The concept itself is simple enough. Actually reaching this conceptual point took a while. Complex mathematical models have been, and are, common; deceivingly simple models only have their roots in the late forties, and took the advent of the microcomputer to really get up to speed.

1) Von Neumann machines

Although any discussion of the history of agent-based modeling must begin with von Neumann machines, von Neumann machines are not agent-based models—or, in fact, models of any kind at all. They are theoretical constructs, but they represent a paradigm shift that laid the ground for the study and construction of artificial life, and later bottom-up modeling.

In the late 1940s, mathematician John von Neumann suggested a machine capable of reproduction. The device he proposed would follow precisely detailed instructions to fashion a copy of itself. It would then gift the copy with a copy of the instructions, and thus create a fertile offspring. Actually building a von Neumann machine would have been a staggering task. But von Neumann's friend Stanislaw Ulam, also a mathematician, suggested that the machine be built on paper, as a collection of cells on a grid. The idea intrigued von Neumann, who drew it up—creating the first of the devices later termed cellular automata.

Although von Neumann never finished his mathematical proof, the implications were clear: the basis of life was information.

more about von Neuman machines

2) Conway's "Life"

The concept that life can exist as information led to some fascinating avenues of research. One was John Horton Conway's game "Life." Unlike von Neumann's machine, Life operated by tremendously simple rules:

Life occurs on a virtual checkerboard. The squares are called cells. They are in one of two states: alive or dead. Each cell has eight possible neighbors, the cells which touch its sides or its corners.

If a cell on the checkerboard is alive, it will survive in the next time step (or generation) if there are either two or three neighbors also alive. It will die of overcrowding if there are more than three live neighbors, and it will die of exposure if there are fewer than two.

If a cell on the checkerboard is dead, it will remain dead in the next generation unless exactly three of its eight neighbors are alive. In that case, the cell will be "born" in the next generation.

Conway and his team ran game after game, making every alteration of every cell by hand, studying a variety of configurations. Later, Conway opened up the game to competition, offering a prize for anyone who could prove that Life was capable of generating an infinite population given a finite initial configuration. The winning team programmed the game into a computer—the first known use of electronic computers to create artificial life.

Although current research deals with larger questions, Life still has devotees, with a number of web pages devoted to it. The simulation is a frequent topic of discussion. Its effect on the path to agent-based modeling is profound.

Links about Life:

Lifepage | Paul's Page of Conway's Life Miscellany | Conway's Game of Life

3) Modeling the real world—Boids

The life on Conway's gameboard was pure information, the rules incredibly simple—but the implications were complex. And, it would lead others to wonder, if such complex movements can result from such simple rules, might not real-world behaviors be the result of surprisingly simple behavior?

The use of artificial life to create bottom-up models of the real world followed from there. Sugarscape is such an enterprise. Another, earlier one was Craig Reynolds's "boids," short for "birdoids." To the human observer, bird flocking appears to follow some centralized control due to the sheer size and complexity of the flock.

Reynolds suspected that flocking was a decentralized activity, and created a computer model to investigate his theory. His agents, which he called boids, each followed a few simple directives; the behavior that emerged was eerily like flocking in nature. So much so, in fact, that the boids were used as a starting point for computer animators who needed to animate flocks for Hollywood films.

For more about boids, see Craig Reynolds's boids page.