Twins in Naturalistic Context: Highlights from the International Society for Human Ethology
The International Society for Human Ethology (ISHE) held two 2025 biennial summer institutes, in Valparaiso, Chile (July 14−18) and in Vienna, Austria (July 21−25). These meetings attract students and scholars from around the world to enjoy five days of lectures, workshops, oral presentations, and posters. (The main meeting of the society is also biennial, with the next one scheduled for 2026.) As a trustee of the society, I was fortunate to attend the sessions in Valparaiso and to present my recent research on twins and tacit coordination. I also introduced one of the plenary speakers, Dr Jaroslava (Jarka) Varella Valentova, from the University of São Paulo (USP) in Brazil, who discussed the evolutionary significance of appearance modifications. In addition, I chaired the research session that followed. A number of presentations in Valparaiso, many from researchers at the Twin Panel at USP, involved twins.
Work by Pedro Alves Sironi, Lilian C. Luchesi, and Emma Otta (University of São Paulo [USP]) discussed the use of thermographic imaging to assess anxiety in Brazilian twin children during their dental appointments. The sample included 74 twins who were evaluated at pretreatment and 2 months later during treatment. MZ twins showed consistency in thermal response across the two timepoints, whereas DZ twins showed minor variation. The procedure aligns with previous studies of anxiety in pediatric settings.
A poster by Lara Espindola da Silva, Emma Otta, Lilian Luchesi, Isabella Ferreira, Rafael de Albuquerque, and Tania Lucci (USP) reported findings from a study of rivalry and conflict in twin relationships and how these measures impact friendship. The friendship networks of 60 twin pairs (26 monozygotic [MZ], 22 dizygotic same-sex [DZSS], and 12 dizygotic opposite-sex [DZOS]), between the ages of 7 and 14 years, were assessed. Twins completed the Pictorial Representation of Friendships (PRF) and the Twin Relationship Questionnaire, which examine rivalry, conflict, dependence, closeness, and dominance. Greater parent-reported conflict and rivalry were linked to increased interpersonal distance between twins. Data collection and analysis are still ongoing. The first author, Lara Espindola da Silva, won the award for the best poster. She is shown in Figure 1 standing with me at the final dinner shortly after her award was announced.
A developmental assessment of Brazilian twins in a home setting was conducted by Elisangela dos Anjos Paula Vieira, Jennifer Leão Correia, Raquel Gonçalves Maia Ribeiro, Fraulein Vidigal de Paula, Tania Kiehl Lucci1, and Emma Otta, from USP. Infant twins were assessed at 3 months and 6 months of age using the Bayley Scales of Infant and Toddler Development (Bayley-III). Adaptive behaviors and socio-emotional behaviors were rated by caregivers via standardized questionnaires, and cognitive, language, and motor skills were directly assessed directly by trained researchers. All infants scored above average in all measures, with the exception of language, in which 50% were in the low average range and 5% were in the borderline range. Future research will compare the developmental trajectories of MZ and DZ twins as the sample size expands.
A naturalistic twin study of early peer interactions was presented by Jennifer Leão Correia, Elisangela dos Anjos Paula Vieira, Raquel Gonçalves Maia Ribeiro, Fraulein Vidigal de Paula, Ricardo Prist, Tania Kiehl Lucci, and Emma Otta, from USP. Six-month-old infant twins were videotaped in their homes for a 3-minute period without external interference. The aim was to capture socially directed behaviors; for example, glances toward the co-twins’ head. Reciprocated social behaviors were also recorded; for example, looking at the sibling while vocalizing. Potential associations between early social behaviors and later developmental outcomes will be examined as the study progresses.
Lilian C. Luchesi, Pedro A. Sironi, Tania K. Lucci, and Emma Otta collaborated on a study of how environmental sensitivity and twin type affect anxiety in a dental setting. (This work relates to the research on dental anxiety in twins, reviewed above.) Previous work indicates that the degree to which individuals perceive and react to environmental stimuli has an estimated heritability of 47%. The sample included 34 pairs of twin children, assessed at their initial dental evaluation and at a follow-up visit 6 months later. The best-fitting model, based on the Akaike Information Criterion, indicated effects of twin type and interaction with emotional sensitivity on the children’s response to being evaluated by a dentist. Understanding the factors affecting dental anxiety is important because virtually all young children visit the dentist, an unfamiliar experience.
Facial expressivity of twins in a standardized situation was examined by Thays Machado da Cruz, Rosana Suemi Takumaru (Federal University of Espírito Santo; FUES), Julia Gomes Freitas, and Tania Kiehl Lucci (USP). This study included 28 pairs of twins (N = 56), ranging in age from 6 to 15 years; 16 pairs were monozygotic (57.1%) and 53.6% were female. Twin children waited alone for the dentist to arrive, during which time their facial expressions were recorded for one minute. Individual differences were observed across all measures. It was suggested that even under standardized conditions, there is an interplay of genetic and environmental influences on facial emotional expressivity.
Alexya Milagre, Dandara de Oliveira Ramos and Rosana Suemi Tokumaru (FUES) presented a study of twin birth patterns and related factors in Espírito Santo, Brazil. The twin birth rate in Espírito Santo was 10.44/1000 live births between 2016−2021. Maternal age and marital status emerged as significant predictors of twinning. The likelihood of twin births increased with maternal age and was lower among mothers in consensual unions or no recorded marital status. Regarding registration, 74 individuals with twins or responsible for twins accessed the registration site between April 2023 and September 2024.
In conjunction with my former graduate student, Damion Garcia, I presented an update of a continuing twin study of tacit coordination (TC). TC, as defined by Thomas Schelling (Reference Schelling1960), refers to circumstances in which ‘two parties have identical interests and face the problem not of reconciling interests but only of coordinating their actions for their mutual benefit when communication is impossible’. The sample included 78 MZ twin pairs and 54 DZ twin pairs, ranging in age from 11−72 years. As expected, MZ twins outperformed DZ twins, and greater coordination was expressed in the coordination versus individual condition. These findings promise to refine theories concerning genetic contributions to coordination and collaboration.
I identified one twin paper presented at the ISHE meeting in Vienna. The work was conducted by Dr Maria Markodimitri (University of Crete, in Greece), who hosted me in May 2025, as I described in the previous issue of Twin Research and Human Genetics (Segal, Reference Segal2025). Dr Markodimitri conducted a longitudinal naturalistic investigation of early imitative exchanges between twins, parents and twin infants, and grandparents and twin infants;’ free dyadic interactions and the emotional state of the partners were assessed before, during and after the imitative sequences. A pair of DZOS twins was observed. No significant differences between twins and non-twins were found in imitation frequency. It was also found that twins show intersubjective abilities from early infancy and that adult participation affects the type of imitation and its frequency. Imitation took place mostly in a communicative context before and after the imitative episodes. It was proposed that imitation, as a social and emotional process, functions as a bridge for communication and relationship-building, even very early in life.
I have always believed that twins tell an informative human developmental story just by acting naturally. The naturalistic and semi-naturalistic studies presented at ISHE 2025 exemplify that view. I look forward to attending future conferences held by the International Society for Human Ethology (ISHE.org).
Twin Research Reviews
Twins With Feingold Syndrome
Feingold syndrome comes in two varieties: Type 1 and Type 2 (MedlinePlus, 2018). Both types involve characteristic features of the hands (shortening of the second and fifth fingers; inward curvature of the fifth finger), head (small head; small jaw), and feet (fusion of the second and third or fourth and fifth toes). Type 1 usually includes blockage of the digestive system, a feature that is not present in Type 2. The two types are distinguished by their genetic underpinnings — mutations in the MYCN gene are associated with Type 1, and mutations in chromosome 13, involving deletion of a chromosomal region that includes the MIR17HG gene, are associated with Type 2. Both genes are involved in growth and development, especially prior to birth.
I became interested in this condition after being contacted by a mother whose 18-month-old male twins were diagnosed with Feingold syndrome one year ago. The twins are monochorionic-diamniotic and were classified as MZ, based on their placentation and concordance for the MCYN gene. They were born at 34 weeks’ gestation to a 34-year-old mother. The delivery was by emergency cesarean section because one twin’s heartbeat was falling, and the other twin’s MCA (middle cerebral artery) Doppler reading was poor — this procedure is used to assess fetal cardiovascular distress, anemia, and/or hypoxia. The twins also showed intrauterine growth restriction (IUGR); their mother experienced hypertension, but not pre-eclampsia. The twins are the only children in the family. Some of the characteristic features of the twins’ condition are shown in Figures 2 and 3; note the curved fifth finger (Figure 2) and the single palmar crease on the hands (Figure 3).
Figure 1. Dr. Nancy L. Segal (L) with poster award winner, Lara Espindola da Silva. Photo credit: Conference attendee.
Figure 2. Monozygotic male twins with Feingold syndrome. Note the inner curvature of the fifth fingers. Photo credit: The twins’ mother.
Figure 3. Monozygotic male twins with Feingold syndrome. Note the single palmar crease across the hands of both twins. Photo credit: The twins’ mother.
I discovered just one other case of Feingold syndrome involving twins (Burnside et al., Reference Burnside, Molinari, Botti, Brooks, Chung, Mehta, Schwartz and Papenhausen2018). A summary of six cases included one member of a DZ pair; the sex of the co-twins was not given. The pregnancy was complicated by the poor growth of the patient, in contrast with her co-twin who was clinically normal. Like the MZ male twins described above, she had a single palmar crease. She also displayed delayed physical and cognitive development, hearing loss, and distally tapered fingers. The patient passed away at the age of 24 years, due to pneumonia linked to respiratory failure. Interestingly, a pair of MZ female twins in the Minnesota Study of Twins Reared Apart both had fifth fingers that curved inward. This condition, known as clinodactly, generally results from abnormal growth and development of the small bones of the finger (Boston Children’s Hospital, 2025).
Twins’ Language Delays
Language delays among young twins have been well documented. Reasons for these delays have focused on twinning effects, namely the close proximity of twins as they develop, restricting their exposure to optimal language learning environments (Thorpe, Reference Thorpe2006; see also Segal, Reference Segal2017). Parents tend to be more controlling of the twin situation, direct less language to each twin than a non-twin child typically receives, and afford each twin less time to respond. However, a study by Foran et al. (Reference Foran, Evans and Beverly2021) found that pregnancy complications were implicated in increased young twins’ difficulties with expressive speech. (Expressive speech refers to language that communicates one’s thoughts and needs, either by verbal or nonverbal means; Brown et al., Reference Brown, Asp, Carter, Spiller and Bishop-Deaton2020; Leonard, Reference Leonard2009). Moreover, twinning effects were more significant for gesture than for spoken vocabulary — in fact, 29 out of 88 16- to 18-month-old twins (33%) placed below norms established for gesturing. Parents and teachers should stay alert to young twins’ language development and seek intervention when warranted.
Breastfeeding Twins
Breastfeeding has been associated with better growth and development of infants, increased emotional mother-infant communication, and reduction in maternal breast diseases. A research team from China investigated the breastfeeding of twins and the underlying explanatory factors that affect its frequency and outcomes (Wang et al., Reference Wang, Li, Xiang, Guo, Peng, Wang and Chen2023). The study obtained online information from 420 mothers who had delivered twins between January 2019 and December 2022, at the Chongqing Health Center for Women and Children/Women and Children’s Hospital of Chongqing Medical University, in Chongqing. Mothers were between 22 and 37 years of age. The period of interest was within 6 months after delivery. Exclusive breastfeeding was reported by 21.9% of mothers, while nonexclusive breastfeeding was reported by 78% of mothers. Twin infants of mothers in the exclusive breastfeeding group, relative to the nonexclusive breastfeeding group, showed significantly fewer feeding difficulties, less infant diarrhea, and less lactose intolerance. Mothers indicated significantly less breast tenderness and postpartum depression, and greater partner support. The researchers concluded that these factors, and others, need to be considered to promote breastfeeding of twins.
Twins with Olmsted Syndrome
Premature twins with Olmsted syndrome were presented as the ‘Case Report of the Month’ in Advances in Neonatal Care (Anderson & Paradise, Reference Anderson and Paradise2022). Olmsted syndrome, first described in 1927, is a rare skin disorder, documented in just 46 infants as of 2012, and 77 cases as of 2016. Characteristic features are hyperkeratosis (thickening of the skin’s outer layer) and keratoderma (marked thickening of the soles of the feet and palms of the hands). Other clinical features observed in infants include alopecia (hair loss) and scaly erythema (flaking of the skin and scalp). Olmsted syndrome is a progressive and debilitating condition.
The female twins under study were born prematurely at 28 weeks, 2 days gestation, and delivered by cesarean section. They were also the first pregnancy of a 25-year-old mother. The twins were most likely MZ, as they were monochorionic-diamniotic.
Most cases of Olmsted syndrome arise sporadically, but some cases have been associated with genetic mutations. One such mutation, TRVP3 located on chromosome 17p13.2, was identified in the twins via genetic testing, indicating autosomal dominant inheritance. The researchers noted that further genetic assessment was denied by the family’s insurance, which was unfortunate because it was of interest to determine if the parents were carriers. However, a dominant gene is expressed regardless of the paired allele at the corresponding locus — carriers have one recessive detrimental gene that is not expressed, so the twins’ parents could not be considered carriers. Treatment of the twins involved the application of hydrating emollients and administration of exfoliating scrubs.
In the News
Loss of Texas Twins
Eight-year-old identical female twins, Hanna and Rebecca Lawrence, were sadly lost in the 4 July 4, 2025 floods that devastated parts of Texas (CBS/AP, 2025). The twins were attending Camp Mystic and were sleeping in a cabin close to the Guadalupe River, the greatest danger point. They had just finished second grade. Their parents, John and Lacy Lawrence, issued this statement to the press: ‘Hanna and Rebecca brought so much joy to us, to their big sister Harper, and to so many others. We will find ways to keep that joy, and to continue to spread it for them. But we are devastated that the bond we shared with them, and that they shared with each other, is now frozen in time.’ The twins’ older sister, Harper, was also at the camp, but survived because she had been assigned to a more senior cabin located on higher, safer ground.
A Singular Musical Sensation
Identical twin, Laufey Lin Bing Jénsdótti, has been labelled ‘a phenomenon’ by New York Times critic, Ben Sisario (Reference Sisario2025). Laufey and her sister, Lin, now aged 26, were born in Iceland to a Chinese mother and Icelandic father, distinguishing them in appearance from others in their community. The twins also spent some of their growing up years in Washington, D.C. Laufey, the performing twin, combines jazz, pop, and classical music in ways that have attracted millions of fans and countless collaborators, including Barbara Streisand — Laufey will sing with Streisand on Streisand’s latest album, The Secret of Life: Partners, Volume 2. Laufey will also be releasing her own album, A Matter of Time.
Laufey’s twin sister, Junia Lin, serves as her creative director, sometime violinist, and full-time correspondent. Little was said in Sisario’s article about their relationship, but an earlier essay was revelatory. There was a period when Junia Lin’s career was uncertain, while her sister’s career was moving ahead (Holtermann, Reference Holtermann2024). Junia confessed to feeling jealous but insisted that those feelings have subsided as she has become deeply involved professionally with her sister’s musical performances. ‘I don’t feel jealous anymore,’ she said. ‘I think I did last year, when I had just graduated college, and I felt more lost. Like, I was job hopping, and she was building her career. But so much has changed in just one year that I no longer have those feelings.’ Junia’s comments tell us that identical twins, despite their social closeness, can experience times of discomfort with one another. At the research level, it is important to study twins over time, to capture the representative tone of their twinship, rather than moments that are isolated and atypical.
Deliveries of Conjoined Twins in Myanmar and India
On July 9, 2025, conjoined female twins were delivered successfully by cesarean section at Bilin General Hospital, in Myanmar’s Mon state (Maung, Reference Maung2025). The delivery took place in a small rural facility that included just one doctor. This was the fifth pregnancy of the twins’ mother, Naw Chaw Gei. The babies and their mother appear to be in good health. Other conjoined twins have been born in Myanmar, such as Aye Aye and Ei Ei Neyin. These twins, now 34 years of age, were surgically separated in 1991 or 1992, and have gained considerable fame (perhaps locally; I was unaware of them). According to a website (Myanmar Model Girls, 2016), the twins were attached at the chest, making them thoracopagus, one of the more common varieties of conjoined twinning. Their shared hobby is photography, but they have not yet started their own business. The twins often argue, but prefer going to places together.
A rare, conjoined twin delivery occurred on July 22, 2025, at the Maharaja Tukojirao Holkar (MTH) Hospital, in Indore India (EVT Bahrat, Reference Bahrat2015). The female twins, delivered by cesarean section, were classified as dicephalic parapagus, indicating that they have separate heads and a single body. This form of conjoined twinning is estimated to occur in just 1/60,000 births. The twins’ mother was 22 years of age and sought medical assistance due to experiencing labor problems. Both babies weighed approximately 2.80 kilograms at birth (6.17 pounds) and appeared healthy; it is unclear if this figure refers to their separate or combined weight at birth. The article noted that male infants are more common among conjoined twins, but the opposite is true, at a ratio of 3:1 (Mian et al., Reference Mian, Gabra, Sharma, Topale, Gielecki, Tubbs and Loukas2017).
Major League Baseball Pairs
Two pairs of twins, one set identical and one set fraternal, have achieved baseball greatness (Harrigan, Reference Harrigan2025). The members of both sets were part of the 2025 major league baseball draft, held July 13−14. The Parker twins, Jojo and Jacob, appear to be a fraternal pair based on their facial differences. Both are outfielders — Jojo has a left-handed swing when he is at bat, but is listed as right-handed, while Jacob is right-handed. Jojo, whose prospect ranking was 9, will join the Toronto Blue Jays; Jacob, whose prospect ranking was 109, will join the Arizona Diamondbacks. Being on different teams will be the first significant separation for the twins who both expected to play for Mississippi. The twins say they are each their brother’s biggest fan and greatest competitor, largely crediting their athletic achievement to their twinship (Castrovince, Reference Castrovince2025; see also MLB, 2025 and ESPN, 2025).
The Witherspoon twins, Kyson and Malachi, appear to be identical, given their closely matched facial structures. The twins are both right-handed pitchers from the University of Oklahoma. Kyson, whose prospect ranking was 10, will join the Boston Red Sox; Malachi, whose prospect ranking was 121, will join the Detroit Tigers. The twins describe themselves as each other’s cheerleaders, both on and off the field. They have pushed each other to be the best player they can become—they will most likely continue to do this from hundreds of miles away (Borek, Reference Borek2025).
