Joon Sung Park, creator of the legendary Stanford Generative Agents paper (the Smallville experiment) and founder of Simile, explains how AI can now simulate human behavior at 85% accuracy—as in, the simulation predicts what someone would do almost as well as they could predict their own behavior.
Smallville origin story: 25 agents in a tiny town, each with a persona, daily routine, and relationships. No scripting. One agent (Isabella, the cafe owner) spontaneously decided to throw a Valentine's Day party, went around inviting people, gathered materials, and on the day of—people actually showed up. Klaus, who got invited, decided to ask his crush. This was emergent behavior from the agent architecture: memory + planning + reflection.
Before Smallville, there was Social Simulacra (2022): simulating an entire subreddit with thousands of personas to see what behaviors emerge. Built on GPT-3, which was janky and pre-instruction-tuning, but the promise was visible.
The product today: Simile partners with Fortune 500 companies like CVS. You define the population, Simile collects real data through RL-trained interviews ('Tell me the story of your life' in 15 minutes) and Gallup partnership surveys, then creates agent simulations that answer any question about that population.
The say-do gap: LLMs are trained on what people say online (attitudinal data), but there's a real gap between what people say and what they do. Simile's behavioral models—trained on repositories of RCTs (randomized controlled trials)—close this gap.
CPU vs GPU of intelligence: today's frontier models (GPT, Claude) are like the CPU—rational, objective, superhuman at math. Simile is building the GPU—models that represent the diversity of human values, preferences, tastes, and irrationality. You need both.
Convergence vs divergence: some simulations converge (network structure always forms hubs regardless of small errors, like PageRank). Others diverge (elections, wars). For convergent questions, compounding errors still converge. For divergent questions, run 100 times and show the distribution of possible outcomes.
The grand vision: simulation is the Hubble telescope for social science. Thomas Schelling won a Nobel Prize with rudimentary red-dot-blue-dot agent models showing how segregation emerges. Now we can simulate with agents that have the full richness of real humans.
The ultimate future: simulations that cost $100M to run once and take months—but solve fundamental questions of society. When does bank fraud happen? Can nations cooperate on climate? What are the signals of democratic collapse?
I am somebody who is quite inspired by science fiction. And when you read science fiction that covers societies that have progressed far enough in its technological maturity, you always see two pillars. You have some version of AGI, and you have some version of simulations that really help guide the society.
Smallville was basically a game town of 25 agents living in it. They would wake up the morning, do their routines, go to work, have relationships, and have emergent phenomena like having parties.
We demonstrated that using our architecture and the models, we can actually predict people's behaviors 85% as accurately as people replicate their own.
Turns out, people are irrational. We have subjective values, preferences and tastes. So you actually start to see divergence in model size going up and the performance in its ability to predict and simulate human behavior.
Today's models are akin to the CPU of intelligence. Simile's model is much more akin to developing something closer to the GPU of the intelligence unit.
The say-do gap: there are things that people say, and then there are things that people actually do. And the gap is real.
Simulation can be the Hubble telescope for human society. How can simulation really unlock our understanding of humanity and social sciences?