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Analysis: AI Fallibility And The Chick Parabola

Analysis: AI Fallibility And The Chick Parabola

November 12, 2010 | By Soren Johnson

November 12, 2010 | By Soren Johnson
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[In this design analysis, originally published in the September 2010 issue of Gamasutra sister publication Game Developer magazine, Spore/Civilization IV designer & programmer Soren Johnson looks at the fallibility of AI and how games should tailor themselves to its capabilities.]

On March 11, 2009 during the Three Moves Ahead strategy gaming podcast, freelance journalist Tom Chick introduced a phenomenon which has come to be known as the Chick Parabola:

"My experience with Empire: Total War is this parabola of fondness. At first I don’t like it, so I’m at the bottom of the curve. I don’t like it because they do a terrible job with their documentation – it’s got a terrible manual; they want you to play through this scripted campaign if you want to learn anything; the tool-tips are really screwy. So, I’m hating it.

But then I’m playing it, and I’m learning it, and I’m liking it, so I’m climbing up that parabola. At the very top of the curve, I think, “Hey, I sort of figured it out. I like this game.” But then I start to discover that the AI is terrible, that it’s a dumb game, and I start coming down the far end of the parabola, and I am no longer fond of Empire: Total War.

Commonly, there’s this curve where I enjoy a game, and then I master the system, and then – unless it’s got a good AI – I lose all interest because I realize that mastering the system is where the challenge ends. Once I reach that point, the game is dead for me, and I hate that! That’s when the game should really start to take off."


Many veteran gamers will recognize this feeling from their own experiences – the rising enjoyment that comes from learning an interesting game system followed by an inevitable deflation as the challenge slowly disappears.

Sometimes, a simple technique or exploit becomes obvious that renders the rest of the game balance irrelevant. However, usually the culprit is a weak adversary as the artificial intelligence cannot grasp certain core game mechanics to offer the player a robust challenge. The problem is that the game’s designers have made promises on which the AI programmers cannot deliver; the former have envisioned game systems that are simply beyond the capabilities of modern game AI.

Symmetry Matters

Still, not all games suffer from the Chick Parabola. Many are so fundamentally asymmetrical – Super Mario Bros., Grand Theft Auto, World of Warcraft, Half-Life – that the AI is simply a speed bump that can be easily tuned to provide the right level of challenge. The games which suffer the most are ones where the computer is forced to play the same game as the human.

These symmetrical games – StarCraft, Street Fighter, Puzzle Quest, Halo – have a unique challenge in that each game mechanic must not simply be judged on its own merits but also by asking whether the AI can reasonably understand the option and execute it successfully. Unfortunately, asking this question often disqualifies many interesting ideas.

Artificial intelligence is notoriously poor at handling issues of trust and betrayal, of long-term investments, of multi-front wars, and of avoiding traps obvious to any human. The question of trust, in particular, has torpedoed multiple attempts to make a viable single-player version of the classic board game Diplomacy, which relies so acutely on being able to read one’s enemies, one’s friends, and one’s supposed friends.

Thus, to avoid the Chick Parabola, designers of symmetrical games must weigh carefully the implications of various game mechanics. An interesting play option which over-taxes the AI runs the risk of making the game more interesting in the short-term – as the player learns the system – but less interesting in the long-term – once the player masters the system and can use the mechanic to run rings around the artificial intelligence.

Of course, designers of symmetrical games built primarily for multi-player – such as the Battlefield series or the fighting genre – can choose to sacrifice single-player longevity for multi-player depth. Non-conventional weapons are fine if we assume that veterans of the game are only interested in playing the game with each other.

The human brain is remarkably flexible, with the ability to easily process novel mechanics which are orthogonal to the rest of the game. This approach has many advantages; Valve has been able to radically change the multi-player-only Team Fortress 2 with each character update (giving the Demoman a sword and shield, for example) without having to worry about toppling over an increasingly rickety AI.

Designing for the AI

However, symmetrical single-player games need to be designed as much for the artificial intelligence as for the humans themselves. Even if painful, designers must be willing to leave some of their most orthogonal – and often most creative – ideas off the table for the sake of the AI. Game design is a series of trade-offs, and empowering the AI is important for avoiding the downward slope of the Parabola.

Nonetheless, creative developers can solve this problem at the design stage before it even reaches some doomed AI programmer. One game mechanic that pushed Chick over the edge with Empire: Total War was amphibious invasion. The AI was simply incapable of coordinating its land and naval forces together to launch a coherent and effective invasion of an overseas target. Smart players would quickly learn that if the AI could not attack amphibiously, then the strategic balance can be gamed easily. Maybe England’s troops are not such a threat after all?

This problem is not unusual; strategy games with transportation units almost always suffer from ineffective artificial intelligence. Coordinating land and naval units to be ready in the same place and at the same time – along with the necessary escort ships – is a non-trivial task.

Rise of Nations, Big Huge Games’s historical RTS, presented a blunt but effective solution to this problem; land forces which approach the shore simply turn into boats to carry themselves across the water. Once they reach their destination, the boats transform back into the original land units. No transportation ships ever needed to be built or managed at all.

With one simple stroke, Brian Reynolds, the game’s designer, removed a classic AI problem from the game, enabling water maps to remain interesting for veteran players. The design may have sacrificed the “realism” of requiring the player to build transport ships along with other naval units, but the upside was extending the game’s longevity significantly.

Furthermore, many design changes meant to bolster the AI by simplification often have the side effect of making the game itself more enjoyable for the player. Quite a few players did not miss having to build and herd transports in Rise of Nations.

Civilization 3 and Civilization 4 introduced global unit support and city production overflows, respectively; both changes helped the AI manage its resources but also made the game more enjoyable for the average player by drastically reducing micromanagement.

Tough Choices

The designer’s biggest challenge comes when a mechanic which is demonstrably fun or core to the game’s theme needs to be simplified or dropped. Occasionally, a game can get away with assuming that a certain option will be human-only; in the original Civilization, Sid Meier added nukes to the end-game but didn’t allow the AI to use them.

He reasoned that because the super-weapon came only at the end of a game with such scope, players who used them were not abusing the game; they were simply having a bit of crazy fun at the end.

Further, if the designer wants to maintain a mechanic that the AI can’t use, cheating is not a viable solution for balancing away the AI’s disadvantage. Allowing too many human-only systems effectively turns a symmetrical game into an asymmetrical one, which will eventually affect the strategic balance.

In the Empire: Total War example, once players know that the AI will never launch an effective amphibious invasion, the rest of the game changes immediately. Maybe players don’t need to bother defending their coastal territories?

Maybe land-based allies are more important than water-based ones? Maybe the AI can be tricked into wasting its resources on futile invasions? Most importantly, the player is no longer playing like a queen – she is playing like a gamer who knows that the AI doesn’t work, one who is on the downhill side of the Parabola.

Ultimately, the designer may have to make a tough choice – drop a beloved mechanic or risk shortening the replayability? Many options do exist to extend a game’s longevity outside of pure balance – scripting a variety of scenarios, supporting procedural content generation, providing robust mod support, developing post-release content, and so on.

However, for robust replayability, nothing compares to pure strategic depth with a competent computer opponent. Sacrificing the game’s longevity to provide a few moments of fun for the human is essentially eroding the design at the foundation. As Chick puts it, when the player finally learns a system, “That’s when the game should really starts to take off.” The joy of learning is a big reason why games are fun, but no one wants to study for a test which doesn’t exist.

[For those interested in receiving similar columns from Johnson and other major columnists monthly as part of the world's leading magazine for video game creators, Game Developer is currently available for print subscription, and also in web-readable and PDF form via the Game Developer Digital service.]


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