Functional Significance of Twinning

The presence of twins in human populations worldwide poses an evolutionary puzzle. Specifically, the functional significance of multiple birth babies is curious, given that twins are often born prematurely, posing health risks to mothers and infants. Clearly, the female uterus is optimally designed to gestate one infant, rather than two or more. This conundrum is especially relevant in the case of parents producing monozygotic (MZ) twins who lack the genetic variation of individual siblings — MZ co-twins are essentially untested genetic duplicates.

Twins are present in all human populations, although their frequencies vary. The MZ twinning rate of .3 to .4% is fairly constant across cultures, whereas the varied dizygotic (DZ) twinning rate is responsible for the elevated twin birth frequency in African populations (17.1/1000 births, or 1.71%), relative to North American populations (16.9/1000, births, or 1.69%), and Asian populations (9.2/1000 births, or .92%); note that these figures partly reflect the effects of in vitro fertilization (IVF), which increases both MZ and DZ twinning, especially among western nations, with DZ twinning increased to a greater degree. Explanations for these population differences also include genetic factors, hormonal influences, and dietary practices. Several evolutionary-based theories and analyses of human twinning are reviewed below.

The Insurance Ova Hypothesis, proposed by Anderson (Reference Anderson1990), views DZ twinning as a byproduct of selection for multiple ovulation that can produce twins. DZ twinning is seen as a reproductive error; however, multiple ovulation reduces the risk of fertility failure, compensating for the production of defective embryos.

The Insurance Ova/Pre-Implantation Model, described by Ball and Hill (Reference Ball and Hill1999), extends elements of Anderson’s (Reference Anderson1990) work. It posits that if DZ twinning is a byproduct of a mechanism that compensates for genetic defects; a logical prediction is that rates of defective embryos and rates of twinning should covary. The authors constructed a graph showing a rise in both Down’s syndrome births and twinning, based on data from 25 nations. An exception is Nigeria, which has a very high natural twinning rate, partly explained by Nigerian women’s unique reproductive biology (Segal, Reference Segal2017).

When I was writing my first book, Entwined Lives (Segal, Reference Segal1999/Reference Segal2000), it occurred to me that DZ twinning might represent the ‘last-ditch effort’ of relatively older women to transmit their genes into later generations. The possible birth of a genetically impaired child would pose a risk, but the risk would be worth taking if children resulted. Excited, I telephoned my friend and colleague, the late Dr Linda Mealey, to discuss my idea. I was disappointed when she referred me to the work of L. Scott Forbes (Reference Scott Forbes1997), a Canadian investigator who had already published this idea. Forbes spoke of a ‘relaxed maternal screen’ in which mothers are less sensitive to offspring quality, noting that a defective child is better than no child. Finally, Helle (Reference Helle2008) contributed the Terminal Investment Hypothesis, in which there would be increased investment in the last reproductive effort in the absence of future reproductive opportunities. Consistent with this view, Pison and Couvert (Reference Pison and Couvert2004) observed that DZ twins are often the last-born children in their families.

With few exceptions, the presence of MZ twinning has been infrequently addressed. For example, Gleeson et al. (Reference Gleeson, Clark and Dugatkin1994) proposed models in which MZ twinning might be rare, or common but appear rare. A colleague and I wondered if parents of MZ twins might produce higher quality children, that is, perhaps their fitness exceeds that of DZ twins and full siblings. We used data from the National Longitudinal Study of Adolescent to Adult Health (Add Health) to evaluate these ideas (Kanazawa & Segal, Reference Kanazawa and Segal2019). The sample included just one individual from the following twin and sibling sets: MZ (n = 425), DZ (n = 950), and full siblings (n = 10,2990. Our fitness measures were examiner-rated attractiveness and self-reported health. We found that the mean attractiveness rating for the MZ twins was significantly higher than the ratings for the other two subsamples that did not differ from each other. We found the same pattern for the mean latent health ratings, although the MZ twin difference approached, but did not reach statistical significance. These findings are encouraging yet should be viewed cautiously — limitations to the study included attractiveness ratings by just a single examiner and the absence of a physical exam to assess health.

Twins have been increasingly incorporated into the research programs of many biological and social science disciplines. These efforts have yielded information unavailable otherwise — at the same time, there is still a great deal that remains unsolved.

Poster Presentations

Three posters at ISHE were contributed by members of the Twin Painel (Twin Panel), at the University of São Paulo, in Brazil, while the fourth came from a faculty member of the Department of Preschool Education, at the University of Crete, in Greece. Only selected findings from these posters are presented.

Early Touch in Twins

Graduate student Jennifer Leão Correia observed 15 pairs of 3-month-old twins with respect to self-directed and co-twin directed touch. She found that female pairs engaged in the most co-twin directed touch. Differences between monochorionic and dichorionic pairs were not found. Jennifer received an honorary mention in the poster competition. Pedro Henrique Dônola de Melo used twins to examine sex-specific pathways in the early biological roots of emotional regulation. Males showed higher liability/negativity scores than females, suggesting greater male susceptibility to environmental insults.

Dr Lilian Christina Luchesi reported on environmental sensitivity and anxiety in twins as they arrived for a dental visit. She found that individual differences in behavior were more important in explaining behavioral outcomes than the twins’ dental treatment. Dr Maria Markodimitritaki, from Greece, examined socio-emotional dynamics in early twin relationships. She found that the twins’ behaviors were organized around distinct but related dimensions, and that shared experiences played an important role in early socio-emotional development.

A Final ISHE conference highlight was a guided tour of the Prehistory Museum (Musée National de Préhistoire) where the conference took place. A model of the remains of the twins of Krems, Austria was on display, as shown in Figure 1; the remains are an estimated 31,000 years old. The twins were located at the Wachtberg-Krems site in Krems, Lower Austria. It was determined that the twins were MZ; it was also indicated that the smaller co-twin (shown in front) died at 1−2 days or life, and the larger co-twin died at approximately 6 weeks. Their mother apparently suffered from malnutrition. The twins’ cousin, age 3−4 months, was also identified, but was not on display (Teschler-Nicola et al., Reference Teschler-Nicola, Fernandes, Händel, Einwögerer, Simon, Neugebauer-Maresch, Tangl, Heimel, Dobsak, Retzmann, Prohaska, Irrgeher, Kennett, Olalde, Reich and Pinhasi2020; Fourment, 2026, personal communication).

Figure 1.

Model of the Krems twins, displayed at the Musée National de Préhistoire, Les Eyzies, France. Photo credit: Dr Nancy L. Segal.

Congenital Chimerism

Chimerism — the presence of multiple DNA lines in a single individual that derive from different sources — is allegedly rare, although some cases may escape detection. The possibility that chimerism occurs more often than supposed is strengthened by its association with DZ twinning, and the fact that in vitro fertilization (IVF) has increased the frequency of DZ twin conceptions (Leach, Reference Leach2026). Given the foregoing, a recently released, first case study of congenital chimerism in a woman carrying both XX (female) and XY (male) cell lines is of considerable interest (Gao et al., Reference Gao, Li, Xiao, Ren, Ma, Zhang and Yan2026).

The case involved a Chinese female murder victim whose unusual DNA was discovered following detailed forensic analysis. Possible sample contamination was ruled out, followed by tests repeated 17 times to confirm the unusual results. It seems that the egg released by the victim’s mother was fertilized by two separate sperm, a process that would prevent embryo viability and development. In the present case, it appears that one sperm carried an X chromosome and the other sperm carried a Y. In contrast, some people show signs of ‘microchimerism’ in the form of different colored eyes or skin tone patches. Microchimerism can come from the exchange of genetic material between the mother and fetus during pregnancy. However, the murder victim in question showed chimerism throughout her body, but without external signs. In effect, the woman was her own twin.

Parental Favoritism of One Twin

Research by Maravilla et al (Reference Maravilla, Lyding, Clifford, Goldsmith and Lemery-Chalfant2026) showed that differential treatment of twins is linked to psychopathology in the less favored twin. Assessment of parent-twin relations was conducted via an experimental task (card game) and reports from parents and twins. Genetic influences on parental hostility and intrusiveness were observed, suggesting that children’s characteristics evoked parents’ responses. I believe it is critical to distinguish between differential treatment and favoritism. Differential treatment does not imply that parents are being unfair to one child while favoring the other child — in fact, efforts at treating children alike may be insensitive to the special interests and talents of one of both children (see Segal, Reference Segal2024). If, however, children perceive differential treatment as favoring one twin over the other, then parents should rethink their rearing patterns and modify them if necessary.

Parasitic Twins

Heteropagus (parasitic) twins are rare occurrences of a malformed twin attached to a normally developing host. Such twins are considered conjoined in that the parasite is attached to the normal twin or autosite. An unusual case of a male fetus with two attached masses was recently reported by Morovic et al. (Reference Morovic, Pedrero and Alcarra2026). The infant was delivered by cesarean section at 32 weeks’ gestation to a 33-year-old mother with no history of medical complications or exposure to teratogenic substances. The infant appeared to be healthy; however, he experienced considerable bleeding and airway obstruction from the attached masses. His physical condition was treated by surgical resection at age 7 days. Not all such cases are considered fetus-in-fetu (the presence of a mass inside the body of a normally developing infant); however, the present case warranted such classification despite the absence of an enclosing sac. That is because there was external covering of skin, a finding of differentiated structures of the mass, and the presence of blood vessels.

Twin Study of Primal World Beliefs

It often seems that twin researchers have exhausted all twin-related topics in their studies — until another first-time study appears (Perizonius et al., Reference Perizonius, Wesseldijk, Clifton, Ullén and Mosing2025). It was striking to see a twin study focused on the origins of primal world beliefs — that is, ‘beliefs about the general character of the world’; for example, whether the world is a safe place. The study included a twin sample drawn from the Swedish Twin Registry. The study also examined associations between primal beliefs and the Big Five Personality traits of openness, conscientiousness, extraversion, agreeableness, and. neuroticism.

Genetic effects underlying primal beliefs ranged between 21% and 49%, with little support for shared environmental influences. Correlations between primal beliefs and personality traits were ‘low to moderate’, suggesting that despite their association, primal beliefs are distinct constructs. Given that this is the first twin study in this area, further efforts are encouraged.

Twins’ Matching Lives

Identical twins, Bill and John Bowdler, from Birmingham, England, attracted media attention because of their unusually parallel lives (Sheffield, Reference Sheffield2026). The twins, at age 82, live together in their childhood home, wear matching outfits on a daily basis, and work at the same jobs. They have only been separated two times in their life — on one occasion they were placed in different classes in grade school, and on another occasion they were apart for 3 months when John was hospitalized for an appendectomy. Most recently, both twins were diagnosed with a cataract on their right eye, and both will undergo the same surgical correction.

The Bowdlers are the first pair of twins to be treated at their clinic and have impressed their physicians with their similarities. However, their matching physical and behavioral preferences are generally expected by twin researchers who are familiar with identical twin development. It is also worth noting that the similarities listed by the BBC, the news outlet that reported their story, are not unusual — uniquely matching traits, such as washing hands before and after using the toilet, or always leaving one quarter of a piece of toast uneaten, especially when observed in reared-apart twins, are more striking (Segal, Reference Segal2012).

Birth of Three Triplet Sets

Physicians at The Allegheny Health Network’s West Penn Hospital, in Pittsburgh, Pennsylvania, witnessed the birth of three triplet sets on three consecutive days (Colosi, Reference Colosi2026). Caden, Eliana and Rory were delivered on June 10 to Mallory and Cory Quinn; Dean, Charlie and Griffin were delivered on June 11 to Matt and Monica Wilden; and Mila, Clara, and Lena were delivered on June 12 to Julia and Jeff DeFrancisis. Two of the three sets were conceived naturally. In fact, triplets were born at the same hospital in April 2026, and a fifth set is rumored to be born there soon. Clusters of multiple birth infants are occasionally observed to be delivered near in time, yet explanations for such events are speculative.

Chinese Twins Reared Apart

Chinese twins, Marissa Doop and Zoie Berkley-Andre, raised by different Minnesota families since they were 13 months of age, were childhood friends (Kantor, Reference Kantor2026). They and their families were always aware of their striking similarities, including date of birth, orphanage in China, ACT scores, college major (chemistry), habits, mannerisms, and gestures. In addition, like 25% of identical twins, one twin is left-handed (Zoie) and the other twin is right-handed (Marissa), one twin’s wisdom teeth are on the left and right up and down sides (Zoie), while the other twin’s wisdom teeth are in opposite positions (Marissa), and one twin’s hair is parted on the left (Zoie) and the other twin’s hair is parted on the right (Marissa).

Despite the twins’ matching and reversed traits, their families and friends generally joked about their being twins until they learned about my study of separated twins from China (Segal & Pratt-Thompson, Reference Segal and Pratt-Thompson2024). My study prompted the then 16-year-olds to undergo a DNA test that revealed their true relationship — identical twins. Now, at age 27, Marissa and Zoie view their twinship as nothing extraordinary since they have known each other for most of their lives — they do not believe that anything will change between them.

A Landmark Conjoined Twin Separation

Nineteen-month-old craniopagus (joined at the head) Nigerian twins were successfully separated by surgeons in 2025 (England, Reference England2026). The twins, named Mercy and Goodness, born in June 2023, were flown to the United Arab Emirates (UAE) for their landmark operation. The surgical team included doctors from the UAE, England, Brazil, and the UK — the London-based Gemini Untwined, co-founded by Dr Noor ul Owase Jeelani, developed the advanced techniques that were used in the operation. The twins were first scanned to obtain details about their brains to enable surgical rehearsals prior to separation. An ‘Open Book’ procedure was also applied in which gravity is used to support the brain, keeping pressure to a minimum in order to reduce risk. The twins have since recovered and have returned home.