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People procreate. Some do it a lot.Some really want to do it before time runs out.Some don't want to do it at all.
Yet, in fiction, people only procreate if it's relevant to the plot, and that includes Backstory. Some characters just don't seem to have siblings, and authors use this for every reason between heightening the angst of Parental Abandonment to excusing romance Genre Blindness. This applies even moreso in adoptive families— parents rarely adopt more than one child in fictionland, nor have both adopted and biological children unless the plot calls for it.
In a World of No Grandparents, all parents are also only children, which means no one can raise orphans except butlers, strangers or the school of hard knocks. The orphan in question usually is also an only child.
In shojo romances, the Plucky Girl heroine has seemingly avoided all interaction with the opposite sex until meeting the Troubled, but Cute school bully. Plucky Girl doesn't have any brothers, and if she does, they are either too young to provide any model of what she is dealing with or they are far older and are in the parenting role, thus they also don't count.
The Chosen Ones, Last of Their Kind and other such fulfillers of ancient prophecies not only suffer from Only Child Syndrome, their entire heritage suffers from it. When a man curses a family's name or goes after descendants for revenge, don't be surprised if 100, 200, 1000 years later, there is only one descendant. All generations previous must have felt bad for the eventual fate of The Hero and only had one child each to make the revenge short. Presumably in a more realistic story, if being the descendant of some ancient ruler means that one has a right to the throne, for every Farm Boy who becomes king, there are 373 cousins who Missed the Call. One wonders how they feel about this.
Most examples of Parental Abandonment in a World of No Grandparents also features Only Child Syndrome. Oh, the Wangst.
Expect some people with Only Child Syndrome to have a few subtleties. Such as, for example, most people who can point out Only Child Syndrome in Real Life can say an Only Child is often used to never having to compete with anyone else for parental attention, or feeling like they're the only person against the world. Or even a subtle disrespect or having never truly learned to live or communicate (in the worst cases) with others their age. This is in spite of the fact that it has been repeatedly shown in scientific studies that there is not a measurable difference in the personalities of only children as compared to siblings (at least not most of the time - there're a lot of perfectionists in the only child world). Sometimes this also happens with Middle Child Syndrome, too. Only children also tend to mature faster.
Some stories use Harem Genre situations to replace the character's lack of a family. Fans may convert a sibling complex onto characters, since siblings are so damn rare. Either for that reason or to explain away attachments for Shipping's sake; the unwanted girl doesn't always have to die for the ship.
Contrast Middle Child Syndrome.
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Anime and Manga
Averted in Black Jack. You would think that an Anti-Hero Doctor with notable Parental Abandonment issues would be an only child, right? Well... no, actually. He has a half-sister from his father's second marriage. They just don't keep in touch because she doesn't like him very much.
In Ichigo Mashimaro, Matsuri and Miu are the only ones who are explicitly only children; Nobue and Chika are, of course, sisters, and Ana... well, who knows? In episode 16, Nobue mentions this in connection with Matsuri's parents knowing to get her the new Harry Potter book. In episode 22, Nobue reflects on why Miu often feels left out.
Justified in JoJo's Bizarre Adventure - Joestars, whether by blood or by marriage, have a really unfortunate tendency to die before having more than one child. Even when they aren't killed, something gets in the way (Jotaro's father is a famous musician who's always on tour - and when we say "always", we mean "he never appears in the series"). Averted with Joseph Joestar, who had a legitimate child (Holly, Jotaro's mother)... and then had an affair with a Japanese college student that resulted in Josuke. His wife was pissed when she found out.
Every mermaid in Mermaid Melody Pichi Pichi Pitch is either an only child, or they're all sisters. Because most of the other races are off doing their own thing and mermaids have a thing about not revealing their identity to humans, the only way that most of them reproduce is by having their pearl create a new mermaid when they die. Caren and Noel refer to themselves as twins despite never even having met before the story starts just because they were born two minutes apart, and some of the songs refer to Aqua Regina as a symbolic "mother", but the Fridge Logic says they must all be only children.
Of the 12 Konoha genin in Naruto, only Kiba, Sasuke, and Hinata have any siblings, and then it's only one (an older sister, an older brother, and a younger sister respectively), while Gaara, Kankuro, and Temari are all siblings. A strange occurrence, considering that their clans are supposed to produce new soldiers to defend their entire nation. Of course, we know nothing about the family life of many of them, and there is a high number of dead parents so it's likely a lot of them didn't even have the chance to have many kids.
Also, it's flat-out stated at the end of the series that Neo Queen Serenity only ever has one child in the future (Chibi Moon).
Miaka and almost all the Suzaku Seven in Fushigi Yuugi have siblings, while Yui and the Seiryuu Seven (sans Amiboshi and Suboshi) were only children (we're not exactly sure about Tomo, Miboshi and Ashitare, though). However, the Suzaku Seven's siblings were either dead, killed off halfway through the story, or unimportant and thus were never mentioned. Miaka's older brother Keisuke had a minor role in the manga but played a more important role in the anime.
This seems to be the case in Mahou Sensei Negima!. Of the class of 3-A, only four characters have a confirmed sibling - the Narutaki twins (each other), Makie (younger brother), and Zazie (twin sister). Ayaka would have had one, but he died at birth, and Natsumi has to pretend Kotaro is her little brother, but she doesn't see him this way.
Negi has Nekane, though it is revealed she is actually his cousin. This does imply Nagi had one though, most likely an elder brother.
In One Piece, Boa Hancock is the only major character known to have biological siblings, as it eventually is revealed Ace is Luffy's adopted brother. There are a few minor siblings like the Nyaban Brothers, the Decalvan brothers, Hotori and Kotori etc. The series does much to say that it's not necessarily yourblood that makes you family, though.
In My Bride Is a Mermaid, Class 2-1 has 8 cast members in it (Nagasumi, Sun, Mawari, Saru, Lunar, Class President, Kai, Akeno). Of them only Saru and Akeno have any siblings (just one each) and four are explicitly only children.
Most lead Pretty Cures from the franchise are only children, with the exception of Nagisa and Saki. However, Tsubomi gets a newborn sister in the final episode and Love and Setsuna become adopted sisters after the first half of the season. Hibiki is actually the only lead Cure who suffers from being an only child while Miyuki, Mana and Megumi don't seem to care.
In Pokemon Special, there is a rotating cast of 19+ main characters, yet only one is confirmed to have a sibling: Green and his older sister Daisy, and even then she's most likely only there because she exists in the games.
Batman and all his proteges (though he adopted most of them; the various Robins seem to think of each other as brothers, for betteror worse).
Superman is doubly so - he was the only child of the Jor-Els (though granted, they might have had more if it weren't for the slight impediment of being dead), and he's the only child of his adopted parents. Understandably justified as his birth parents died, along with his entire planet when he was a baby and the Kents couldn't have kids of their own.
Wonder Woman's birth creation into a league of immortals doesn't raise as many eyebrows as to why few children are born. Since they don't die, if they had new children showing up as often as humans do, their home island would be standing room only after a few centuries.
Granted, Pre-Crisis Diana had a Black twin Sister. No, really.
Peter Parker (Spider-Man) is an only child but justified because his parents died a short while after he was born so they didn't even have the chance to have another kid.
All the Runaways are only children, but this is justified because each set of parents agreed to give their designated place in paradise to their offspring. Because there were only six places, the six couples had one child apiece. Victor is also an only child for obvious reasons. The same can't be said for Xavin and Klara, but then again, it's never been explicitly stated that they didn't have siblings back on Tarnax VII or in 1907.
Young Avengers: Hawkeye has an older sister, and it's implied that Patriot comes from a large family. Wiccan mentioned two younger siblings in the first run, but they have since failed to appear in an capacity whatsoever, so it may as well be played straight for him.
Averted once more by Fantastic Four; Reed Richards is the only member to be an only child. Jonny and Sue are siblings and Ben had an older brother who died when they were teens. Reed and Sue also have two children.
In Empath: The Luckiest Smurf, Papa Smurf reveals to Empath that he is Papa Smurf's only begotten son. However, Empath's mother did have another child through another father, which turned out to be Brainy, giving Empath a half-brother.
Bootstrap Bill only had one kid in Will Turner, something that makes the plot of the first Pirates of the Caribbean movie.
From book to the first movie of The Sisterhood of the Traveling Pants they removed one of Tibby's younger sibling so that she only has one, Bridget's brother so that she's an only child and Lena's sister so that she can go to Greece by herself. Lena's sister who we see in the second movie must have been jealous.
Almost all of the major characters in Harry Potter, including the title character and protagonist himself, are only children.
Bella Swan, the protagonist from Twilight, is an only child. Although Edward Cullen has adoptive siblings, Edward is biologically an only child as well.
The Wheel of Time contains a particularly egregious example. Throughout the first three volumes, Perrin never mentions any siblings and he explicitly says that he had no sisters. Then in the fourth book the trollocs come in and kill his entire family, including a boatload of siblings, some of whom are indeed female. Basically, they didn't exist until they became necessary to the plot.
In the Belgariad, Riva Iron-Grip's family has a tendency to produce only children, due to divine intervention. Garion, at least, is only alone because his parents were killed when he was an infant. (It's implied at the end of the Malloreon that he'll break the tendency - by having one son and legions of daughters. After all, the Prophecy stated the Rivan King would have only one son. It said nothing of daughters...) Belgarath the Sorcerer lampshades this.
In Polgara the Sorceress, it's stated several times that the line of hidden kings on occasion had multiple children (but not necessarily multiple males). Polgara presumably was only concerned with the eldest male child.
The main line itself flows through the only survivor of the Nyissan massacre, the king's "youngest grandson." Hopefully this was a one-off and all other all-male lines are extinct. We don't need another random challenger.
The protagonist of Tamora Pierce's Song of the Lioness quartet has a plot-significant twin, but her royal and noble supporting cast are almost all only children. In the case of Jonathan and Roger, it's explicitly for medical reasons: the Queen could bear no more children, and Roger's mother died, a condition stated to be very common. The follow-up series Protector of the Small averts this trope and has siblings up the wazoo.
Terry Pratchett and just about all of his protagonists. Though, considering how the Disc runs on narrativium, this may just be because the universe is aware of the trope. Meanwhile, Esme Weatherwax has an Evil Twin, and Tiffany Aching has about six older sisters and an Annoying Younger Sibling. Oh, and Mustrum Ridcully has a brother too.
Technically all of the wizards are an eighth child of an eighth child, but Ridcully's brother is the only sibling who really comes up.
In the Harry Potter universe out of the three main characters there is Harry, whose parents died when he was a year old, Hermione, who has no siblings and the Weasleys who are easily a case of Massive Numbered Siblings. Then again, maybe that's why Harry and Hermione marry who they do.
Death Eaters only have one child if any—just enough to satisfy plot needs: Nott, Malfoy, Crabbe, Goyle, Rosier, Mulciber, and Avery all have only one son. And these lines never go into three generations like real life would suggest. None of the original Death Eaters (Tom's era) have grandchildren in Harry's era, while none of the younger Death Eaters (Lily's era) have confirmed fathers who were Death Eaters (although there are some surnames that appear in both eras as Death Eater names).
Wizards live longer — much longer — than Muggles, though, so it may be that not enough time has passed for the third generation of Death Eater names to show, or they went to Durmstrang, or simply attended Hogwarts during some time period between Tom's school days and Harry's and then got out of the family business.
The majority of kids born to the characters in the Star Wars Expanded Universe are lone children or part of a male-female twin set. Wedge had two girls and Leia and Han had twins and then a boy, but they're the exceptions; for that matter, those lone children are overwhelmingly boys. Actually, a lot of Star Wars characters fit that pattern.
Justified in the case of Tobias, as his father Elfangor had died, and his mom had amnesia.
The book Charley by Jack Weyland has a typical Mormon son (except that he has no siblings) who marries an only child and together they have exactly one child.
The only characters in The Infernal Devices who aren't only children are Tessa, whose search for her brother Nate who eventually betrays her, and is actually her cousin kicks off the plot, the Dark Sisters, Gabriel, whose siblings aren't seen until at least the second book, and Will whose younger sister comes to the Institute at the end of The Clockwork Prince. Aunt Harriet, who was Tessa and Nate's mother's sister, is dead. Everyone else's parents are either deceased or simply not there, leaving them only children.
The Elenium takes a humorous twist to the "never had to compete" bit mentioned in the trope description. Patriarch Bergsten is big and impressive note Sparhawk decides he'd really rather not fight Bergsten, thanks anyway because he was an only child and didn't have to worry about sharing food with other siblings.
The Reynard Cycle: This is justified in the case of Hirsent. She had three children, but the first two died.
In Charlie and the Chocolate Factory, not one of the five Golden Ticket finders has siblings. (This also seems to be the case with Willy Wonka, who states he has "no family at all.")
Live Action TV
Almost all the main characters in both Buffy the Vampire Slayer and Angel seem to be only children: Buffy (pre-Dawn), Xander, Willow, Cordelia, Fred, Faith, Spike and Wesley all seem to have no siblings. Only Angel (sister), Gunn (sister), Tara (brother), and Andrew (brother) are known to have had siblings - and two of those four were either already dead or were killed off straight away!
Andrew, interestingly, originally had to keep being introduced as "Tucker's brother," even though Tucker was a villain of the week who wound up being much less important to the series and only appeared in one episode.
In The Sarah Jane Adventures every protagonist is an only child including Sarah Jane herself. Her son is a special case, as he was grown in a lab by aliens.
Sky - Luke's new adopted sister in Series 5 - has similar-though-not-matching reasoning for being an only child (biologically, anyway). On top of that, Sarah Jane's parents died when she was a baby, which justifies her lack of siblings. Meanwhile Maria's dad and Clyde's mum are divorced from partners with whom they didn't work out and Clyde's dad ran off with his wife's sister, whom it's revealed he impregnated, giving Clyde an unseen half-sibling/cousin. The only one with no excuse whatsoever is Rani, and to be fair not all parents in the present-day want more than one child anyway.
Nearly all companions in Doctor Who are only children: Rose ( until Jackie and alt!Pete get together, but her baby brother is mentioned only once, and she's an only child for the majority of her tenure), Mickey, Donna, Amy, Rory, and Clara are all only children. The exceptions are Martha (a brother and a sister) and technically Captain Jack (who, as it turns out, had a brother, but this fact is only revealed in his spin-off Torchwood).
Adric had a brother. Emphasis on had.
River has a younger brother, although, like in Rose's case, we only hear about him in what is the epilogue for her parents' storyarc and it's unclear if they ever met. Then again, given her parent's dangerous, unsteady lifestyle, and how it got you can understand why her parents might have refrained from having another kid untill their space travel days had come to an end.
Sort of justified, however, in that someone with little ties to their place of origin to begin with would simply be more likely to follow an extraterestial oddball on a joyride through time and space than someone whose siblings or parents would get worried.
In Saved by the Bell, Kelly and Slater are the only children to have siblings. Jessie does end up gaining a stepbrother. Zack, Screech, and Lisa are all only children.
Paige Matthews of Charmed grew up as an only child and when she grew up, she discovered that she had three biological half-sisters.
Very much averted in The Big Bang Theory. Howard is the only main character in the series who is explicitly stated to be an only child. Sheldon has an older brother, George, and a twin sister, Missy; Leonard has an unnamed older sister and a younger brother, Michael; Penny has an unnamed brother and sister; and Raj has a younger sister, Priya, three unnamed brothers, and another unnamed sister. Bernadette, Amy, and Stuart have an unspecified number of siblings as well. Numerous aunts, uncles, cousins, nieces, nephews, and grandparents are also mentioned throughout the series.
Almost every main character on The Vampire Diaries is an only child with the exception of Stefan, Damon and Matt. Caroline, Bonnie, Tyler, and Jeremy are all only child. Elena is technically an only child as well even though she grew up with Jeremy as an adopted brother (Jeremy is actually her biological first cousin)
Almost all of the core characters on Teen Wolf are only children. Scott, Stiles, Allison, Lydia, Kira, Jackson, Malia and Liam are all only children. Derek and Isaac have siblings, but Isaac's brother is dead, and Derek's two sisters were either murdered or Put on a Bus to South America. If Derek had any other siblings, they likely died in the fire along with the rest of his family before the series began.
Technically, Malia had a younger sister when she was a child, but she died in a car accident. And since Malia turned out to have been adopted, she wasn't biologically related to the girl anyway.
Also averted in Friends: Joey has seven sisters, Rachel has two sisters, Phoebe has a twin sister and a half-brother, Monica and Ross are brother and sister, leaving Chandler as the only only-child. And that's justified seeing as parents hated each other, his Dad was gay and they barely noticed the son they did have.
On Zoey 101, Zoey is the only one of the main characters to have a sibling, her brother Dustin. Lola mentions a younger sister in passing (season three's "Surprise") who is never mentioned again and never seen, and none of the other characters seem to have siblings at all.
This is discussed in an interesting way in Everybody Hates Chris. Chris is jealous of Greg when he comes to stay with them because he's an only child and he expects all of the attention and does everything by himself, whereas Chris usually has to take care of his younger siblings. Near the end of the episode, Chris confronts Greg, and Greg says that the reason why he does everything by himself is not because he wants to, but because he has to, being an only child.
The fact that Claire from Heroes is both adopted and an aversion, having a little brother Lyle, produced vast speculation over his origins because it's so rare for a family with adopted children to have more than one.
Smallville begins with the lonely Clark. Lex has also grown up without siblings. Lana's parents died when she was young. Pete and Chloe also appear to be without siblings. There is no reference to Kara having had any. Lois might be the one exception having grown up with a sister.
Jimmy has a younger brother. (Also named Jimmy.)
House has the title character and as far as we know Chase, Thirteen, Taub and Kutner. It takes a while to mention Cameron's brother, Foreman's brother, Wilson's brother and Cuddy's sister.
In Degrassi, the vast majority of the Loads and Loads of Characters that have appeared over the years are only children: Snake (older brother), Liberty (younger brother), Craig (half-sister), and Holly J (older sister) are some of the few exceptions.
They seem to be averting this in the post Season 10 episodes. Out of the characters still on after season 13, Drew had a stepbrother, Alli has an older brother, Miles has a brother and sister, Tristan has a stepbrother, Maya has an older sister, etc.
Rory is her mother's only child in Gilmore Girls but that might be because Lorelai made sure she didn't have another child out of wedlock. Of course, the basic premise of the whole series is that it focuses on a mother-daughter team.
In an exception to the vaunted realism of My So-Called Life, five of the seven youth characters (Brian, Sharon, Rayanne, Rickie, and Jordan Catalano) are only children, and the remaining two (Angela and Danielle) are each other's only sibling. These characters would have been born c. 1980, which was the beginning of the "Echo Boom", when Boomers settled down and started having families, and the birthrates rose.
Merlin zigzags the trope. At first, the core four characters (Arthur, Merlin, Gwen and Morgana) are all apparently only children. Arthur's mother died in childbirth, Merlin's father was run out of town before he ever knew he had a son, Gwen's mother is nowhere to be seen, and Morgana has a dead father and a Missing Mom. Arthur and Morgana are half-siblings through their father, and Gwen has a brother named Elyan who's spent the majority of his life running around, leaving Merlin the only one without siblings.
That '70s Show had Hyde, Jackie, and Fez as only children. Eric had sister Laurie and Kelso eventually had brother Casey make an appearance, while Donna had a sister but was reconned into being an only child.
Evidently averted in Murder, She Wrote, as Jessica Fletcher seems to have a different niece or nephew in almost every episode.
Carnivāle provides a justified case of this, at least according to the "Gospel of Knauf" that outlines the series' mythology. One of the pitfalls of coming from a line of historic Avatars (one of the super-powered humans at the heart of the series' mythology) is that a woman is rendered infertile after giving birth to an Avatar—who, as per the rules of the universe, can only be the firstborn male child of his family. Hence, Avatars can only have older female siblings, and are very often only children.
Most of the main characters of Community seem to be only children, with the exception of Annie (younger brother), Jeff (younger half-brother), Pierce (half-brother), and Chang (brother and twin sister,who he ate in utero).
Most of the titular agents in Agents of S.H.I.E.L.D. seem to be only children: Fitz explicitly states that his only relative is his mother, and Skye was raised in an orphanage and never kept a foster family long enough to gain siblings that way. Simmons mentions her parents often enough to make it seem like that's the extent of her family unit, particularly since when she thinks she's about to die her wish is to pass messages on to her parents and Fitz, but no-one else. Coulson and May's parents are, respectively, frequently mentioned and featured in cameos, without any siblings in the picture, and Coulson later described as a classic only child. Meanwhile, Triplett appears to be the sole heir to his legendary grandfather's stash of cool vintage S.H.I.E.L.D. tech.
The only Aversions are Ward and Koenig: Ward mentions two brothers and a sister (the two brothers have played important roles in his back-story and his older brother becomes a major player in Season 2, though the sister has yet to figure), while Koenig appears to have a suspiciously well-stocked army of Back-Up Twins.
This is all possibly Justified in that S.H.I.E.L.D. has a reputation for hiring loners, and lacking an extended family network surely helps in this regard. (Note that Ward, the only main character to mention having siblings, makes it explicit that they no longer have anything to do with one another.)
Romeo Montague and Juliet Capulet from Romeo and Juliet are both only children. Tybalt, Mercutio and Benvolio are presumably only children as well.
The Cousland family has two children, both of them grown, and a single grandson. All of them save the elder son (and possibly the second child, one of the possible origins) are killed in the prologue. By the end of the game Fergus is the heir and sole member of the family.
The Guerrin family consists of two brothers, one of them unmarried, and a son barred from succession because he is a Child Mage.
Rendon Howe has three adult children: one becomes a Grey Warden and thus is unable to procreate; his only daughter has one son; and his second son is dead by the end of the game.
Loghain Mac Tir has an only daughter, the Queen.
Queen Anora never had any children with her husband, King Cailan, who was likewise the only son of King Maric, who was the only son of Queen Moira, who was the only daughter of King Brandel, who was the only son of King Vanedrin. And, of course, Alistair is the last scion of the Theirin family, which does not bode well since he is a Grey Warden.
Even the minor families of Ferelden seemingly have only one or two children, and some even appear unmarried or childless. How this feudal, hereditary monarchy can survive such an impending power vacuum is never addressed. It's possible that, because this is a video game where the focus is not dynastic politics, many of these families are actually larger, or have extended, female-line branches that are simply not seen, but for a series that revels in creating a huge codex of extraneous lore, that would be a strange oversight.
Almost all of the Disney Princesses are only children. However, in the original poem 'The Ballad of Hua Mulan', Mulan has a Younger Sister, an Elder Sister and a Little Brother (which is who they named they dog after in the movie) who is not old enough to join the army, but old enough to kill pigs by the time she comes home. Also, the real Pocahontas probably had many brothers and sisters as Powhatan had quite a few wives. Belle is given an unwanted suitor instead of the two jealous sisters from the original tale. Cinderella still has her mean stepsisters, though. Ariel is an exception, having six older sisters.
Curiously, every child who had a prominent appearance in Danny Phantom is shown as being an only child. The only one who isn't is Danny himself. He has an older sister and if you wanna stretch it, an Opposite-Sex Clone whom he treats like a little sister.
Sam: If you don't mind, we'll be over there doing the "glad-I'm-an-only-child" dance."
On Ed, Edd n Eddy, Edd, Rolf, Kevin, Jimmy, Johnny and Nazz are all only children. Only Ed/Sarah, Eddy/his brother and the Kanker Sisters are siblings.
In King of the Hill, the three main kids—Bobby, Connie and Joseph—are all raised as only children, as were Hank and Luanne. Hank eventually gets two half-brothers, though (one born in his father's old age, one he just never knew about), while Joseph has at least one half-sister but doesn't realize it.
Of the six main characters in Recess, three (Gretchen, Gus and Mikey) are only children. TJ and Vince have elder siblings who have appeared in the show, and Spinelli has an older brother in jail.
In the early days of The Fairly Oddparents, the only ones shown to be siblings were Vicky and Tootie. Since then, a character is an only child unless a story involving siblings is written. Several seasons later Sanjay was revealed to have a step brother that hasn't been mentioned since, and Timmy wished for an older brother that he inevitably unwished. In season six Timmy got a godbrother in Poof. Up in Fairy World, Wanda was eventually revealed to have a sister, and in a currently unaired-in-America episode, Cosmo has a brother.
Partially averted on South Park: while most of the minor characters seem to be only children (except for Craig, who has a younger sister), out of the five "main boys" three have siblings, though Butters and Cartman grew up as only children.
Uh, it seems Cartman can now be crossed off that list of only children. And Scott Tenorman...
Aside from the Simpsons family, The Flanders', Apu and Manjula, the Hibberts and Sherrie and Terri's family the rest of the couples have one child only. With the exception of the above, most of the kids from the elementary school are only child.
This is justified in an issue of Simpsons Comics that ends with all the adults in Springfield becoming sterile.
The fourth season contestants fit this more closely; Zoey is the only one specifically stated to be an only child, but the only one who mentions a sibling is Scott, who has a sister.
In Metalocalypse, Pickles is the only member of Dethklok with a sibling.
Phineas and Ferb seem to play this halfway—there's the title characters and their sister Candace, Jeremy / Susie and Irving / Albert as siblings, but Isabella, Baljeet, Buford, Stacy and Vanessa all get lots of screen time and give no indication of having a sibling. Adults having siblings seems to be more common—Lawrence has at least one (Ferb has six cousins from what we know to be Lawrence's brother, and Scottish cousin whose relationship to Lawrence has not been revealed), Linda has Tiana and Doofenshmirtz has Roger.
Most of the babies in Rugrats were only children, but this changed after the respective releases of the first two movies: Tommy gained Dil as a younger brother after the first movie, and Chuckie gained Kimi as a stepsister after Rugrats In Paris. Among the main babies, Angelica is the last only child remaining (Phil and Lil are twins, and Susie is the youngest of four siblings).
In ĄMucha Lucha! Rikochet, Buena Girl and the other characters have no brothers or sisters. Only The Flea has a baby sister named Pulgita.
On The Adventures of Jimmy Neutron, not a single one of the main characters (Jimmy, Cindy, Carl, Sheen or Libby) has a sibling. Jimmy's lack of a brother was even the impetus of the plot of the episode "I, Brobot".
Ned's Newt: There is no mention of any of the kids having siblings.
On Codename:Kids Next Door out of Sector V, Numbuh 1 is the only operative to not have any siblings. Numbuh 5 has her older sister Cree and mentioned having an older brother in passing, Numbuh Four has a baby brother named Joey, Numbuh 3 has her younger sister Mushi, and Numbuh 2 has his younger brother Tommy.
Of the main eight kids on The Magic School Bus, five are only children (Ralphie, Tim, Arnold, Phoebe, and Keesha). Dorothy Ann has a younger sister (Evan), and Wanda and Carlos have younger brothers (William and Mikey).
Averted for the most part in My Little Pony: Friendship Is Magic; Rarity has her younger sister Sweetie Belle, Pinkie Pie has her sisters, namely her older sister Maud Pie, Twilight has her older brother Shining Armor (along with sister-in-law Cadence and even surrogate younger brother Spike) and of course, Applejack has her large family, inculding her older brother Big Mactinosh and her younger sister, Apple Bllom. They are pretty important supporting characters and even have a few episodes of their own.
Interestingly enough, the pegasi Fluttershy and Rainbow Dash have next to nothing revealed about their family in general, and even then, we have Rainbow Dash having a surrogate younger sister in Scootaloo and a flashback revealing who might possibly be her father (the stallion was revealed to be named Rainbow Blaze)
Laney Penn from Grojband is the only member to be an only child.
Since 1979, China's one child policy has prevented urban married couplesnote With numerous exemptions, such as: you can have two kids if you live abroad, if you've divorced and remarried, or if you're an ethnic minority. from having more than one child. This has led to coinage of the term Little Emperor Syndrome to describe the social implications of such a phenomenon when it has an effect on a nationwide level. The multi-generational aspect of this trope is averted, however, as parents who were both only children themselves are allowed to have two children.