with other men is treated as bi or gay.