Dynamic Difficulty

Video games attract all kinds of people, from the casual gamer to the hardcore tournament champion. But a game that provides a satisfying play experience for one may be frustratingly difficult or yawningly easy for another.

Thus, some games adjust their difficulty level behind the scenes to match the player (ignoring the fact people can just adjust difficulty levels).

When this is invoked, it's important to implement a ranking system so the better players can feel special. That way, casual players feel good about beating the game, and veterans still feel challenged trying to get the SSS-level and beating the True Final Boss.

On the other hand, some players inevitably prefer kicking ass on Easy and will get annoyed that the game won't let them, while others would rather play on Hard even if if means repeating That One Boss 20 times and will feel cheated if the game "lets them win." More opportunistic players will quickly learn to intentionally play just well enough to continue but no better to keep the difficulty down.

This is very similar to Rubber Band AI. Can cause problems such as Empty Levels. The opposite of Unstable Equilibrium, where the game gets harder if the player is doing badly. Not to be confused with Schizophrenic Difficulty, where the difficulty goes up and down at random, regardless of the player's performance.

Level Scaling and Difficulty by Acceleration are subtropes.

Action Adventure Games

 * Lego Star Wars II, Indiana Jones, and Batman feature "Adaptive Difficulty" which merely affects the amount of LEGO studs you lose upon death depending on how well you play, which can go up to a very high amount.
 * To keep things interesting, Diablo II scales the hitpoints of the enemies according to the number of players in the game.

Adventure Games

 * Snatcher had a shooting range which could be accessed for fun. If you get consistently high scores on the shooting range, the game amps up the horror of the shooting set-pieces.

Beat Em Ups

 * God Hand will adjust the difficulty up a level (1, 2, 3, and Die) if the player lands enough hits on enemies, increasing enemy strength and durability. It will then scale the difficulty back down if they take too many hits. You gain more rewards for defeating more enemies at higher difficulty levels. The game has "normal" difficulty settings, as well - the difficulty level never rises above 2 in Easy Mode, and Hard Mode has you always on Level Die.

Driving Games

 * The Need for Speed Underground series plays this one twice. First, the more you tune your car, the faster the other cars will be. Second, there's the Catch Up feature that makes the enemy cars go faster as you pass them, and slower as they pass you. In theory, the latter makes the game more balanced; in practice, it's nothing but Fake Difficulty.
 * To the extent that one of the recommended strategies if you're having difficulty is to remove all your upgrades, as the lower speed makes it much easier for you to react to obstacles and avoid mistakes. It's like playing in slow-mo.
 * Gran Turismo 4 uses "A-Spec Points" to encourage you to choose slower, less powerful cars to win the race: the more powerful your car is, the smaller your score will be. If, for example, you manage to win against a squad of sleek German machines with a puny little Pontiac Sunfire, you'll win 200 points; but if you win the race with the superpowered Polyphony Formula Gran Turismo, you'll win 1 single point.

Edutainment games

 * Many edutainment games start you off on the lowest difficulty setting by default. Then depending on how well you're doing, it would start to throw harder challenges at you. If you have trouble on a certain difficulty level, it keeps you on that level.
 * It's actually possible to exploit this in JumpStart typing. You actually have to type to a certain level of words-per-minute to unlock Coach Qwerty from the trophy closet, and the activities are merely to practice typing. What really sets the difficulty was how you typed in the initial trial, and then it'd set your goals. It's therefore possible to finish the game in a ridiculous amount of time by simply typing at 1 word per minute so the goals are set low, and then proceed to steamroll through it all.

Fighting Games

 * Mortal Kombat vs DC Universe features this type of AI. Lose, and it will edge down the difficulty slightly. Lose more, and it keeps sliding the difficulty down until you can win and continue progressing through the story. The final boss is not exempt from this difficulty edging, either.
 * Most of Capcom's fighting games will make it so that the opponent gives more and takes less damage the more you win. This occurs on any difficulty setting.
 * In The Simpsons Arcade Game, the amount of enemies on screen depends on how many people are playing. In the boss fight against Smithers, if there are more than 2 players he won't throw any bombs you can throw back.
 * Smackdown vs. Raw does this two ways. After a few wins the computer will kick into overdrive and engage The Computer Is a Cheating Bastard mode to make you lose, no matter how much it has to break the game to do so. On the other hand, winners and losers seem to be chosen before a match starts, so if you're facing a CPU opponent and they are slated to lose then they will really slack off and you have to carry them throughout the match.

First-Person Shooter

 * SiN Episodes: Emergence featured the Personal Challenge System, designed to adapt itself to the player's skill level and varies the skill, numbers and toughness of enemies faced in accordance the player's performance. It was claimed that thanks to this, a proficient FPS player and a brand new FPS player would be able to finish the game in roughly the same amount of time. However, a bug present on release in the system caused the game to never ease up on players making it overly challenging and unforgiving. This has since been corrected.
 * The game's balancing system worked off of invisible triggers you ran past, among other things. Going over one recorded the time since you'd gone over the last one and deactivated it. Going faster obviously made the game harder. What went wrong is that one of the levels had left in it an extra trigger used for testing, which made the game think you were going faster than you really were in one part... And it didn't deactivate, and was in the middle of an area with a fight so there's a good chance you'd end up going back and forth over it several times, causing the game to think you'd run through half a dozen checkpoints in 20 seconds, thus cranking the difficulty into the stratosphere and making the difficulty-lowering functions a drop in the bucket.
 * These invisible triggers are also autosave points, and loading from an autosave would lower the difficulty, while loading from a quicksave would not. This still applies (though not nearly as much) in the bugfixed release, though in the original version the way most people played (quicksaving and quickloading) meant that the game never let up, as opposed to dropping difficulty slightly each load from an autosave point.
 * Unreal Tournament has the option to turn adaptive bot AI on and off, and it gets quite eerie sometimes...
 * Valve's game Left 4 Dead features a "director AI" that spawns Infected based on how easily the players made it past the previous encounter. If the skirmish ends with the players healthy and having used few ammunition, the director sends in a horde. If the players are dying and low on ammo, the director only sends in a few. If you are playing on Expert and are doing poorly, it says "why aren't you dead yet?" and redoubles its efforts. Whimper.
 * Killing Floor increases the number of Specimens per wave and their health depending on the number of players.
 * Borderlands also makes enemies more powerful the more players there are.
 * This is, however, a straight case of Statistically Speaking. The enemies just have more HP and shields. They still do the same amount of damage as their level would inflict on a a single-player.
 * Halo: Reach has enemies not only more trigger happy depending on the amount of players, but also more aggressive in taking ground, more accurate and generally more competent.

Four X

 * The Expansion Pack to Galactic Civilizations II can be set to feature massive events that intentionally destabilize the playing field from time to time to keep things interesting. Additionally, the player can also tweak the personality, competence, and starting alignment of the factions.

Light Gun Games

 * The arcade Lethal Enforcers started with a certain amount of time a baddie took to hit you (a bit under three seconds). This gradually decreased over time and reset only after you took a hit. The upshot was that you'd have to shovel in a ton of tokens to get to the end of the game no matter how good you were. Lethal Enforcers 2 was even more brutal, with a shorter starting time and a much faster decrease.
 * Many Light Gun Games amp up the difficulty if there are two players present. Which means if some little kid decides to jump in on your game despite your protests, the game becomes much harder because the presence of an annoying kid who doesn't know how to play has made enemies more numerous and difficult to destroy.
 * For a specific example: The House of the Dead III. With 2 players playing, zombies double in number, and bosses take twice as much shots to cancel their Break Meters. This is especially bad with The Fool's swiping attack, which requires 6 shots in less than 2 seconds to cancel, and that's only in 1-player mode. In 2-player mode, you need 12 shots (6 per player), so unless the other person is competent at the game, he or she will be bogging you down and making you lose a life EVERY TIME, NO MATTER WHAT.
 * Also, for the whole series: Playing good or poorly in a stage will change the boss' speed.
 * Brave Firefighters, though not exactly a "true" gun game (you use simulated firehoses instead of guns), operates on a similar principle. If you play in 2-player mode, the time bonuses for clearing segments quickly are lower.

MMORPGs

 * World of Warcraft generally doesn't have dynamic difficulty - the bosses remain constant no matter how good or bad geared players are. Which makes most bosses a cakewalk for a well-geared group or even killable by only a single player who leveled to a higher level Cap from the next expansion. Still, some of them employ numerous One Hit Kills and other crafty mechanics which the players will still have to avoid no matter how well-geared they are.
 * Vehicle fights were originally a static difficulty - vehicles didn't scale with the player's gear, so the battle didn't change in the slightest. They were later updated to scale, though still slower then the players themselves do.
 * EVE Online features wormholes, where the amount of enemies depends on the number and size of the player ships.
 * And in a particularly egregious example, don't ever try to bring a carrier (a capital ship) into the fight in wormholes. Each time you do, there is an extra spawn of a few sleeper battleships. Unless you are doing this specifically to farm them, this is tantamount to suicide.

Multiple

 * Most games developed by Platine Dispositif feature a "Phase" system, where higher phase levels increase the number and speed of bullets the enemies shoot at you. Getting hit or using bombs decreases your phase level, while defeating mini-bosses (or bosses) increases it. Demonic Mode, which is Harder Than Hard, starts you off at the highest phase level, and it never decreases.

Platform Games

 * Games in the Crash Bandicoot and Ratchet and Clank series will often disable environmental hazards and/or move checkpoints if a player dies too often on one particular obstacle.
 * Ratchet and Clank also has a leveling system, and experience points are kept when you die. If you die a lot, you will end up getting stronger than if you played through the game without dying or revisiting old areas.
 * The Mega Man Zero series assigned you a rank after every mission based on how well you completed it. If you fought a boss while having an A or S rank, the boss used new, stronger attacks which Zero would gain for himself after winning the battle.
 * The Spyro the Dragon series also scaled difficulty depending on the skill of the player. Weirdly, this resulted in an individual playing on the lowest difficulty setting to be incapable of completing the third game 100%.
 * In New Super Mario Bros Wii if you die on a single level enough times, a green block will appear. Hit it, and you will activate Super Guide mode, where the computer basically beats the level for you.
 * Note this is only passing the level, and usually the bare minimum for doing so. You still have to do it yourself if you want the extra goodies or full completion.
 * The Super Guide returned, with slight variations, in Super Mario Galaxy 2 and Donkey Kong Country Returns.
 * Perhaps the earliest advertised use of this is in the Bitmap Brothers' Gods, which would at predetermined points give the player a "help bonus" if he was doing poorly, or spawn additional enemies if he was unhurt.

Puzzle Games

 * In Master Mode of Tetris the Grand Master 2 and 3, certain game mechanics behave this way:
 * TGM2 requires the player to clear each section in under 65 seconds to get a Master or Grandmaster grade (the latter being the ultimate goal of the game), and failure to do so in any section caps the player's grade at S9 for the session. However, clearing sections in 63 seconds or less can raise the bar for subsequent sections for that session. The time limit for section 6 is 65 seconds or the average time for the first 5 sections plus 2 seconds, whichever is lower, and the time limit for each subsequent section is 65 seconds or the previous section time plus 2 seconds, whichever is lower.
 * In TGM3, if you complete a section fast enough, you will get a "COOL!" message, which gives a bonus to your grade. The first two times you do this, it also causes the speed level to increase by 100. Meaning, if you get a COOL!, then advance to level 200, the game will act as if you're at level 300. Do this for the first two sections and you'll hit maximum drop speed at only three-tenths into the game (as opposed to halfway through if you don't get any COOL!s). In addition, the time limits to get a COOL! are also set as "X seconds or 2 seconds more than the previous section's time", with lower values of X for later sections.

Real Time Strategy

 * The Homeworld series scales enemy fleets to match yours, to a certain extent. In the first game, it's easy to have an overwhelmingly powerful fleet anyway by abusing the Cap system by capturing enemy ships. In Homeworld 2, capturing ships is no longer practical and enemy fleets scale so heavily in comparison to yours that building more ships makes the game harder, not easier. Amusingly, a well-known exploit is to retire your fleet at the end of a mission -- the next one will be staggeringly easy as you can rebuild your fleet in a few scant minutes.
 * In Age of Empires III, the level of your computer-opponents' Home Cities will scale up to match yours (or an average of all player-owned cities in use). This seems to also come with a slight increase in computer competence as well.

Role-Playing Games

 * The Battle Tower in Pokémon becomes harder the better you are. However, rather than just increasing AI skill, it also stacks the opponent's teams against you and manipulates the RNG.
 * The difficulty of a level in Nethack is based on the average of your character level and the dungeon level.
 * In Dragon Age Origins, at higher levels the enemies will start using attacks like Scattershot, Crushing Prison, and Curse of Mortality. These are dangerous enough when the player uses them; in the hands of the enemy they are lethal.
 * In Dragon Slayer, each time you kill a monster, the Clown Car Grave that spawned it will spawn the next tougher one.

Rhythm Games

 * Guitar Hero 5 has a battle mode called "Momentum", which increases the difficulty every 20 combo (starting at medium), and drops you back down after 3 misses in a row. Warriors of Rock adds "Momentum+", where it increases every 10 combo instead, and automatically drops the leading player back down to beginner difficulty if Star Power is deployed.

Shoot Em Ups
To make matters worse, every time someone plays the game, the starting rank goes up, and each time the attract mode loops the starting rank decreases. The idea is the more people are playing it the harder it gets, so it can eat more quarters, but if people stop it backs down so a new player will have an easier time and get hooked. However, people figured out that resetting the arcade game also reset the rank. So Raizing fixed this in their next game, Armed Police Batrider. The rank at poweron was the maximum possible starting rank instead!
 * Raiden, Raiden Fighters and Fire Shark have the tank/gunboat enemies. If you do well enough (survive long enough, amass loads of points, get lots of powerups/bombs), the tanks/gunboats that initially have poor reaction time as well as Painfully Slow Projectiles start shooting you with faster and more accurate shots as soon as they come onscreen. If you're doing well enough, you'll start meeting up with so-called "Sniper Tanks".
 * On a similar note, Battle Garegga also exponentially increases in difficulty by increasing enemy aggressiveness and health due to a number of factors, such as picking up too many items you don't need and shooting too much. The only way to reverse this? Dying. Cue crying from more conventional shoot-em-up players.
 * In modern Shmups, this is known to fans as "rank" and very common. This has been a feature in shooters since the Japanese release of Zanac in 1986. That game had artificial intelligence that adjusted itself to your playing style. However, the AI wasn't all that bright, and could be subverted by simply firing less.
 * Zanac for the NES got this ass-backwards. The AI level decreased if the player could defeat a boss within the given time limit but increased if the player could not defeat a boss within the time limit. Thanks, Compile.
 * Triggerheart Exelica calls this the VBAS (Variable Boss Attack System): basically, this means that the more point medals you collect during the level, the more forms the boss of that stage has and the harder it is. If you have enough, you even have to face a Bonus Boss afterwards.
 * In Darius Gaiden, getting a shot powerup will increase the difficulty to the stage's default difficulty.
 * Touhou has this is the 4th to 6th games. It gets remarkably less complicated each game. In the 4th game (Lotus Land Story) pretty everything that could be considered playing well raises it, but the opposite is also true, meaning that letting point items fall off the screen makes the game easier. The next game (Mystic Square) has fewer things increase the rank, but the only ways to decrease it are dying and bombing. Embodiment of Scarlet Devil has the rank just keep going up over time and can only normally be decreased by dying. And for some reason it resets to the middle level with each stage. None of the other games use rank, though Imperishable Night lets you start with more lives if you keep continuing.
 * Not exactly rank, but part of this trope: The AI in the story mode of the versus shooters, Phantasmagoria of Dim. Dream and Phantasmagoria of Flower View, blatantly cheats; there's a period of time at the start of each fight where the AI simply refuses to get hit. Every time you lose to it this time goes down by a bit. This is especially noticeable for PoFV's Final Boss; it's three minutes long, meaning that most people lose their first fight before even damaging her.
 * The Gradius series has both this and its infamous Unstable Equilibrium, the worst of both worlds.
 * In Do Don Pachi dai ou jou, using a Hyper will cause enemies to fire more and faster bullets.
 * And in DoDonPachi Dai-Fukkatsu Black Label, you get a new Super Mode known as "red mode" in which continuously using it will gradually crank up the difficulty, indicated by a special gauge off to the side of the regular HUD.
 * Tumiki Fighters makes the game harder if you try holding on to a large number of enemy ships (thus maximizing your score). If you lose these ships, the bullets immediately thins out.
 * Warning Forever adjusts to the player's tactics. If a player focuses on destroying a particular segment of a boss, the next boss will have that section reinforced.
 * If the boss manages to kill the player, it'll have more and better guns of the same type next time.
 * The faster you destroy groups of enemies and mini bosses in Ikaruga, the more enemies are generated. The screen is also cleared of bullets when you die, and dying also decreases the difficulty of certain bosses, especially Tageri. The third boss's difficulty is unfortunately bass-ackwards, as it spins faster the longer the fight takes (although it times out after a while).
 * The Ray Series also does this.
 * In Chaos Field, if you switch from the Order field to the eponymous Chaos field, your weapons become more powerful, but the bullets greatly increase in speed and density.
 * The difficulty of the bosses in Rez increases depending on the player's skill in shooting down enemies.
 * Daioh has a similar situation where the difficulty increases depending on how well you do. The farther you go without dying, the more likely it becomes Bullet Hell.
 * Dark Star One is a Double Subversion: as you collect more power-up artifacts, the game starts throwing more and stronger enemies at you, but if you exploit turrets and the Shield Boost the game actually gets easier. However, the Flak Cannon missions force you to deal with the scaled enemies without all your power-ups, and can be nearly impossible if you've gathered every artifact to that point.

Simulation Games

 * After Burner Climax adjusts its difficulty depending on how many enemies you shoot down in a level. If you get more than 50%, you'll earn a star (up to five). Dying brings the star level down by one, and using a continue resets it to zero. More stars = more enemies bombarding you with missiles.

Sports Games

 * The AI opponents in Wii Sports combine Dynamic Difficulty with Rubber Band AI. Get enough home runs in baseball, and they start to make it harder to do in subsequent games, for instance.

Third-Person Shooter

 * Resident Evil 4 does this, the fewer times you die the more enemies will appear and will be a tad smarter but if you kept dying they would decrease in number and stand still while you aim at their head for a minute, also ammo and health items would decrease/increase. This would stop players getting frustrated at sections where they are repeatedly dying but arguably it removed the challenge.
 * Resident Evil 5 has a similar dynamic difficulty mechanic, which has been discovered to use a hidden points system. Basically, the game has 11 separate "sub-levels" for the difficulty level, changing how much damage enemies deal, and how much damage they take from your weapons. Attacking and killing enemies adds points in small amounts (anywhere from 2 points for each hit you deal, to 100 points for a critical headshot), while taking damage or dying subtracts points in larger amounts (from 400 points for a small hit, to 1200 points for dying and continuing). For every 1000 points added or subtracted, the game shifts the difficulty up or down one level. Each of the four difficulty modes (Amateur, Normal, Veteran and Professional) has a minimum and maximum setting, and although some of them overlap, Professional mode is in a league of its own. At the lowest setting, enemies deal half as much damage to you, and take 2.5 times normal damage from your weapons. For the sake of comparison, Veteran mode's highest setting is the second-highest overall, where enemies deal three times normal damage to you, and take 88% normal damage from your weapons. The highest setting, which Professional mode stays at all the time, makes enemies deal ten times normal damage to you.
 * The official guide had an entire section devoted to this system, and how to game it.
 * Max Payne claimed to do this. The effect was pretty noticeable on the console versions, but the PC version seemed to be permanently stuck on "as hard as possible". Bear in mind, it had this in addition to three difficulty modes.

Turn-Based Strategy

 * X-COM let you select a difficulty level to begin the game at, and then moved it up or down a notch once per month depending on your score that month. This was rather undermined by an unfortunate bug, which reset the difficulty to the lowest level when you loaded a saved game. Saving and restoring frequently could make the game arbitrarily easy!
 * The monthly timer had very little effect even without saving and loading; since the game was made up of two executables, it had to save & quit each time you switched from a land battle to the world map (or vice versa). In short, it's impossible to play more then one battle before the difficulty resets to the lowest level - if you patch the game to fix this (using, say, Xcom Util), or use the Windows-based re-release, then the difficulty stays static.
 * The Super Robot Wars series has Battle Masteries, which are optional, and occasionally difficult, goals to reach in a mission. Depending on how many Battle Masteries (also known as Skill Points) you have at certain points in the game, the difficulty will scale back to Easy, or up to Hard mode.
 * Sting Entertainment's two flagship GBA games, Riviera: The Promised Land and Yggdra Union: We Shall Never Fight Alone, reduced enemy Hit Points and other stats (notably, the Rage bar in Riviera) when the player lost. In Riviera, if you lost enough, you wouldn't have to fight the enemies at all (although this didn't work on bosses, for obvious reasons). Yggdra Union would just scale down until a certain minimum point, and then stop (although practically, the player is getting stronger all the time because of how the game handles experience).
 * Early in the Wing Commander series, there was supposed to be an adjustment of AI skills, in the games before Difficulty Levels. However, for many there's often little to no notice of much of a difference, in any of the games where this trope was in effect.

Non-Video Game Examples

 * On the Japanese game show DERO!, the Bomb Room round worked this way. All questions were multiple-choice, and each time the team gets a question wrong, the number of choices on all subsequent questions was also reduced by one. If you did poorly enough, you'd end up facing 50/50 choices, albeit for relatively paltry potential rewards.
 * Some of the other rounds seemed to do this as well, albeit in a less obvious manner. For example, in the Block Word Quiz section of the Beam Room round, a team that was doing especially poorly would often get puzzles with answers only 2 characters long instead of 3.

Truth In Television

 * Real World Example: RN and EMT certification exams have a binary branching system, where your performance on each question will determine the difficulty of your next question. Because of this, if you correctly answer enough questions at the beginning of the exam, they will pass you without requiring you to finish.
 * The computerized GRE also does this, though you do need to finish answering all the question; it just impacts your final score.
 * Many computerized standardized tests do this.
 * In recent years, so do driver's ed tests.