Ditto Aliens

"Luke Rattigan: How...do you tell each other apart? General Staal: We say the same of humans."

- Doctor Who

The tendency for all aliens, within a given species, to look almost identical. This is contrary to human expectations, where the diversity of appearance within even single families of humans is remarkable. With extraterrestrials there is no evidence of this racial or ethnic diversity. Everyone from the same species will look almost exactly alike.

The technical reasons for this stem from the aliens' real-life origin. If they're Rubber Forehead Aliens, then the rubber forehead, if it is of sufficient weirdness, makes every actor who wears it look alike. If the aliens are Serkis Folk, the modelers got lazy and only designed one computer model (this is also why this is common in video games). If the aliens are Big Creepy-Crawlies, well, all bugs look the same anyway.

If anyone tries to point this out, an alien character may retort, "to me, all humans look the same." Alternatively, they may accuse you of racism. Counterintuitively, this is one way in which Human Aliens are more realistic than the more "complicated" types.

A potential explanation is that aliens tell themselves apart by non-visual signals. Many real animals, who may or may not know each other visually, get lots of information just from scent, but this is rarely used as a device.

May sometimes also occur in Fantasy.

Interesting side note: Some evidence suggests that humans have trouble discerning differences between faces that belong to a group they are unfamiliar with. "They all look the same", therefore, has some basis in fact. This is the basis of the All Asians Are Alike trope. However, this effect fades with time and exposure to different individuals of said different groups. Therefore, a good-old-boy living in Korea will, after initial difficulty, eventually be able to tell his neighbors apart quite well.

Additionally, neurological studies have shown that the human brain has an area that specializes in recognizing and differentiating human faces. When this area of the brain is damaged, it leads to a condition called prosopagnosia. People with this condition are able to recognize when someone has a mustache, large nose, brown eyes, etc., but is unable to recognize the face as a whole. Some describe it as "everyone looking like golden retrievers."

See also In the Future, Humans Will Be One Race. Compare Planet of Hats. Contrast Cast of Snowflakes, where even the aliens look different from each other.

Anime and Manga

 * The Emilys in Soukou no Strain looked exactly the same and even shared the same consciousness.
 * The alien race Tony belongs to in Axis Powers Hetalia all look like typical grays, though some have antennae. This is played with in Paint it White, where Tony's data claims that Earth is one of the most backward worlds in the whole galaxy because of its diversity and constant fighting. The Pict targeted Earth as part of its Assimilation Plot because it was rumored to be one of the most underdeveloped. Everybody on Earth is different and has their own ideas and views on the world, while they get along in harmony because they are almost exactly the same as each other allowing them to advance further in their technology and naturally get more "evolved."
 * This is also played with when it turns out that the Pict are indiscriminate of whatever world or being they come across

Film

 * Averted in Star Wars where most of the aliens given multiple member screen time are diverse. This includes suits (Twi'leks, Wookies, Iridonians, and Ewoks) and computer generated (such as Gungans and Yoda's people). An exception is the Gungan celebration at the end of Phantom Menace, we see a group of absolutely identical CG Gungans dancing in impossible lockstep. Like everything else about the Gungans, it was intended to be funny, but it totally breaks suspension of disbelief.
 * Averted in Avatar: The Na'vi all have noticeable difference in facial features.
 * Cocoon: Played straight with the Antareans, to such an extent that in the second movie.

Literature

 * Out of the Silent Planet by CS Lewis: This trope is touched on, and then averted in this sci-fi novel. A space traveler from earth finds he cannot tell the individuals of each local race apart, but after spending time with them, notices the differences.
 * In Harry Turtledove's Worldwar/Colonization series, which takes place over nearly a century, neither humans or the alien Lizards ever quite get the hang of even telling each other's genders apart.
 * In Doris Egan's Two-Bit Heroes, the heroine, Theo, is on a planet where the overwhelming majority of people have dark hair and eyes. She's bewildered when the locals insist that she closely resembles another offworlder -- at one point, someone tells her, "You could be twins" -- despite Theo having auburn hair and the other woman being blonde. Everyone having the same hair color has led to them recognizing one another by cues that have nothing to do with coloration, to the point where they don't even think about color as an identifying feature.

Live Action TV

 * Babylon 5 provides many counter-examples.
 * The Minbari all have different head crests, starting with caste differentiations and then extending down to individual characters, and each Narn had a unique facial structure and spot pattern. Even Centauri (who were the most human-looking and thus already easily distinguishable to viewers) used their hairstyles to advertise their status. When any other race appeared in quantity, they also were all individuals.
 * Even non-speaking background characters from the minor races were often diverse, although sometimes more due to Art Evolution (the makeup changing over time). Just for a few examples: Drazi sometimes have spiked cheeks, but many don't, and the scale patterns on their head are different - some even have smooth heads. The Abbai's crest comes in different lengths, and their colour ranges from orange to reddish-pink to beige, with different spot or blotch patterns. The Hyach come in different shades of orange from a near-beige to a near-red, and their scale patterns are different each time (some more pronounced, sometimes extending over the bridge of the nose). Markab sometimes have underbites, sometimes overbites. One of the Brakiri is bald, while most of his race have hair.
 * Pak'ma'ra seem to be impossible to distinguish for other people, which mostly results from the fact that the masks completely cover the actors faces and hide all facial features. Within the show, the similarity of individual pak'ma'ra is used to employ them as spies and secret couriers. They all look the same, never seem to talk to anyone, don't have any conflicts with other races, and are also said to smell horribly. As a result they are ignored by everyone, are almost impossible to be individually recognized, and security personnel are very reluctant to perform searches on them, which makes them perfect spies.
 * The family Zathras (all of them) is (are) the exception to the aversion. They're all played by the same actor, and all have the same name (albeit pronounced in several inaudibly different ways). The whole thing is played for levity, mostly.
 * Doctor Who: Several of the alien races, usually because the costume designer stretched the budget by making all the masks from one mould. The most popular of the Doctor's adversaries (Daleks and Cybermen) are intentionally homogenuous, which adds to their creepiness.
 * In The Sensorites, the humans' inability to tell the Sensorites apart inspires one Sensorite to impersonate another with no more disguise than a change of clothes; the imposture fools everyone he meets, including some of his fellow aliens.
 * That's right, the Sensorites were all physically identical, to the point that even they themselves could not tell each other apart except for their clothing, and not one of them had ever noticed this fact in all of the species long history.
 * An odd exception: the Sontarans, explicitly described as being a race of clones, didn't all look alike, firstly because a variety of different actors played them and secondly because the costume and make-up was mildly revised for nearly each story in which they appeared. (The first and second Sontaran stories, The Time Warrior and The Sontaran Experiment averted this by having the same actor player play the three Sontarans seen in that story. Even though they did have a continuity mix-up in the latter story.) Despite there still being slight differences between the two Sontarans whose un-masked faces we see on scene in "The Sontaran Stratagem" the Ditto Aliens phenomena is lampshaded in the typical way "we say the same of humans."
 * Star Trek Deep Space Nine
 * Odo on always said he had difficulty imitating humanoids (unlike other Changelings), and when someone said they thought he had perfectly imitated a seagull, he responded "I doubt the seagulls would agree."
 * After Sisko ends up taking the role of a 21st century historical figure, Quark fails to notice the resemblance even when it's pointed out to him, saying "All humans look alike."
 * A Star Trek the Next Generation episode had Wesley mistake a visiting alien officer for a friend of his from the Academy. Said alien explained that members of his race who come from the same "geostructure" look identical. When asked how the aliens told each other apart he replied "We just do."
 * This may be a Lampshade Hanging, as the alien was played by the same actor. It's possible it was intended to be the same character, until the actor came on set and explained why that wasn't possible.
 * Klingons in Star Trek are the archetype of the Rubber Forehead Aliens, but are a good counter-example to this trope. Their ridges are family traits and the exact pattern is as unique as a fingerprint.
 * Star Trek Enterprise managed a slight aversion with the Andorians, who, while mostly being various shades of blue, also feature a race of white-skinned Andorians called the Aenar. In the Distant Finale, we see that blue Andorian Shran married one of these Aenar, and their child is an interestingly unique Teal color.
 * The Asgard in Stargate SG-1 and Stargate Atlantis are all identical, though it is justified by the fact that they are all clones. (In the finale Daniel Jackson says they tell them apart from their voices). Other alien races such as the Wraith, are similar to one another, but have enough differences between themselves to be unique.
 * That would be a Lampshade Hanging or a Justified Trope or something, because the actor who plays Daniel Jackson also does the voice of an Asgard character.
 * Averted hard in Farscape where pretty much all the alien species seen have distinct differences among individuals.
 * Buck Rogers in The 25th Century: Played straight in an episode of where one race has no diversity of appearance beyond male/female.

Tabletop Games

 * Dungeons and Dragons: Played with with the Eye Tyrant (Beholder) race. To other races, they all look alike... but to the Beholders, that slight difference in the exact shade of red their skin is or just how long their teeth are that the other races overlook is a sign that the other Beholder is an abomination against nature that must be killed on sight. A Beholder would probably shocked and disgusted that humans or elves can't tell its purity from the debased nature of the other breeds... if it actually viewed anything that's not a Beholder as anything more than annoying vermin

Toys

 * Transformers: Exception -- Due partly to the Merchandise-Driven origins of the franchise, most Transformers look suitably different, have different weapons and abilities, and even those who share an alternate mode may transform differently. A small handful don't even have a humanoid robot mode. It's not uncommon for Transformers to be upgraded into new bodies by any number of means, either, so the trope has arguably been completely reversed here: Not only do they not look identical, sometimes they won't even look like they did last time someone saw them!

Exception to the exception: The plastic injection mold being one of the most expensive parts of the toymaking process, Hasbro and Takara tend to create multiple characters as recolors of the same physical design, so in the fictions some characters are model-mates with others. In G1, Starscream, Thundercracker, and Skywarp are all the same design, as are Thrust, Ramjet and Dirge. Rumble and Frenzy are the same model, as well. Then there are some specific "races" of mass-produced Cybertronian Mecha-Mooks, like Sharkticons, Sweeps, the Vehicons from Beast Machines, and some other examples.

Video Games

 * In Solar Winds, all the aliens of the nearby warrior race all look the same, as they use the same sprites. However at one point you are disguised as an alien through some Camera Spoofing, and it's explained they merely look identical to your (human) eyes. If you then run into the alien that the spoofed footage is based on, he'll notice immediately that you stole his face and attack you.
 * Used throughout Star Control, and lampshaded by the Zoq-Fot-Pik: "You must meet with our leaders. They are wiser... more powerful beings! ...They look just like us, though."
 * In the Mass Effect Expanded Universe, Kahlee Sanders notices that every single quarian looks exactly alike, and then when she thinks of it, nearly all members of alien races look identical. Then she notes that humans are beginning to follow the same trend, since in this universe, due to many interracial relationships, many humans are becoming more and more similar as all the races begin to mix together. She theorized that a couple hundred more years, humans will become Ditto Aliens.
 * Mass Effect actually averts this trope in that most of the aliens of the same species do look different, either by having different facial proportions or different coloration/markings. For example, asari have skin tones ranging from blue to deep purple, turians range from grey to reddish brown and krogan have different colored head crests and different shapes of eye while salarians have different skin patterns and variously shaped 'horns' on their head. True, some do look identical, only generic NPCs that reuse the same character model.
 * This is pointed out by Mordin Solus, who claims that humans have a much greater genetic variety than other races.
 * Lampshaded in the first game, when a human attempting to get a refund says to the turian clerk, "I know it was you, I remember your face." The turian is somewhat impressed that the human can tell aliens apart at all.
 * Spore, though earlier stages at least have "baby" versions with more exaggerated features. Civ and Space, though? All members of a given species are identical, including the outfits.
 * The Legend of Zelda: This fantasy series uses the Ditto Aliens trope a lot. When Gorons and non-lethal Zoras were introduced in The Legend of Zelda Ocarina of Time, each had exactly one model to go around - the only ones who looked any different were the Zora king and princess, and the Gorons' tribe leader. The gorons at least had deviation in size, but it was still the exact same model, just enlarged or shrunken as needed. This is in stark contrast to the Hylians, who all looked unique.

This was improved a bit in The Legend of Zelda Twilight Princess -- the Zora had two different models for standard citizens. A number of relatively important gorons got unique models as well, but the ordinary ones still all looked alike.
 * Originally, Pokémon did this with all of its Mons and most NPCs. In the newer games there are now differences between genders of Pokemon. Also, since GSC mook trainers have names.

Web Comics

 * Given a Lampshade Hanging in Schlock Mercenary, when a local sapient uses "they all look the same to me" line when referring to the mercenary group, offending the titular character Schlock, who is not even remotely bipedal. Hung with an even larger lampshade in a later strip of the same comic, where the alien members of the mercenary crew are complaining that a group of new recruits are all Terrans, and 'all look the same to them'. Given that at this point in the series history, 'Terrans' includes humans, various sapient varieties of ape (pretty much anything Chimpanzee-sized and larger, up to and including Gorillas), along with sapient elephants...
 * Uryuoms from El Goonish Shive seem to be Ditto Aliens as well. They don't even have different genders. However, they are big on fashion and cosmetics, so apart from the basic face, they can still be differentiated.
 * In one Starslip Crisis strip, when the crew is on Earth, two humans mistake Mr Jinx for their own cirbozoid employees. When Mr Jinx corrects them, one of them haughtily replies that cirbozoids all look the same... followed by a panel showing them standing next to each other, showing them to be nearly identical (mostly due to the strip's art style).
 * Averted in The Inexplicable Adventures of Bob. We've seen crowd scenes of dragons, bigfeet, and Fleenians, and they've always been pretty well individuated.

Western Animation

 * Exo Squad plays with this; it is mentioned several times that Terrans can't tell Neosapiens apart, to the point that every Neo has to have a unique "broodmark" tattooed on his/her head in order to distinguish them. However, they all look quite different.
 * Roughnecks: Starship Troopers Chronicles: All the characters are Serkis Folk, but the humanoid "Skinny" aliens are all the same model in (occasionally) different clothes. As are the bugs, within a given breed. The "Skinny" Colonel T'phai uses the "all humans look alike" line at one point.
 * On Futurama, the aliens of Omicron Persei 8 take the "humans all look the same" philosophy to ludicrous extremes by not being able to tell the difference between a woman with one eye and a orangutan wearing clothes, a wig, and an eyepatch.
 * Usually avoided in Ben 10, if only because you rarely see two aliens of the same species even if the scene is in a crowded space-prison, although you occasionally see an alien like the ones Ben can change into. There is a literal Ditto Alien, though, in Ben's Ditto form which can split into multiple copies. In Ben 10 Alien Force, however, the DNAliens and the Highbreed all look the same, and Ditto has been replaced with the sonically-empowered Echo-Echo.
 * Actually, the Highbreed had differing patterns for the eyes on their face, which are sometimes asymmetric. One episode featured extended contact with a specific Highbreed individual, who returned for the finale.
 * In one episode of Alien Force, a dragonlike alien claims that all primates look alike to him. He was probably referring the the similarity Kevin and Gwen bear to his enemies the Forever Knights, but at the time Ben was in the form of a four-armed blue monkeylike creature.
 * You mean Spider Monkey
 * Averted in Invader Zim. Irkens vary widly in looks, while still obviously being the same species.
 * Star Wars: The Clone Wars plays this very strangely in-universe. Most Gungans are fairly different looking, yet Anakin and Padmé decide that Jar Jar looks enough like an incapacitated Gungan boss to pose as him for the boss's followers, whom to further complicate this are at this point quite annoyed at Jar Jar and the Republic in general. And it works.

Real Life

 * This exercise demonstrates why this may well be Truth in Television -- while we're very well wired (for good reason) to distinguish between other members of our own species, without particularly noticable distinguishing features we're far less able to differentiate between individuals even of other commonly encountered species native to our own world. As such, it's no stretch to imagine we'd have difficulty with creatures from another world entirely.
 * However, people who regularly live or work with animals will usually learn to recognise individuals of that species after enough time.
 * People from one race are often unable to differentiate between members of another (unless they are well known to them). It's called the Cross-race effect.
 * More generally, an estimated 2.5% of people can't tell faces apart at all.