Edge Gravity

Early video game designers figured that if a player walked toward the edge of, say, a cliff, it was only reasonable that if they kept going when they got there, they'd fall off (typically to an untimely death). This sounds reasonable enough and makes sense.

But try it in real life. Go on, we'll wait.

It's not as easy as it looks. Reasonable though it may seem, it's actually very very hard to step off of an edge. It's something that the human body has an autonomic aversion to doing, as it requires moving your center of gravity out in front of your feet. Which means that it is, in fact, unrealistic for it to be trivially easy to walk off a cliff.

Edge Gravity is the property of the edge of a platform to actively resist the player's motion over it.

Edge Gravity can be implemented as simply as having the motion of a player character halt momentarily when an edge is reached; it may be indicated by having the character teeter on the edge. Games which use analogue controls may require that the player extend his control to maximum to cross the edge, or even press the "jump" button (reflecting that while you might want to jump off, or climb down, you're unlikely to want to step off the edge). Alternatively, a game may let the player go straight off the edge, but automatically spin around and hang from it. Or, to make absolutely sure, it could flat out slap an Invisible Wall in front of you.

Early Sierra Adventure Games were notorious for their lack of Edge Gravity (and for being condescending toward any player who fell victim to a steep drop). Avoiding such drops was often difficult because of the camera angles and difficulty of depth perception in a 2D game with 16 colours.

These are most commonly found in third-person 3D Platformers. First Person Shooters and 2D Platformers generally don't, though Donkey Kong Country is an interesting exception.

2D Platformers can generally simulate the effect with careful level design. Some kind of harmless obstacle is placed in front of important edges. This means that a careless (or extremely fast) hero won't walk into a Bottomless Pit or take a Leap of Faith without the player meaning to. Edges that aren't so important (because falling has no serious consequence) can thus be navigated at full speed with no cost to the player, making for a smoother playing experience.

If the Edge Gravity is unreliable, it can be a form of Fake Difficulty, and if it's so powerful as to be impossible to overcome, it can work as an Insurmountable Waist Height Fence.

If this happens while not on a platform edge, you just hit an Invisible Wall instead.

Action Adventure

 * In Beyond Good and Evil - you simply can't jump down or over an edge unless the game expects you to. (If you try, Jade will instead do the "grab the edge" routine, after which you can either climb back - or try to jump down once again, resulting in Jade looking around, seeing that the fall would have been fatal, and maintaining her grip.) This applies to vertical obstacles as well, sometimes leading to Insurmountable Waist Height Fence situations.
 * Link in The Legend of Zelda Twilight Princess will, if forced to walk off an edge, usually grab the edge and hold on for dear life. This makes it difficult sometimes to get him to actually jump off when you want him to, such as if jumping to a lower level in an area.
 * Additionally, in most top-down Zelda games past the original, Link will stumble a little at the edges of pits, giving the player a chance to get back onto safe ground. Especially amusing was the fact that this applied to all pit-type spaces, resulting in Link stumbling on thin air.
 * In The Legend of Zelda Spirit Tracks, you are actually attracted to lethally empty voids, and approaching any edges in the game is a mortally bad idea. Unless you want to jump to a different platform, which is only accomplished by running at the edge at high speed. Yeah, it's pretty dumb.
 * In Famous has many edges that are impossible to cross unless you jump. You can't just walk onto the street from the sidewalk, you must make a five foot leap.
 * Shadow of the Colossus has mild edge gravity on edges from which falling would do moderate to severe damage: Wander instinctively backs up from them if he isn't running fast, but you can get him to go over them by jumping. (Of course, that's not always a good idea...)

Action Game

 * Die Hard is an especially Egregious case. If you try to climb down the roof of the building on a rope, our hero refuses to do so saying he'd have to be desperate to try it, and yet he'll happily walk through an office window to his death. (Pointed out by The Angry Video Game Nerd).
 * Averted in the X-Men arcade game, as shown here, complete with a Narmtastic scream that wouldn't be out of place in King's Quest!

Adventure Game

 * Astonishingly present in the second Leisure Suit Larry game, where otherwise there are a lot of things that can kill you. You even get points for almost falling off the cliff, though they get taken away immediately afterwards.
 * In the first Broken Sword, one location on a mountaintop has an inexplicable "walk off the edge" option. If you try it, George understandably refuses, so at first it looks like just another of the game's many joke options. However, not long afterwards,

Fighting Game

 * The Super Smash Bros series of games exhibit this along with unique character animations when at the "event horizon". The game also features a variation of this trope that causes character to latch on to any nearby ledge when falling, a sort of Ledge Gravity.
 * Edge gravity is oddly variable in the series. A character performing a dash attack or an evasive maneuver has infinite edge gravity; however, a walking or running character has almost none (which is good, as it allows for air-based edge-guarding); certain attacks have no edge gravity (which can actually cause suicides, as with Fox's and Captain Falcon's side specials); and in some cases pressing Down + A for a Down Smash while on the edge will often cause the character to instead Fast Fall into a Down Air off the edge, which is invariably fatal. Can be avoided via C-Sticking, but still annoying.
 * Present in Dissidia Final Fantasy, dodging towards an edge will hang the character on it, you'll have to dodge again, or jump before dodging, if you want to get out of this (however, you can just walk off, but dodging is a no go)
 * It should be mentioned that the AI completely forgets about the edge gravity and will only dodge once, getting caught on the edge and almost always gaurunteeing a hit (this is completely acceptable to most players, since they otherwise have unbelievable reaction time.)

First Person Shooter

 * Golden Eye 1997 007 didn't just have edge gravity, it was literally impossible to step off an edge, even a foot high one (and no jumping, either). With the only exceptions being the very first level which required a bungee jump at the end, and the final level where . Its Spiritual Successor Perfect Dark didn't, but still had no jumping.
 * There are other places that lack edge gravity, like when jumping into the bunker in the first surface level. And in the Aztec level, there's a big chasm you can fall down that leads to a different part of the level. There's a slab you can walk on to cross the chasm, but you can't walk off the side of the bridge into the pit, even though there are no rails.
 * Star Wars Jedi Knight: Jedi Academy lacked this, and sometimes the more acrobatic enemies would actually jump off cliffs to their doom.
 * In every game of the Unreal series, the player can't fall off a ledge when not running.
 * In Serious Sam as well.
 * Bio Shock has a few docks with no railing, but you can never walk off into the water.
 * In Left 4 Dead and Left 4 Dead 2, if your character steps off a ledge or something similar, (s)he will dangle off the ledge for a period of time; getting back up on the surface requires a teammate to help you up. This period only lasts for a certain amount of time, however. If no help is offered before that time runs out, you fall and die. However, if you go really far over an edge, such as a long jump off the side or being tossed/knocked off by a Tank, it's an instant fall and instant death.
 * Because of the game mechanics, if your grip completely gives out, you're going to die no matter what. This includes hanging over a 2 foot drop that you would most likely survive from!

Platform Game

 * In Tomb Raider, the player can choose whether to walk (by pressing Shift while playing on PC or the R1 button while on PS 1) or run. When walking, Lara will not walk off an edge.
 * The same thing can be done in Prince of Persia.
 * Tomb Raider games made after the developer change (Legend, Anniversary, Underworld) will have Lara spin around and hang after running off an edge. Even if you want to drop from a height that wouldn't hurt her (much), she will cling to any available ledge like a lifeline; a lifesaver at times, but often results in Camera Screw.
 * Nicely averted in Prince of Persia: Sands of Time by making the Prince jump off the edge, spin around and dangle off the edge by his hands; this actually proves a useful maneuver when ducking wandering creatures, or dodging activated traps.
 * It carries over to the second and third games, possibly the reboot too.
 * Bunny Must Die is one of the few 2D platformers that actually has this. Sadly, it mostly contributes to the feeling that the main character was a Sherman tank in a former life.
 * Inverted in Castlevania: Falling from the edge of a platform results in falling down instantly at terminal velocity, as opposed to the gradually accelerating fall that happens if you jump down the edge.
 * Also happens in Mega Man 1, if you fall down from a magnet beam after walking on it for a while.
 * Mega Man suffers from exactly the same effect as Castlevania, where if you waltz off the edge of a cliff you typically rocket downwards, but if you jump you have a slower descent.
 * The magnet beam effect is actually a glitch; the magnet beam forgets to check your downward acceleration, so the game thinks you've been falling for quite a while when you step off.
 * Played with in the 2D Adventure Game A Boy and His Blob, where walking off an edge would give the player 2 seconds of realising he was not walking on solid ground ... cartoon style.
 * Toejam and Earl (the first one) averts this; your player will teeter for a while before caving into the player's commands and falling off. If they're using running or rocket shoes though, they'll take a big leap instead.
 * Lampshaded in the tutorial of 1213, when being instructed to jump over a pit. Don't worry! You can't fall off by walking over the edge. That would be very silly of you, wouldn't it.
 * Lode Runner 3D had a rather gimmicky implementation, in that you could only fall off corners, not edges.
 * Super Monkey Ball and some of its sequels have a variant: Most platforms have a very small rail at the edge, about 1/10th the diameter of the ball in height and width, though they become increasingly infrequent as you progress through Expert mode. Running into a rail while moving slowly and/or at a shallow angle may cause it to hold you on the platform, but don't count on it in most circumstances. The game also has unique monkey animations for when the ball is right on an edge without a rail or on top of the rail itself.
 * Several Sonic the Hedgehog games use this trope.
 * Sonic Adventure 2 allows Sonic or Shadow to stop and teeter for a second if you steer them into an edge and they aren't rolling. Interesting in that they can go from crazy fast to full stop instantly, which you can't do manually.
 * Sonic Heroes is annoyingly random about its edge gravity. It'll prevent you from running off the edge, except when it abruptly doesn't. However, one circumstance that is guaranteed to toss you off the edge is combat. If a power character attacks, he'll frequently move forward with the punch, which the edge gravity isn't programmed to notice. Since it's the game where the 3D Sonic's overabundance of bottomless pits reached critical mass, expect to die from punching several times.
 * In the 16-bit Sonic games like Sonic the Hedgehog 2 and 3, as long as Sonic (or Tails or Knuckles) isn't running at full speed, they will sometimes come to a complete stop if they reach an edge, regardless of if there's a bottomless pit or not. They all have unique animations if they're right on the edge. Sonic, in fact, has two in the second game; he'll balance on one leg if you're only near an edge and will be on both tiptoes leaning over with his arms windmilling for balance if you're right on the brink.
 * In Sonic Unleashed, Sonic the Werehog will automatically grab ledges if he walks off of them, but only specially designated ones that glow. There is a downloadable stage that is almost entirely this.
 * In Shadow the Hedgehog, this is played with--if you're running into a cliff at full speed, Shadow will stop and tumble a bit before standing upright. If you're thrown at the cliff and are unable to stop (ie hitting a spring or driving to the edge) he'll just fly off.
 * Harry Jr. in Pitfall 3D Beyond the Jungle won't willingly walk off a cliff...once. If the player tries again, though, he'll happily fall off the edge (sometimes this takes several tries, though). Unfortunately, Harry won't 'bump back' from the edge if he's walking on a container or box.
 * Kirby 64 The Crystal Shards: When balanced on an edge, Kirby will wheel his stubby little arms, teeter back and forth, and exclaim "Woah!" in the most adorable voice ever.
 * Bug! has this, on platforms with edge "borders". You can't walk or jump off a bordered side of a platform, unless you did a high jump (via a spring, or by Goomba Springboard).
 * Super Mario 64 and Super Mario Sunshine will cause Mario to grab onto a ledge automatically if he ever steps off. This becomes an essential mechanic in the latter, with cliff-climbing sessions with ledges too narrow (or obstructed) to walk on but still grabbable.
 * Banjo-Tooie also allowed Banjo to grab onto a ledge if he fell off one. Oddly, Sergeant Jamjars had to teach Banjo how to do it.
 * If you are playing a pirated copy of Mirrors Edge, anytime Faith attempts to leap off buildings/rooftops she'd slow way down: Mirror's Edge: Gravity Edition

Puzzle Game

 * The game EDGE, by Two Tribes, is all about this trope. You have to use edge gravity to hang onto platforms you can't stand on in order to solve puzzles.

Real Time Strategy

 * Sacrifice was set on a series of flying islands, with Insurmountable Edge Gravity to stop anybody falling off. (The game mechanic included neither jumping nor climbing, and each island's surface was composed entirely of slopes that could be traversed merely by walking, no matter how steep.) Flying creatures, however, could travel beyond the edges -- meaning they could take the straight route across concavities that ground-based creatures had to go around -- and if one was killed by a ranged attack while over the edge, its body would vanish into the abyss.

Roguelike

 * In the game Dwarf Fortress's Adventure Mode, the player can walk off a cliff... provided they hold Alt while they press the arrow key, and then confirm the move on top of that. On the other hand, dodging an attack automatically makes you move to a random adjacent square and can easily result in jumping off a ledge.

Role Playing Game

 * Particularly noticeable in Final Fantasy X-2. Yuna will wobble and gasp when she gets too close to the edge of a cliff, or when a jump is mistimed.
 * Probably added as a courtesy to players of the original Final Fantasy X, in which you couldn't walk (or jump) off of anything at all. X-2 takes place in the same world, but with 3D movement tacked on.
 * She actually does walk off a cliff accidentally in a cutscene though, while dizzy from vertigo. Guess where the camera goes during this scene too...
 * Followed to the point in Final Fantasy Crystal Chronicles: The Crystal Bearers when the player tries to run Layle off a cliff to a fatal drop; he will fall for a bit then use his gravity-bending powers to pull himself back up from the last thing he fell off of.
 * In Gothic, characters can walk towards some edges at less than running speed and will stop short of walking off it. While this does a good job at keeping characters from falling off high areas to their death, and saves on some NPC stupidity, this also make it difficult to slowly walk to and drop down to any narrow areas just below where you currently stand.
 * In Ultima VIII: Pagan, there were three speeds to move at: a careful tiptoe, a walk, and a run. You could only actually fall off a cliff at a full run. Attempting to tiptoe (and maybe walk) off a cliff stopped movement, and had the character teeter at the edge, saying "Whoah!"
 * The "tiptoe" speed could be used to force the Avatar to fall off (after his "whoah") as long as you were doing it absolutely perpendicular to the edge of the cliff - given the slightest chance he'll walk around instead. Which isn't as useful as it sounds since there are several places in the game where you have to fall down a hole, which can be frustratingly difficult to arrange.
 * The first 3 Wild Arms games have the character teeter if you want towards an edge. You can then choose to jump down or not. If you're running, however, you automatically jump down. However, if there's a Bottomless Pit below the edge, you just fall. Go figure.
 * In The Elder Scrolls II: Daggerfall, moving over a ledge normally involved your character jumping from it. As most holes in dungeons were one span across, trying to force your character to fall down them without landing on the other side or clipping into either wall was more difficult than it should have been. You couldn't sneak (speed reduction) off of ledges either.

Simulation Game

 * All three Lost in Blue main characters go "Whoaaa!" when they get too close to the edge of a high cliff.
 * Minecraft plays this interestingly, you have no edge gravity during normal movement, but if you crouch down it is impossible to fall, and you can even move a few inches beyond where you would normally fall from.
 * What makes this weird is that if you release the crouch key while being a few inches beyond the block you're "standing" on, you'll simply fall down, often to your death.

Sports Game

 * A very interesting version in the Tony Hawk games. When going vertical on your skateboard (or bike or what-have-you) you will frequently turn around corners, so that you come down perfectly - well, as long as there's ramp left to land on, anyway.
 * In SSX for the Xbox 360 and PS 3. If you're moving toward a bottomless pit that's off the beaten path, the player character will deliberately resist your control and try to steer away. You'd better listen to them.

Stealth Based Game

 * Metal Gear Solid 2 and Metal Gear Solid 3 have a similar system. If the player character dives or rolls off the edge they will fall, but if they run or walk off, they will grab the edge and hang there. Irritatingly, certain traps are made by changing the Edge Gravity of a given set of floor...
 * Hitman, most of the time.
 * Assassin's Creed provided you with just about every conceivable option. Walking to an edge (generally) made Altair stop. Running to an edge made him visibly teeter and either regain his balance or grab onto the edge. Specifying the Le Parkour mode would make him leap off the edge without hesitation -- hopefully to a platform or handhold.
 * The Splinter Cell series counters its near-suicidal aversion of Edge Gravity with Sam's ability to dangle off most anything he can fit his fingers on, so long as you press the D-pad in that particular direction. In the first iteration (at least), alert guards didn't even have the ability to aim down at Sam, nor did they consider stepping on his hands.

Survival Horror

 * The eerie alternate world of Silent Hill is loaded with cutaway floors that are impossible to fall through. This was averted in Silent Hill 3 where Heather will sometimes stop and teeter at the edge of a Bottomless Pit, but other times lose her balance and stumble in. Good way to skip the opening nightmare sequence.

Wide Open Sandbox

 * Contributes to That One Boss in No More Heroes. Rank 6 fights you on a beach, which she has filled with pitfall traps. When you run over one the first time, Travis falls in (and has to climb out fast before a grenade the boss tossed in goes off). Once the pit is exposed, however, it has no edge gravity - running into it forces another climbing session, as does running too close to it.
 * You cannot run into the ocean. You can drive into it, the game takes you back to the motel.

Other

 * A weird non-videogame example occurs in X Men Origins Wolverine during the climatic battle on the top of the coolant tower. Punched, kicked, tripped, backdropped and blown up, characters always land on the top of the wall, which is less than a meter wide.