There are long-running beliefs that twinhood is hereditary and twins skip a generation, and these beliefs bleed into fiction. A family that has a previously-introduced set of twins (or triplets, or so on) can expect to have at least one more set waiting in the wings—even when this is nearly impossible according to real-life genetics.
The presence of established twin relatives, identical or fraternal, can often set up the birth of more twins or a long-lost twin showing up. If one of the parents is a twin, having twin children is often used to bring their story full circle (especially if the children are a male/female pair). Often, the newer or younger set of twins parallels the older one in some way: they bear strong similarities to the previous set that can extend to personality and behavior, take different life paths, or have an entirely different sibling relationship. In other words, the twin children are used as not so subtle Foils or Mirror Characters to the parents.
"Twins run in the family" will be used to explain all sorts of improbable twinning, such as a family having both fraternal and identical twins or multiple sets of identical twins, male fraternal twins going on to father twins, or sets of twins many generations removed from each other. A fictional species may even go as far as having a majority of births be multiples as a type of Bizarre Alien Reproduction.
In real life, there is no scientific proof that twins skip a generation, and as far as scientists know, identical twins are a spontaneous, random occurrence that doesn't run in families. Fraternal twins can run in the family, but only through the mother: some women are genetically predisposed to hyperovulationnote . Artificial insemination and a previous birth can also increase the chances of a multiple birth. Fiction, however, will often treat identical twins and fraternal twins similarly.
- In Another twins run in Mei's family. Besides Mei and her Separated at Birth twin sister Misaki their birth mother Mitsuyo was also a twin, and Mei was adopted by Yuikyo, the other sister, after she suffered the Tragic Stillbirth of her own child and financial problems in Mitsuyo's family meant they couldn't afford to care for two children.
- In Sekirei, the twins Hikari and Hibiki are both in a relationship with their Ashikabi Kaoru, and the epilogue reveals that the two give birth to two sets of twin boys.
- "Baby's First Baby", a fake baby doll by artist Darren Cullen, satirises the idea of kids roleplaying as parents by depicting a pregnant baby doll whose fetus is also pregnant. It also comes with a fake ad for twin baby dolls who are both carrying pregnant twins.
- The Flash: Prior to his death in Crisis on Infinite Earths, Barry Allen fathered fraternal twins—Don and Dawn—with his wife, Iris. It was later revealed that Barry himself had a long-lost identical twin brother who had been Separated at Birth and had been raised by the Thawne family, eventually becoming the villain Cobalt Blue. Since Don and Dawn inherited his superspeed, this led to the line in The Life Story of the Flash: "Twins ran in the Allen family ... literally."
- In Legion of Super-Heroes, twin births are the norm on Winath, to the point that children born without a twin, like the villain Lightning Lord, are outcasts in Winathian society.
- Young Avengers has a complex example. Scarlet Witch is the fraternal twin of Quicksilver, and Speed is the reincarnation of one of her magically-conceived identical twin sons. He mirrors Quicksilver's powers; his "brother" Wiccan mirrors Wanda's. In one Bad Future, Kate Bishop is pregnant with Speed's twins.
- Godzilla: King of the Monsters (2019): Dr. Ilene Chen, Monarch's chief mythologist and resident Ms. Exposition, not only has a twin sister, but also twin daughters, her mother and aunt are twins and her grandmother and great-aunt are twins. The novelization takes this even further, confirming that twins run in Ilene's family. Word of God reveals that this is meant to settle Ilene and her sister Ling into the roles of the Shobijin, Mothra's twin priestesses, so in this case there's a supernatural justification.
- Twins: At the end, the twin brothers have married twin sisters, and each couple has twin babies.
- The Belgariad: At the end of the Malloreon, Polgara (who had a twin sister) has twins; Belgarath says it's a family trait.
- Dragonriders of Pern: In Dragonsdawn, Barr meets up with Sallah after years apart. Barr tells Sallah that she now has five children, including a set of twins; after she gave birth to the twins, her husband informed her that he was a twin.
- The Fall of the House of Usher: Madeline and Roderick are Half-Identical Twins. One interpretation is that this sets up the twist that the Ushers are "all one line"; they were all born from BrotherSister Incest between twins and the "taint" upon their house is as a result of this.
- Heralds of Valdemar: The main characters of the books are identical twin girls with an older brother. Of the past five generations of their mother's family, three have consisted of identical twin girls with an older brother.
- Night World: Hinted at with the Harman family. Edgith's younger siblings, Elspeth and Emmeth, are fraternal twins, and Edgith's daughters were also twins. Hellewise, the ancient founder of the family, was a twin too.
- Secret Histories: Justified in the climax of the first book as the Awful Truth behind the Drood family. Their deal with an Eldritch Abomination causes all Droods to be born with an identical twin, who is sacrificed to the entity in exchange for magical power.
- In The Thirteenth Tale, Isabelle Angelfield, who is the fraternal twin sister of the violent possessive Charlie, has identical twins, Emmeline and Adeline. A heavy amount of Incest Subtext exists between Charlie and Isabelle, and the twins are noted to behave a lot like Charlie, so the townsfolk speculate heavily that Emmeline and Adelaide are twins born of Twincest.
- The Thrawn Trilogy: Leia is Luke's fraternal twin sister. In The Last Command, she births twins: Jacen and Jaina.
- The Trebond family in the Tortall Universe displays this, starting with Alanna and Thom in Song of the Lioness and later on with Alanna's own children Alianne (protagonist of Trickster's Duet) and Thom. Alianne herself ends up with triplets.
- Wolves of the Calla: in Calla Bryn Sturgis, the main setting of the story, this is taken Up to Eleven as almost all births are twins, with having just one child being a rarity. It is lampshaded however that this is not a natural phenomenon, and the abundance of twins is what makes Calla a target for the Wolves, who want 1 child of each pair of twins every 23 years in order to drain them from the innate material responsible for Twin Telepathy.
- Michael and Lindsey from Arrested Development are established as being twins when they're introduced. It's later revealed that their father has a twin brother himself. Later subverted when it's revealed that Lindsey is actually adopted.
- The Brittas Empire: The titular Gordon Brittas has a twin brother (Horatio Brittas), becomes father to no less than two sets of twins over the course of the series and is apparently part of the fourth generation of twins in his family.
- Desperate Housewives: Lynette and Tom have identical twins Porter and Preston at the beginning of the series, which Lynette blames on twins running in Tom's family. When Susan says that she thought twins were determined by the mother, Lynette tells her to shut up. So when Lynette has a Surprise Pregnancy in Season 5, she gets pregnant with twins again, although one of them dies in utero.
- Eerie, Indiana: This is invoked in the "Foreverware" episode. The plot involves a family whose members have been sleeping in giant tupperware boxes to stunt their aging for three decades. The episode ends with the family accelerating the "spoiling" process instead slowing/stopping it, aging the twin sons thirty years overnight. When found doing work outside their house by someone unaware of the family secret, they pull a Pretending to Be One's Own Relative and claim that twins run in the family.
- Friends: The final two-parter sees Erica, the woman whose baby Chandler and Monica are set to adopt, go into labour. The three of them are shocked during the birth to discover that it's twins, and as they discuss it, Erica notes that, "Twins actually run in my family!"
- Jane the Virgin: Petra gives birth to identical twin daughters, which sets up the reveal later in the season that she herself is an identical twin. And the finale pulls one last twist by revealing she is actually a triplet.
- Helena and Sarah on Orphan Black are revealed to be identical twins within a group of identical clones, who were mostly implanted in different birth mothers—but Sarah and Helena's embryo split into two. This likely explains why they don't have the sterility and associated ovarian cancer the other clones were designed with, since genetic transcription errors occur when an embryo splits. Helena then carries identical twin boys of her own later in the show.
- The Drake family on Pretty Little Liars. Mary Drake was revealed to be Jessica DiLaurentis's twin sister, which sets up the reveal that Spencer, since revealed to be a Drake, has an Evil Twin.
- On Riverdale, Cheryl is the Angsty Surviving Twin to Jason. In hindsight, Cheryl and Jason being twins is a hint that twins run in their family.
- Their father Clifford is revealed to have a long-lost identical twin brother in season 2.
- Jason himself fathers fraternal twins with Polly—Juniper and Dagwood. Cheryl attributes this to twins running in the Blossom genes, even though this would be dependent on the mother's genes, not Jason's. However, it's established that the mother is in fact Jason's third cousin, so could have gotten that part of the Blossom genes, or Cheryl could have been unclear on the genetics and is known to be very arrogant about her family heritage.
- Sabrina the Teenage Witch: The "Family Secret" that Sabrina has to learn to earn her Witch's License is that every member of the Spellman family is born with a twin, and in all witch twins one of them is inevitably the Evil Twin, who will be imprisoned in the Twin Cities in the Other Realm. Sabrina's twin is named Katrina, and there's a series of tests to determine which one of them is evil (foregone conclusion: it's Katrina). However, the "twins" are not actually siblings but inexplicably identical relatives from different branches of the family.
- WandaVision uses "twins begetting twins" for a different kind of plot twist. Wanda is a fraternal twin, and gives birth to fraternal twin sons when she had just been expecting one child. But seeing her sons reminds her of her brother, who had gone unmentioned since his death — which in turn causes the fragmented sitcom reality to unravel.
- In Fate/hollow ataraxia the Edelfelt family of magi has a unique bloodline ability that lets two members inherit the family's magic crest at once. During the third Holy Grail War the Edelfelt sent twin sisters to represent the family, and in the present day the granddaughter of one of the two, Luviagelita, also has a twin sister.
- In Dan and Mab's Furry Adventures, the Cubi clan of Zezzuva specifically has this as their clan quirk. Zezzuva herself gained her position from her twin sister by killing her and using her power to become clan leader.
- Members of the Severin family in Muted always birth daughters in pairs, with the exception of Avaline, who is an only child. We learn later in the story that this wasn't always the case, as she did have a twin but her own mother Athalie sacrificed her daughter as the price to have a demon murder her own sister and her children.
- The World of David the Gnome: While explaining the idiosyncrasies of gnomish biology, David points out that gnomes only have children once in their lives and they're always twins.
- Gravity Falls: Dipper and Mabel are Half-Identical Twins, which foreshadows the reveal that their Great-Uncle Stan is a fraternal twin.
- Hulk and the Agents of S.M.A.S.H.: In "Days of Future Smash: The Tomorrow Smashers", the Hulk is sent into the future, and encounters a new team of Smashers. Among them are two red gamma mutate twins named "Thad" and "Betsy", who are heavily implied to be Red Hulk's descendants. After he returns to the present, Hulk nonchalantly suggests that twins run in Red's family, which Red confirms (then confusedly asks how Hulk knew that).
- The Simpsons:
- One story in "Treehouse of Horror VII" has Bart be the conjoined twin of an apparently Evil Twin called Hugo, following his identical twin maternal aunts Patty and Selma.
- The Flash Forward episode "Future Drama" has Nelson get both Always Identical Twins Sherri and Terri pregnant. Later, both are shown to have given birth to twins.
- Highly exaggerated case in another Flash Forward episode, "Holidays of Future Passed". Apu and Manjula are briefly shown with their now adult octuplets, who themselves each had a set of octuplets.
- A town in Brazil called Candido Godói is notorious for an unusually high rate of twins, 10%, causing people left and right to be baffled. The most explanatory hypothesis for this reason is local genetic traits. It has been reported that twin births were particularly noted when the first immigrants of the town, which happened in the early twentieth century, included seventeen sets of twins. On top of all that, the population can trace their ancestry to the Hunsbrück region of Germany, which also has a higher than average twinning rate.
- The Derrico family of Doubling Down with the Derricos has a total of 14 children, and of those there are two sets of twin boys, a set of quintuplets (three girls and two boys), and triplets (two girls and a boy). Of the six pregnancies the mother has had, four have been multiples.