Twins’ Book Series: Behind Their Popularity and Beyond Their Pages
My Twin Studies Center at California State University, Fullerton occasionally (but regularly) receives donations in the form of books, articles, videotapes, CDs, DVDs, and various twin memorabilia. These materials have been provided by directors closing their twin centers, retirees downsizing their libraries, and individuals pursuing interests in twins and twin studies. Among my treasures are a signed copy of Monozygotic Twins Brought Up Apart and Brought Up Together (Shields, 1962; also signed by Shields’s colleague Irving I. Gottesman), a vast collection of news clippings concerning the monozygotic (MZ) Dionne quintuplets of Canada (bequeathed to me by a Dionne quintuplet fan in Minneapolis), and an original wood carving of the name ‘Jim Twins’ (crafted by the separated MZ pair that launched the Minnesota Study of Twins Reared Apart). These items represent a fraction of the materials I have amassed.
A number of years ago, I received two book series that can be considered ‘classics’ when it comes to fictional stories involving twins. They are the Bobbsey Twins, authored by Laura Lee Hope, and the Sweet Valley Twins, authored by Francine Pascal. Both collections were given to me by the late identical twin Kay Cassill, co-founder of the Twins Foundation (1980−1995) and author of Twins: Nature’s Amazing Mystery (Cassill, Reference Cassill1982). The Bobbsey Twins series includes 72 books, the first one written in 1904, and the last one written in 1979 (FictionDB, 2025a). I have 131 of these hefty hard-cover volumes in my Twin Center, but with duplicates some unique copies are missing. In fact, I counted 80 unique books; the additional eight may have resulted from two attempts to restart the series, neither of which enjoyed the same success rate as the original. It is also the case that several of the older volumes were rewritten in 1960 (Books Series in Order, 2025).
The Sweet Valley Twins series includes 118 books, the first one written in 1986, and the last one written in 1998 (FictionDB, 2025b). I have 37 of these slim paperback books that include 26 unique copies, six of which are larger ‘super editions’.
Both series are targeted to readers between the ages of 8 and 12 years. Almost every book has the owner(s) name written inside the front cover, usually printed in pencil in the unaligned, uneven handwriting of a young child. I did not examine each volume, but several Bobbsey Twins books were owned by opposite-sex twins who may have especially enjoyed these stories; see below. The enchanting covers and titles of these series can be viewed at https://www.abebooks.com/book-search/title/bobbsey-twins/first-edition/ (Bobbsey Twins) and at https://www.penguinrandomhouse.com/series/SVT/sweet-valley-twins/ (Sweet Valley Twins).
At the center of the Bobbsey Twins stories were two opposite-sex pairs born to the same family. The members of the older pair were dark-haired Nan and Bert, age 12, and the members of the younger pair were fair-haired Flossie and Freddie, age 6. I was an enthusiastic reader of this series when I was in middle school, but I knew nothing about its fascinating origins. In fact, these books were created by Edward Stratemeyer (1862−1930), identified as an author and book packager (Keeline, Reference Keeline2000) — a book packager is a publishing company that creates book content themselves, assisted by freelancer writers and in-house editors (McKean, Reference McKean2023). Stratemeyer’s professional labels make perfect sense — I recently learned that he wrote the first book in the Bobbsey Twins series under the pseudonym Laura Lee Hope, while the other 71 books were authored by ghost writers working under the auspices of the Stratemeyer Syndicate.
An extensive search did not reveal the source of Stratemeyer’s inspiration to develop a series based on twins. He was, however, the father of two daughters, Harriet and Edna, born two and a half years apart (Geni, 2018, 2022). Perhaps seeing the sisters together reminded him of twins, and/or the public’s fascination with the subject explain Stratemeyer’s development of the Bobbsey Twins series. His daughters managed the syndicate after their father’s death in 1930, although their business relationship was not always harmonious (Keeline, Reference Keeline2016).
The Bobbsey Twins series is noteworthy for possibly having the longest publication run of Stratmeyers’s children’s novels; however, it has triggered less nostalgia than some other series, such as Nancy Drew and the Hardy Boys (also Stratemeyer creations). That is because while the plots in the Bobbsey Twins books appealed to children younger than 8 years of age, the reading level was higher, so parents or older siblings had to read these books out loud. In contrast, older children were able to read Nancy Drew and the Hardy Boys on their own, establishing a more personal connection to the characters and the events (see Keeline, Reference Keeline2016: Bobbsey Twins Formats).
The Sweet Valley Twins follows the lives of adolescent identical female twins, Jessica and Elizabeth Wakefield, who did everything together until they entered Sweet Valley Middle School. I have never read this series, but I was generally aware of its popularity. The initial idea came from an editor’s suggestion that the author, Francine Pascal, write a ‘teenage version of Dallas [the popular television series about the Ewing family that aired between 1978−1991’ (IMDb, 2025). Pascal was intrigued and eventually arrived at the idea to center her stories on twins. She explained that ‘everyone is always fascinated by twins!’ Moreover, her literary agent was a twin, as was her sister-in-law. ‘Then I began to create a life for them, a history, and a family’ (Turner, Reference Turner2025). In answer to this same question about the books’ origins, posed in an undated interview, Pascal replied, ‘I always had a fascination with twins. The trick is to think of Elizabeth and Jessica as the good and bad sides of one person’ (Random House Children’s Books, 2024).
Elsewhere, Pascal revealed, ‘When I came up with the idea for Elizabeth and Jessica, the Jekyll and Hyde twins, I was off and running’ (Maughn, Reference Maughn2024). Her words probably represent the beliefs of many people, especially during the 1980s’ debut of the series, when less was known about twins’ behaviors. Also unfortunate is that it fuels the fictitious notion of each MZ twin as just part of a whole, stripping each twin of their individuality (Segal, Reference Segal2017). This dichotomy was probably perceived as tantalizing and somewhat truthful by readers who likened the Wakefields to twins they had known. For example, if one twin seems more outgoing or athletic than the other, it is tempting to exaggerate the differences between them to tell them apart. It also fascinates people to think that two individuals who look so much alike can differ in their behaviors. Of course, behavior is not in one’s face, as demonstrated by the lack of personality similarity in unrelated look-alikes (Segal et al., Reference Segal, Hernandez, Graham and Ettinger2018).
It is insightful to learn how Pascal presented the physical resemblance of the Wakefield twins. The following excerpt from the first book in the series, Double Love (1983), describes the twins’ identical features, with one exception:
Both girls had the same shoulder-length, sun-streaked blonde hair, the same sparkling blue-green eyes, the same perfect skin. Even the tiny dimple in Elizabeth’s left cheek was duplicated in her younger sister’s — younger by four minutes. Both girls were five feet six on the button and generously blessed with spectacular, all-American good looks. Both wore exactly the same size clothes, but they refused to dress alike, except for the exquisite identical lavalieres they wore on gold chains around their necks. The lavalieres had been presents from their parents on their sixteenth birthday. The only way you could tell them apart was by the tiny beauty mark on Elizabeth’s right shoulder. (Reproduced in Valdesolo, Reference Valdesolo2024)
Of course, it is unusual to find MZ twins as physically similar as Pascal portrays, especially regarding height, but it is possible. The height difference between MZ female twins may be as much as two inches, usually associated with adverse prenatal influences. I suspect that Pascal did examine the literature on twinning to some extent, since one of the twins had a beauty mark that the other twin did not have — this detail is quite correct (Segal, Reference Segal2017; see also Segal, Reference Segal2011).
Pascal’s description of the twins aligns with their being MZ; however, I was curious to know if they would be diagnosed as such by means of a standard physical resemblance questionnaire. Applying the various items and scoring procedure established by Nichols and Bilbro (Reference Nichols and Bilbro1966) classified them as MZ at the highest level. However, this classification was consistent with only one of the three relevant items: Both twins report they are frequently confused by close friends — or one twin indicates that confusion occurs ‘frequently’, and the other twin indicates that confusion occurs ‘occasionally’. (The first two MZ twin items concern confusion by parents when young and confusion by parents recently. This information was not included in the excerpt provided above, but it might appear in one or more of the books — however, most parents do not confuse their twins since they become aware of subtle differences between them that others miss.) Regardless, the twins’ matching hair color, eye color, and height, and their constant confusion by others are consistent with their classification as MZ at the highest level, and inconsistent with their classification as DZ. The scoring rules specify that if any level-1 item applies, the twins are diagnosed on that basis.
Interestingly, the identical fictional twins, Jessica and Elizabeth, chose not to dress alike, underlining a specific and real-life difference that may have made them more captivating to readers. Pascal indicated that many of her story ideas came from watching her own three daughters growing up (Random House Children’s Books, 2024) — whether this behavioral difference was drawn from Pascal’s own experience as a mother is unknown.
It is worth asking why these two series were so popular among young readers. There were many series to choose from that involved captivating nontwins, yet the Bobbsey Twins and Sweet Valley Twins were among the most popular books and are still being discussed today.
I believe that the general appeal of twins is largely responsible for the success of these series. Most people are curious about the similarities, differences, and relationship qualities that twins share, and these books offered opportunities for children to witness twins’ lives up close. Furthermore, male-female twins rarely received the focused attention that MZ twins typically attract — even among todays’ scientists — so the Bobbsey Twins series was probably welcomed by twin brothers and sisters, their families, and those who knew them. The presence of two sets in the same family also lent an interesting and realistic angle to the stories.
In addition to these two series, I also discovered the 26 volume twins’ series authored by Lucy Fitch Perkins, between 1911 and 1991. In my library are seven of these treasures, also gifts from Kay Cassill and also targeted to children — titles are The Filipino Twins, The Eskimo Twins, the Indian Twins, the Belgian Twins, The Puritan Twins and two volumes of The Scotch Twins. Each novel features a pair of male-female twins from a particular country — Perkins’s plan was to present the lives of children to other children around the world, to foster peace and understanding. A publisher persuaded her to write a series of ‘geography reading books’ using a boy and girl as the main characters (Dolan, Reference Dolan2016). I could not determine how and why Perkins chose to turn these characters into twins, but twins’ widespread appeal may have been behind this decision.
Fictional works can be educational if based on fact, often reaching wider audiences than scientific books and documentaries. I am always impressed when novelists pose questions to me to be certain that the twins and situations they create are truthful. The original movie, Parent Trap, which premiered in 1961, was a huge success. This fictional film told the story of young MZ female twins, Susan and Sharon, who were reared apart from an early age when their parents divorced — each parent raised one twin in different parts of the United States. The twins, unaware of their twinship, met by chance at a summer camp, recognized their physical likeness, compared their life histories, realized they were twins, and switched places at the end of the summer. I believe that this movie was popular because the story and the characters are believable — a number of reunited twins in the Minnesota Study of Twins Reared Apart met by chance. While none of the twins switched places, as did Susan and Sharon, the reared-apart twin firemen, Mark and Jerry, did substitute for one another (fairly successfully, albeit briefly) when Jerry met Mark’s father for the first time (Segal, Reference Segal2007, Reference Segal2012). In contrast, the Patty Duke Show (1963−1966), despite its popularity, portrayed ‘identical cousins’, a situation that is biologically impossible even if both sets of parents are MZ twins. The children delivered by two genetically identical couples would be biologically equivalent to full siblings (Segal, Reference Segal2017). The Patty Duke Show was cancelled when United Artists Television refused to start producing the show in color as the ABC network demanded, and because Duke did not wish to travel regularly to Los Angeles for filming (Fandom, 2025). However, I believe that the implausibility of its premise made the show wear somewhat thin.
I favor extended television and podcast series centered on the psychological, physical, and medical issues twins raise and what twin studies tell nontwins about the nature and origins of human behavior and development. I participated in two such series, a two-part program shown on ABC’s Good Morning America, in 2005, and a three-part program shown on NBC’s Today Show, in 2015. It is unlikely that these series, limited in both length and scope, reached the hundreds, possibly thousands, of twins, parents, educators, and physicians who would have benefitted most from viewing them. There are many topics — varieties of twin types, genetics of twinning, and heteropaternal twinning — that could be examined in productions that would both enlighten and entertain.
Twin research is proliferating at a rapid rate. Twin methods are being increasingly adopted by individuals representing diverse fields of inquiry. Twin registries continue to be established around the world, facilitating comprehensive multi-center studies. Human interest stories about twins appear in the popular media on a regular basis. These events will, hopefully, inspire a greater number of scientific-artistic collaborations that will deliver new knowledge and new ideas in a meaningful and enjoyable manner.
Twin Research Reviews
Twinning in Low-Income Countries
Given trends toward older maternal age in low-income countries, the rate of twinning has been projected to rise by 2050 (Lee & Barclay, Reference Lee and Barclay2025). The countries in question are in sub-Saharan Africa and Southern Asia — India is expected to show the greatest increase in twinning rates. The data were obtained from mothers in 39 nations, between the ages of 15 to 49 years. Participants were interviewed about their conceptions and deliveries that mostly occurred between 1980 and 2015. These women were from nations that had limited use of assisted reproductive technologies (ART); in fact, conceptions resulting from ART (what the authors label MAR, i.e., medically assisted reproduction) were excluded from the study. The estimated increases by 2050, relative to 2010, were between 0.4% and 55%, while estimated increases by 2100 were between 3.4% and 79%. Exceptions to these patterns were the nations of Togo and Mozambique, which showed a slight shift toward younger maternal age in 2050, relative to 2010. A caveat, as the authors noted, is that the data were obtained by self-report, injecting a possible bias into the analyses.
The health, educational, and family implications of these projected twinning rates are many. Hospitals and medical staff will need additional equipment and knowledge to better manage the difficulties associated with multiple pregnancies (e.g., prematurity; transfusion syndrome). Teachers and counselors will need to know more about the special issues presented by twins (e.g., school separation; private speech). Parents will require additional resources and support from the community to better manage their growing family (e.g., information booklets; twins clubs). Having this information available in 2025 will allow better planning for the future.
Oxytocin During Twin Pregnancies
Among the risks associated with multiple pregnancies are uterine atony (inadequate contraction of the uterus following delivery), postpartum hemorrhage, and hysterectomy. The administration of oxytocin during elective cesarean section delivery is known to alleviate these complications to some degree, but the appropriate dosage has been uncertain. A double-blind study of 30 patients was conducted by Peska et al. (Reference Peska, Balki, Pfeifer, Maxwell, Ye, Downey and Carvalho2024) in Canada It was concluded that women with twin pregnancies require higher doses of oxytocin than women delivering singletons, that is, an initial 5 IU bolus (5 international units given as a single administration) over a 1-minute period or more during delivery.
Male-Female Twins and Preeclampsia
Preeclampsia is a hypertensive disorder of pregnancy that may involve cardiac complications for mother and fetus. Preeclampsia has also been associated with fetal sex. Few studies have explored this condition in twin pregnancies, prompting researchers to fill this gap in our knowledge of multiple pregnancy (Brown et al., Reference Brown, Noah, Hill and Taylor2024). A retrospective cohort study of 1032 twin deliveries identified from PeriBank, a database associated with the Baylor College of Medicine and Texas Children’s Hospital, was conducted. The twin pairs were organized according to sex: male-male, female-female, and male-female. The relative risk of preeclampsia was elevated for the male-female pairs, relative to the others. However, the mechanisms explaining this finding have yet to be determined. The comparative risks of preeclampsia among MZ and DZ twins, as well as dichorionic and monochorionic sets, also require further examination as existing findings have been inconclusive.
Loss of a Twin From the Controversial 1960s New York City Twin Study
Most readers of Twin Research and Human Genetics will be familiar with the controversial 1960s study of intentionally separated twins (Segal, Reference Segal2021). The study was orchestrated by Drs Peter Neubauer, Director of the Jewish Board of Guardians’s Child Development Center and Viola Bernard, psychiatric consultant to Louise Wise Services, the adoption agency that placed the twins. The multiple birth status of these adoptive children was concealed from their adoptive families who were only told that their child was is a longitudinal study of behavioral and physical development.
In my book, Deliberately Divided (Segal, Reference Segal2021), I profiled each of the twins who were separated and studied or only separated. One pair, Melanie Mertzel and Ellen Carbone (née Lieber), learned of their twinship when Ellen’s aunt visited a pancake restaurant and noticed a waitress (Melanie) who looked exactly like her niece. The twins met for the first time when they were 23. They had nearly 36 years together, some of which did not go smoothly — Bernard believed that separated twins would be well prepared psychologically to engage with their identical co-twin if they first had opportunities to establish their own identity, but that was not necessarily the case. Both twins believed that they would have benefitted from the support most identical twins uniquely provide to one another.
Sadly, Melanie passed away from cancer on March 24, 2025, two months before her 59th birthday. Thus, Ellen underwent a second loss of her twin sister. I feel privileged to have known Melanie during our interviews and conversations. She will not be forgotten.
Human Interest
Identical Quadruplets Conceived Naturally
Rhode Island couple, Rachel and Marco Vargas, became the parents of identical female quadruplets on January 24, 2025 (GMA Team, 2025). The babies were conceived naturally when Rachel decided she wanted to add one more child to her family of four — the Vargas family had one young son and one young daughter. Rachel and her husband were shocked to learn that Rachel was carrying four fetuses. This high-risk pregnancy was managed at the Banner University Medical Center, in Phoenix, Arizona. The babies, named Sofia, Philomena, Veronica, and Isabel, were delivered at 30 weeks and 3 days gestation.
Twins of Cleopatra and Mark Antony
There is a limestone statue, in Egypt. depicting the opposite-sex twins born to Cleopatra and Mark Antony (Draycott, Reference Draycott2018). The twins, named Cleopatra Selene and Alexander Helios, are two of the three children born to this couple. The twins arrived in the fall of 40 BC, although there is no record of the exact day on which they were born or the order of their birth. The twins’ first names could be linked to prominent members of Near Eastern royal families, although it is possible they were named after their parents. Their second names (Selene: Moon, and Helios: Sun) link the children as twins, and to the beliefs and prophecies of the Roman Empire with reference to a forthcoming ‘golden age’.
The statue was displayed in the Egyptian Museum in Cairo after it was discovered in 1918 near the Temple of Dendra on the west bank of the Nile River. The figures were later rediscovered as the twin children of Cleopatra and Mark Antony by the Italian Egyptologist, Giuseppina Capriotti (Lorenzi, Reference Lorenzi2012).
Star Basketball Player is a Twin
Cooper and Ace Flagg, from Newport, Maine are exceptional basketball players (Hale, Reference Hale2025; Medcalf, Reference Medcalf2024). The twins appear to be fraternal, based on their 6-inch height difference and contrasting hair color. Cooper, at 6 feet, 2 inches tall, left high school a year early to join Duke’s Blue Devils team. He distinguished himself during the National College Athletic Association’s (NCAA) March Madness, as his team advanced to the final four; the blue Devils lost narrowly to the University of Houston’s Cougars in the penultimate playoff game. Cooper is slated to be the number one recruit for the 2025 National Basketball Association (NBA) that is held in June. Cooper’s twin brother Ace, at 6 feet, 8 inches tall, has committed to play basketball for the University of Maine’s Black Bears team. Meanwhile, Ace transferred to Greensboro Day School, in Greensboro, North Carolina, just an hour’s drive from Duke — he believes the move will help him achieve his full athletic potential.
Twin Stabbed at Track Meet
Sixteen-year-old identical twin, Austin Metcalf, was stabbed to death by a 17-year-old student at Frisco, Texas’s Independent School District’s Kuykendall Stadium (Heinz & Guerrero, Reference Heinz and Guerrero2025). This tragic event occurred on April 2, 2025, at a track meet, after Austin told the other student that he was in the wrong place. The victim had a 4.0 grade point average, was the MVP (most valuable player) of his high school’s football team and had plans to go to college. Sadly, Austin passed away in his twin brother’s arms. His surviving twin will become the constant reminder to family and friends of the young man who is lost. The investigation is ongoing.
Another Twin Hostage in Gaza Revealed
Nimrod Cohen is an Israeli soldier and twin held hostage in Gaza (Times of Israel Staff, 2023; Jerusalem Post Staff, 2025). Cohen’s family, including his twin sister Romi, learned he had been abducted when they discovered a Hamas video of him late in the day of October 7, 2023, the day Hamas attacked Israel; news of Nimrod’s capture was later confirmed by the Israeli army. The twins were 19 years old at the time of Nimrod’s abduction; they are now 20. On March 1, 2025, the family identified Nimrod in a blurred video, based on a tattoo on his arm. An extensive look at twins separated due to the Hamas attack is available in Knafo-Noam and Segal (Reference Knafo-Noam and Segal2025).
