Research Challenges and Opportunities

A 2015 meta-analysis on the heritability of human traits, encompassing 50 years of twin studies globally (Polderman et al., Reference Polderman, Benyamin, De Leeuw, Sullivan, Van Bochoven, Visscher and Posthuma2015), highlighted a significant underrepresentation of Brazilian research in the international database. Of the 2748 publications analyzed, only 10 were authored by Brazilian researchers, distributed across various fields: two in genetics (Jacques et al., Reference Jacques, Salzano and Penna1977; Rapaport et al., Reference Rapaport, Colletto, Vainzof and Zatz1991), one in medicine (Custódio et al., 2007), five in odontology (Bretz et al., Reference Bretz, Biesbrock, Corby, Corby, Bretz, Wessel and Schork2011; Bretz, Corby, Hart et al., Reference Bretz, Corby, Hart, Costa, Coelho, Weyant, Robinson and Schork2005; Bretz et al., Reference Bretz, Corby, Melo, Coelho, Costa, Robinson, Schork, Drewnowski and Hart2006; Bretz, Corby, Schork et al., Reference Bretz, Corby, Schork, Robinson, Coelho, Costa, Melo Filho, Weyant and Hart2005; Su et al., Reference Su, Corby, Elliot, Studen-Pavlovich, Ranalli, Rosa, Wessel, Schork, Hart and Bretz2008), and two in physical education (Machado et al., Reference Machado, Fernandes, Roquetti and Fernandes Filho2010; Reis et al., Reference Reis, Machado, Fortes, Fernandes, Silva, Dantas and Fernandes Filho2007). Notably, this collection entirely lacked psychological research, despite the strong global interest in behavioral problems, cognition, and social interaction. We envision the Painel USP de Gêmeos as a critical catalyst for advancing twin research in Brazil and Latin America by adopting innovative methodologies and fostering robust interdisciplinary collaboration.

Most twin studies rely heavily on WEIRD populations (Western, Educated, Industrialized, Rich, Democratic). Because heritability is a statistical measure defined with respect to a particular population and environment, estimates can vary substantially depending on sample characteristics. As Uchiyama et al. (Reference Uchiyama, Spicer and Muthukrishna2022) observed, heritability estimates tend to be higher when environmental conditions are relatively homogeneous and lower when environments are more diverse. Hagenbeek et al. (Reference Hagenbeek, Hirzinger, Breunig, Bruins, Kuznetsov, Schut, Odintsova and Boomsma2023) likewise emphasize the importance of recruiting diverse and representative samples in contemporary twin research. A scoping review conducted by the Painel USP de Gêmeos research team identified 133 twin studies in Brazil, spanning fields such as medicine, psychology, dentistry, and physical education. This comprehensive review underscored the pressing need for validated zygosity assessment, larger and more heterogeneous samples, and stronger study designs (Fernandes et al., Reference Fernandes, Ferreira, de Felipe, Segal and Otta2021).

Recruitment, Ethical, and Infrastructural Framework

The Painel USP de Gêmeos employs a comprehensive and multi-channel strategy for recruitment, underpinned by a rigorous ethical and legal framework, and supported by a robust infrastructural system. Recruitment of participants utilizes multiple integrated channels. Mothers of twins are contacted directly via institutional databases at the University Hospitals (HCFMUSP and HCFMRP-USP). Adult participants are reached through the USP Information Technology Superintendency, which sends annual email invitations to all faculty, staff, and students. A dedicated website (https://www.paineluspdegemeos.com.br) serves as a central hub, presenting the goals and importance of twin research and offering a mobile-friendly registration form for adult twins, higher-order multiples, and their parents or guardians. Recruitment efforts are further amplified through active social media communities on Instagram (3853 followers; https://www.instagram.com/paineluspdegemeos/?hl=pt) and Facebook (2400 followers; https://www.facebook.com/paineluspdegemeos/?locale=pt_BR). To strengthen community engagement and attract media coverage, annual Twin Festivals have been hosted at USP since 2016, providing cultural and recreational activities.

The USP Twin Panel serves as an umbrella project, encompassing multiple subprojects that organize specific repositories for research use. Some repositories are publicly accessible, such as the Fonoteca Cesar Ades (FOCA), maintained by the Ethology & Bioacoustics Lab (fonotecacesarades.ffclrp.usp.br), while others are available upon request, subject to approval by the coordinating committee. Data collection spans various developmental stages, including the prenatal period (HCFMUSP and HCFMRP-USP), infancy and early childhood (home visits by researchers), school age and adulthood (laboratory assessments). The observed participant overlap across studies reflects the intentional use of a unified recruitment strategy across an integrated network of university hospitals and academic partners. Although projects are managed independently, running simultaneously or sequentially based on their design and funding cycles, all four datasets employ the same hierarchical identification system for participants and families. This system is built upon consistent, unique identifiers: a specific identifier assigned to each twin (Unique Twin ID), and a common identifier shared by both twins within a pair (Family ID or Twin Pair ID). This consistent structure is the core mechanism that allows for the reliable merging and integration of multi-modal data from the same participants across the different repositories.

The panel implements rigorous ethical, legal, and infrastructural procedures to ensure the protection of participants’ rights, privacy, and data confidentiality. Registration is managed through a secure, encrypted online platform, and all data are stored in protected systems accessible only to authorized users. All research projects undergo review by ethics committees. Personal information is never shared with third parties, and access to data is strictly controlled, with only anonymized datasets made available for approved analyses. Data from the USP Twin Panel and maternity records are stored in secure institutional locations, including the repositories of the USP Information Technology Superintendency and the FAPESP Thematic Project Laboratory. Bioacoustic datasets are preserved in the Fonoteca César Ades (FOCA), which is maintained by the Ethology & Bioacoustics Lab at FFCLRP-USP. In all systems, identifying information is removed, and long-term preservation is ensured. The panel adheres to the best practices in data management promoted by USP and FAPESP, and participant consent authorizes the use of data in future ethically approved research, including external collaborations. Governance, data protection, anonymization, controlled access, and long-term storage enable cross-cohort studies while safeguarding participant rights.

The USP Twin Panel is formally registered with the USP Innovation Agency (AUSPIN) and the Brazilian National Institute of Industrial Property (INPI-BR; Process 913418420). These intellectual property registrations serve as instruments of institutional recognition, protection, and governance. They safeguard the panel’s name, brand, and structure against unauthorized use, ensure transparency and compliance with university innovation policies, and prevent disputes related to authorship or ownership. The INPI registration extends protection nationally, reinforcing institutional credibility and providing a solid legal basis for partnerships with public, private, and international collaborators. The University of São Paulo acts as the institutional holder of the registry, while the consortium of collaborating researchers contributes to governance and scientific development in line with USP policies. This framework guarantees institutional accountability while maintaining flexibility for multidisciplinary and multi-institutional research. Finally, the panel operates in full compliance with Law No. 14874/2024, Resolution 441/2011 of the Brazilian National Health Council, and USP biobank policies. Access to data and biological materials follows regulated procedures that ensure scientific integrity, participant protection, and adherence to ethical standards.

Key Research Projects

The main research projects of the Painel USP de Gêmeos are summarized below. Three major face-to-face data collections have been conducted, each targeting a different age group. For adults, the sample includes 100 pairs (83 MZ and 17 DZ). For school-age children, an ongoing study includes 107 pairs (56 MZ and 51 DZ), each assessed twice, approximately six months apart over a two-year period. For infants, a study in progress includes 10 pairs assessed at 3 and 6 months of age. Data collection for these cohorts began in July 2018, January 2022, and September 2024, respectively. All twin participants contribute to multiple studies associated with four datasets: the Biorepository, the Physiological and Physical Repository, the Behavioral Repository, and FOCA. Researchers contributing to the Biorepository collect biosamples for DNA analyses. The resulting information will be made available to investigators contributing to the Physiological and Physical Repository, the Behavioral Repository, and FOCA.

Twinning: Epidemiological Perspective

Our research group conducts ongoing population-based birth-rate studies using data from the Sistema de Informações sobre Nascidos Vivos – SINASC (Brazilian Live Births Information System; Otta & Fernandes, Reference Otta, Fernandes, Otta and Lucci2021; Otta et al., Reference Otta, Fernandes, Acquaviva, Lucci, Kiehl, Varella, Segal and Valentova2016; Varella et al., Reference Varella, Fernandes, Arantes, Acquaviva, Lucci, Hsu, David, Bussab, Valantova, Segal, Otta and Elito2019). Our team accesses anonymized microdata publicly released by DATASUS, the Department of Informatics of the Brazilian Unified Health System (Sistema Único de Saúde – SUS). Access to identified or sensitive data is strictly restricted to authorized managers at municipal, state, and federal levels, requiring formal registration, responsibility agreements, and strict adherence to confidentiality regulations. Consequently, the Panel is not permitted to contact mothers or twin pairs directly using this registry.

Between 1996 and 2023, a total of 82,174,678 individuals were born in Brazil. Regional disparities are evident, with the highest twinning rates in the Southeast (10.64‰) and South (10.45‰), the lowest in the North (7.50‰) and Northeast (8.87‰), and intermediate in the Central-West (9.43‰). These disparities warrant further investigation. We hypothesize that the higher twinning rates in the more developed Southeast and South regions may be influenced by a combination of factors, including increased maternal age due to delayed childbearing, greater access to assisted reproductive technologies, and improved nutrition associated with higher socioeconomic status and food availability.

In addition to broad epidemiological analyses based on the SINASC database, we are conducting targeted studies that draw on the HCFMUSP database to investigate specific patterns and clinical determinants of health among twins. Established in 1944, HCFMUSP is the largest healthcare complex in Latin America with a high-volume maternity ward and advanced tertiary care. Within the Thematic Project supported by FAPESP, the team from the Twin Unit is investigating the epidemiological profile and outcomes of twin pregnancies. The database currently comprises 2485 twin pregnancies (2005–2022), offering detailed records of maternal, fetal, and neonatal variables that form a solid basis for epidemiological analyses within this and related studies (e.g., Hernandez et al., Reference Hernandez, Francisco, Bittar, Gomez, Zugaib and Brizot2017; Maia et al., Reference Maia, Liao, Brizot, Francisco and Zugaib2016; Miyadahira et al., Reference Miyadahira, Brizot, Carvalho, Biancolin, Machado, Krebs, Francisco and Peralta2018).

The field requires a dual approach, integrating broad epidemiological mapping with targeted, hypothesis-driven studies. Epidemiological research faces considerable methodological challenges, which can be mitigated through the use of innovative methods and the linkage of national databases. A notable example of such innovation is the challenge of linking the Brazilian SINASC, which provides detailed data on twin birth weights and intrapair differences, with the Central Bank’s CPF-based databases (Cadastro de Pessoas Físicas). This linkage would enable the rigorous testing of key socioeconomic hypotheses, particularly those concerning the long-term economic consequences of early-life health endowments. Specifically, it would allow researchers to examine: (a) whether absolute birth weight is significantly associated with adult economic earnings, and (b) whether intra-pair differences in birth weight predict earnings differences between twins. The potential value of this type of linkage is illustrated by research conducted by Rodrigues (Reference Rodrigues2025), which accessed the Central Bank’s CPF databases. In a notable contribution to labor economics, her PhD thesis analyzed a large dataset comprising 378,399 twins born between 1986 and 2001, linked to their annual earnings data from 2019 to 2022. Future studies that employ administrative data linkage could further advance this approach, enabling researchers to isolate the causal impact of early-life variables with greater precision.

Development of a Twin Biorepository

Coordinated by Professor Cintia Fridman, who leads the Laboratory of Genetics and Molecular Biology at the Forensic Medicine Department of FMUSP, the Twin Biorepository serves as a core resource of the Painel USP de Gêmeos. This facility is responsible for the rigorous, systematic collection of biological samples — specifically blood, saliva, and DNA — from both adult and child twin participants. A key function of the Biorepository is to provide definitive zygosity determination through DNA analysis, complementing the initial questionnaire-based or self-report assessments. In a study conjointly conducted by the forensic medicine and psychology teams, the Brazilian version of a 4-question zygosity questionnaire for adult twins was successfully validated (Varella et al., Reference Varella, Fernandes, Fridman, Lucci, Defelipe, Fernandes, Garcia, Antonio, Segal and Otta2023). This instrument, adapted from a tool used for over half a century by the Danish Twin Registry (Christiansen et al., Reference Christiansen, Frederiksen, Schousboe, Skytthe, von Wurmb-Schwark, Christensen and Kyvik2003), assesses zygosity based on overall physical similarity and the frequency of mistaken identity. The study involved 100 same-sex twin pairs (mean age: 30.9 + 12.9 years) who completed the questionnaire and provided blood samples for DNA analysis. Genetic testing classified 83% of the twin pairs as MZ and 17% as DZ. Crucially, the questionnaire demonstrated a 96.7% accuracy in distinguishing zygosity, establishing it as a reliable screening tool.

Comprehensive national data on population twinning rates by zygosity in Brazil are lacking. In our DNA validation study of a questionnaire-based zygosity measure, genetic testing yielded an MZ:DZ ratio of approximately 5:1, a proportion that contrasts sharply with epidemiological expectations. Worldwide, MZ twinning rates are relatively stable at about 4 per 1,000 maternities, whereas DZ twinning rates vary substantially across populations and tend to increase with maternal age. In the United States, Europe, and Australia, this pattern typically results in an MZ:DZ ratio of about 1:3 (Hoekstra et al., Reference Hoekstra, Zhao, Lambalk, Willemsen, Martin, Boomsma and Montgomery2008; Tong et al., Reference Tong2000; see Marceau et al., Reference Marceau, McMaster, Smith, Daams, van Beijsterveldt, Boomsma and Knopik2016 for a review of twinning prevalence studies). Thus, the proportion of MZ twins in our sample was considerably higher than expected based on population-level estimates. This pattern, however, aligns with a well-documented recruitment bias in twin research: approximately two out of every three volunteer twin pairs tend to be MZ (Lykken et al., Reference Lykken, McGue and Tellegen1987; Lykken et al., Reference Lykken, Tellegen and DeRubeis1978). Evidence suggests that differences in intrinsic motivation contribute to this imbalance, with MZ twins showing greater interest in participating in research than DZ twins. As Lykken and colleagues emphasized, overcoming this bias is essential for maximizing the scientific value of studies on health and behavior: ‘The only adequate solution to this problem may be to provide sufficient extrinsic incentive (…) to overcome the strong recruitment bias documented’ (Lykken et al., Reference Lykken, Tellegen and DeRubeis1978, p. 1).

Another validation study comparing a Zygosity Questionnaire with DNA analysis is currently underway. This research is being conducted within the school-age children cohort born at HCFMUSP and aims to integrate data from the project’s biological and behavioral repositories. Parents are completing the Brazilian version of the Danish Twin Registry’s four-question Zygosity Questionnaire. To aid comprehension, a visual version of these questions was developed by a graphic artist for the children. Molecular confirmation of zygosity is being performed via DNA testing from buccal cells collected through a noninvasive cheek swab.

The Biorepository supports a wide range of genetic, epigenetic, and molecular investigations, offering a unique platform for studying the biological mechanisms that underlie similarities and differences between twins. A central concept in this work is heteroplasmy, defined as the presence of more than one variant of mitochondrial DNA (mtDNA) within the same individual. Mitochondrial heteroplasmy is especially informative in the context of MZ twins because it provides a source of genetic variation independent of the nuclear genome. Although MZ twins arise from a single fertilized egg and therefore share nearly identical nuclear DNA, their mtDNA profiles may diverge due to the distinctive features of mitochondrial inheritance and replication. As mtDNA is exclusively maternally inherited and present in multiple copies per cell, mutations may arise in only a subset of these copies, creating a mixture of normal and mutated genomes. Investigating heteroplasmy has implications for forensic identification, but also strengthens the use of twin data to explore genetic variability, maternal inheritance, and disease susceptibility.

The Twin Biorepository has a critical role in advancing both forensic genetics and interdisciplinary molecular research. Flagship studies have investigated molecular approaches to distinguish between MZ co-twins (Fonseca & Fridman, Reference Fonseca and Fridman2022; Fonseca et al., Reference Fonseca, Valle-Silva, Mendes-Junior and Fridman2025; Marano & Fridman, Reference Marano and Fridman2019). With NGS and the Precision ID GlobalFiler™ STR Panel v2, Fonseca and Fridman (Reference Fonseca and Fridman2022) identified unique SNPs in two of 16 MZ twin pairs, though these findings were not replicated with alternative genotype-calling software, emphasizing the need for expanded sequencing. Complementary mtDNA analyses by Antonio and Fridman (Reference Antonio and Fridman2023) revealed limited variability, underscoring the importance of high-sensitivity NGS for reliable twin differentiation. These investigations highlight the Biorepository’s contribution to advancing forensic genetics and molecular twin research.

The Biorepository also enables interdisciplinary studies of the saliva microbiome in twin children, led by Professor Paulo Henrique Braz da Silva at the Faculdade de Odontologia da Universidade de São Paulo. Twin pairs treated at the Pediatric Dentistry Clinic are followed longitudinally, with saliva samples collected at multiple time points to examine microbiome composition, metabolomic and proteomic profiles, and inflammatory markers.

These flagship studies demonstrate the Twin Biorepository’s broader potential as a research platform. Linking genetic, molecular, microbiome, and clinical datasets, it enables novel investigations into developmental trajectories, disease susceptibility, and behavioral outcomes. The Twin Biorepository thus positions the Painel USP de Gêmeos as a model for high-impact, interdisciplinary twin research, bridging basic science and translational applications.

Twin Physiological and Physical Data Repository

The Painel USP de Gêmeos Research Group is developing a comprehensive Twin Physiological and Physical Data Repository designed to systematically store a wide range of physiological (e.g., facial temperature) and anthropometric (e.g., weight, height, grip strength, digit ratio) measurements obtained from MZ and DZ twins. Ongoing analyses derived from this repository are already generating important insights. Chorionicity is established as a substantial influence on birth weight and perinatal outcomes in twin pregnancies; however, its longer term impact on postnatal growth trajectories is a current subject of research inquiry (e.g., Beijsterveldt et al., 2016; Marceau et al., Reference Marceau, McMaster, Smith, Daams, van Beijsterveldt, Boomsma and Knopik2016). Analyses were conducted to characterize weight z-score trajectories from birth to school age among twins born in São Paulo, specifically emphasizing the contributions of chorionicity and zygosity (Correia et al., Reference Correia, Moura, Peixoto, Brizot, Carvalho, Krebs, Gomes, Albuquerque, Lebaron-Kherif, Ferreira, Arissi, Luchesi, Prist, Lucci and Otta2024; de Moura et al., Reference de Moura, Correia, Arissi, Lucci and Otta2024). The longitudinal sample comprised 188 healthy twins (M age = 9.13 years; SD = 2.16). Birth weight was extracted from HCFMUSP hospital records. Follow-up weight assessments were conducted at school age in the laboratory. Reference distributions for analysis were based on singleton birth weights and standard growth curves. The proportion of twins with weight z scores below the median declined markedly over time, from 98.3% at birth to 31.4% at school age. At birth, monochorionic twins displayed significantly greater dispersion of z scores below the median compared with dichorionic twins (2.051 vs. 1.701). This difference in dispersion was no longer evident at school age (1.861 vs. 0.976). Overall, 68.8% of the sample demonstrated improvement in weight status, and no instances of decline were observed. Although MZ and DZ twins exhibited similar intrapair differences in birth weight, by school age, MZ twins were more concordant, whereas DZ twins exhibited significantly greater intrapair weight divergence. Chorionicity exerted a stronger influence on early developmental outcomes than on later ones, a pattern consistent with findings reported in other studies (Hur & Shin, Reference Hur and Shin2008; van Beijsterveldt et al., 2016).

Research on dental health conducted by the dentistry group in 144 twin individuals, demonstrates a significant heritable component to dental caries. Results indicate greater concordance rates in MZ twins (intraclass correlation coefficient [ICC] = .60–.84) than in DZ twins (ICC = .37–.69; Freitas, Reference Freitas, Otta, Crosato and de Lourdes Brizot2025). A study examining the change and maintenance of hygiene and diet habits in twins (N = 38 pairs) following dentist-provided guidance is currently underway (Barbosa et al., 2024). After a 6-month follow-up, an improvement in oral hygiene habits was observed in 71% of the children, while improvement in sugar consumption habits was noted in a lower proportion of the sample (45%). The outcomes of the study of Barbosa et al. (2024) did not show a significant association with zygosity or age. A research is currently underway with data obtained from intraoral scanning and radiographic images of these children. The dental information collected during clinical examinations includes assessments of dental caries, trauma, enamel formation defects, dental fluorosis, and sleep bruxism, as well as quality-of-life measures (see also Freitas et al., Reference Freitas, Pontes, Brondani, Otta, Raggio, Braga and Mendes2023).

A study is also in progress examining facial thermography as a potential physiological marker of dental anxiety. The same sample of school-age twins, whose zygosity was confirmed by DNA testing, participated in this research. The study investigates their physiological reactions during a dental treatment by means of various indices. Data collection materials included the Faces Version of the Modified Corah Dental Anxiety Scale (MCDASf) and the Environmental Sensitivity Scale for Children (David, Reference David2025). Facial thermography was measured with an infrared thermal camera. The children’s self-reported anxiety was assessed via the MCDASf questionnaire. Changes in facial skin temperature were analyzed specifically at the tip of the nose and expressed as the difference between the initial and final temperature readings. These temperature changes were subsequently compared with the MCDASf scores. Preliminary analyses indicate lower nose temperatures in children reporting higher anxiety compared to those with low anxiety. Furthermore, MZ twins exhibited greater intrapair consistency in their temperature responses than DZ twins (Alves Sironi et al., Reference Alves Sironi, Luchesi and Otta2025; Luchesi et al., Reference Luchesi, Cavalcanti, Lucci, David, Otta and Monticelli2024).

Forensic dentistry studies led by Professor Edgard Michel Crosato, including research on cheiloscopy and AI-assisted facial recognition, are exploring innovative methods to differentiate MZ twins (Crosato, Biazevic et al., Reference Crosato, Biazevic, Novaes, Farias, Prist, Brizot, Lucci and Otta2025; Crosato, Otta et al., Reference Crosato, Otta, Mendes, Pontes, Lucci, Novaes, Biazevic, Otta, Crosato and de Lourdes Brizot2025; Crosata et al., Reference Crosato, Otta, Mendes, Pontes, Novaes and Biazevic2024; de Figueiredo et al., Reference de Figueiredo, Fernandes, Rabello, Crosato, Cavalcanti and Bento2022; Herrera et al., Reference Herrera, Mazzili, Melani and Crosato2014). Using image segmentation, feature extraction, and machine learning, Crosato, Biazevic et al. (2025) examined 50 pairs of twin images. The results revealed a high average intra-individual similarity (images of the same twin taken at different times) of 90.3%. In contrast, the average similarity between images of different individuals was markedly lower at 10.8%.

There is a wide range of research interests associated with the Physiological and Physical Data Repository. Research conducted with our sample of adult twins has examined chronotype and sleep–wake patterns, key biobehavioral traits with strong physiological underpinnings (Arruda et al., Reference Arruda, Gomes, Otta, Lucci, Miguel and Araujo2025). The study demonstrated significant heritability for several measures, where ℎ2 represents the heritability estimate. Chronotype, assessed using the Morningness–Eveningness Questionnaire (MEQ), showed a heritability of ℎ² = 0.55. The MEQ-derived Peak Time of Cognitive Arousal presented a heritability of ℎ² = 0.21. Sleep duration exhibited ℎ² = 0.29, and Sleep Dysfunction, measured by the Pittsburgh Sleep Quality Index (PSQI), showed ℎ² = 0.37. The only trait without significant heritability was Overall Sleep Quality, also assessed with the PSQI. The analyses also revealed substantial shared genetic effects (pleiotropy) among several traits: chronotype and peak time shared 49% of their genetic variance; chronotype and sleep dysfunction shared 59%; and peak time and sleep dysfunction shared 60%. Together, these findings underscore the strong genetic contributions to chronotype and related sleep–wake characteristics. They provide important insights into the biological mechanisms underlying circadian rhythm regulation and have implications for the development of personalized health and behavioral interventions. As a natural extension of our research with adult twins, we are planning a new study focused on a sample of school-aged children. This project will be jointly led by Professor John Fontenele Araujo from the Universidade Federal do Rio Grande do Norte (UFRN) and Dr Ricardo Prist from the Instituto de Psicologia da USP. The research will focus on the regulation of the sleep-wake cycle, specifically examining the heritability of the homeostatic and circadian components measured by actigraphy in twin children and their parents.

These studies illustrate the substantial potential inherent in the collaborative use of repositories by multidisciplinary research teams. Taken together, the Twin Physiological and Physical Data Repository functions as an integrated and highly valuable resource supporting advanced research across psychology, medicine, dentistry, genetics, and applied mathematics. By providing this comprehensive data platform, the repository actively advances the understanding of development, health, and behavioral processes in twins.

Twin Behavioral Data Repository

The Behavioral Data Repository is a core component of the twin research infrastructure, providing a systematic and secure platform for organizing behavioral and psychological assessments. It is specifically designed for the storage, analysis, and controlled sharing of complex multimodal datasets. The repository’s power lies in its ability to promote the comprehensive integration of behavioral, psychological, physiological, and genetic data. This unified approach facilitates interdisciplinary research, leading to a deeper understanding of individual differences and developmental processes across the lifespan.

Research within this repository spans multiple subfields and age ranges. For the fetal period, 4D ultrasound and computational algorithms are used to precisely quantify facial expressions and behavior in both singleton and twin pregnancies, utilizing the validated FETAL-7 scale (Bernardes et al., Reference Bernardes, Carvalho, Harnik, Teixeira, Ottolia, Castro, Velloso, Francisco, Listik, Galhardoni, Aparecida da Silva, Moreira, de Amorim Filho, Fernandes and Ciampi de Andrade2021; Bernardes et al., Reference Bernardes, Fernandes, Carvalho, Ottolia, Hamani, Oliveira, Kubota, da Silva, Veloso, de Carvalho, de Amorim Filho, Arenholt, Leutscher and Ciampi de Andrade2024), to provide objective measures of fetal neurodevelopment. Research with newborns focuses on the collection and analysis of acoustic data (Monticelli & Oliveira, Reference Monticelli, de Oliveira, Otta, Crosato and Brizot2025). Cry recordings are being systematically collected from twin newborns in the Rooming-in Unit and the Neonatal Center of HC-FMUSP and HC-FMUSPRP. The research specifically investigates nonlinear phenomena in twin newborn cries. These acoustic irregularities manifest in several forms, including biphonation, where two independent fundamental frequencies are produced simultaneously; subharmonics, which are tones appearing at integer fractional multiples of the fundamental frequency; pitch jumps, which are sudden, abrupt shifts in the dominant pitch; and segments of deterministic chaos or noise, which show a highly aperiodic or turbulent acoustic pattern. The detailed analysis and comparison of these vocal nonlinearity markers between co-twins offer valuable insights into early vocal regulation and physiological arousal. Ultimately, this comparative research aims to help investigators identify potential neurodevelopmental or stress-related differences linked to shared or unique prenatal and postnatal conditions.

In early infancy, the behavioral abilities of twin infants are assessed during home visits (J. L. Correia et al., Reference Correia, Vieira, dos, Ribeiro, de Paula, Prist, Tania Lucci and Otta2025; Vieira et al., Reference Vieira, Correia, Ribeiro, de Paula, Lucci and Otta2025). Ongoing research includes twenty Brazilian twin infants (11 boys; 55%) who completed the Bayley Scales of Infant Development (Bayley-III), at 3 and 6 months of age in their homes. The Cognitive, Language, and Motor domains were evaluated directly by two trained examiners. Regarding birth weight, most infants (75%) were classified as having normal birth weight (M = 2577.60 g, 95% CI [2416.37, 2738.83]); 5% had low birth weight (M = 1675.40 g, 95% CI [1251.10, 2099.70]). Overall, the infants performed above average (scaled scores > 90) across all domains, with one exception: in the Language domain at six months, more than half of the sample (58.3%) scored in the low-average range (scaled scores 80–89). The study successfully demonstrated the feasibility of administering the Bayley-III to twin infants in the home environment while strictly adhering to standardized procedures. This ecologically valid approach provides a valuable framework for systematically studying early development within naturalistic contexts. The ongoing data collection is designed to investigate whether the preliminary finding of early language delays in twins is transient or persistent, and also to compare developmental trajectories between MZ and DZ twin pairs.

Datasets from school-aged children and adults include video recordings of problem-solving and emotion-eliciting tasks (Almeida et al., Reference Almeida, Lucci, David, Lebaron-Kherif, Prist and Otta2021; Lebaron-Kherif et al., Reference Lebaron-Kherif, Lucci, Esseily and Otta2023; Lebaron-Khérif et al., Reference Lebaron-Khérif, Lucci, Esseily and Otta2024), facial expressions (Lucci et al., Reference Lucci, David, Arantes, Oliveira, Prist and Otta2023; Lucci et al., Reference Lucci, Oliveira, Arantes, David and Otta2021), and audio recordings (Campos et al., Reference Campos, Luchesi, Kiehl, Otta and Ferreira2024; Luchesi et al., Reference Luchesi, Cavalcanti, Lucci, David, Otta and Monticelli2024; Otta & Monticelli, Reference Otta and Monticelli2021; Paula et al., Reference Paula, Luchesi, Lucci, Otta and Monticelli2024). These materials enable the investigation of developmental, cognitive, and emotional processes across the lifespan (I. Ferreira et al., Reference Ferreira, Ito, Resende and Otta2024; F. L. Ferreira et al., Reference Ferreira, Lucci, David, Otta, Otta and Lucci2021). By invitation, we participated in a comprehensive meta-analysis led by Professor Meike Bartels of the Vrije Universiteit Amsterdam, as part of her World-Wide Wellbeing Project, which conducts large-scale comparative and meta-analytic investigations encompassing populations with diverse cultural and genetic backgrounds. Our contribution drew on data from Fernandes’ (Reference Fernandes2021) master’s dissertation, which investigated subjective well-being and personality in a sample of 201 same-sex twin pairs (146 MZ and 55 DZ; see also Fernandes & Otta, Reference Fernandes and Otta2024).

A Triadic Experimental Task, inspired by Segal’s (Reference Segal1984) research, was developed for use with school-aged twins (see also Segal, Reference Segal2002; Segal, Reference Segal2017). Each twin was filmed side by side for 10 minutes while solving a puzzle, while the mother or father stood between and slightly behind them and could choose whether or not to assist (I. Ferreira et al., Reference Ferreira, Ito, Resende and Otta2024; F. L. Ferreira et al., Reference Ferreira, Lucci, David, Short, Crispim, Reali, Marty, Rocha, Grinberg, Segal, Segal, Knafo-Noam and Otta2023; Ito et al., Reference Ito, Ferreira and Otta2024). This task was designed to investigate differential parental treatment (DPT) by simulating everyday situations in which a parent must manage limited time and energy while two children simultaneously seek attention. Sixty-nine twin pairs (46.4% MZ; 53.6% DZ) participated in the study with their caregivers — 58 pairs with their mothers and 11 with their fathers. On average, caregivers spent 40.1% of the task time observing the twins, 57.2% interacting with their puzzles, and 27.2% communicating with them. A DPT score was calculated as the absolute difference between the behaviors directed toward each twin. Early health and physical differences predicted DPT. In same-sex pairs discordant for health problems (n = 10), the twin with health issues received more DPT. Height differences also mattered: in pairs differing by more than 4 cm (n = 12), caregivers directed more DPT toward the shorter twin. Current weight differences were not predictive, but birth weight differences were: among pairs differing by more than 20% in birth weight (n = 10), DPT was greater toward the lower–birth weight twin. In sum, caregivers consistently differentiated their behavior toward twins, and this variation was systematically associated with early health status and physical disparities. These findings highlight how medical and developmental factors shape the caregiving environment, even within the same family.

The integrated repository thus provides a rich, multimodal platform for interdisciplinary research on developmental trajectories, maternal influences, behavioral phenotypes, and the interplay between genetic and environmental factors in twins. Researchers involved in the FAPESP Thematic Project value the opportunity to integrate diverse research domains within a coherent framework.

Public Outreach

The USP Twin Panel conducts outreach activities through the Coordination of Education, Knowledge Dissemination, and University Outreach, under the supervision of Professor Claudio Possani. The outreach work focuses on content production for the Painel USP de Gêmeos’ Instagram, Facebook, and website, with weekly publications created by a dedicated team of fellows, undergraduate students, and a journalist. This content is designed to promote the research conducted by the Painel USP de Gêmeos’ team, invite public participation in studies, disseminate findings published in scientific journals, share news relevant to the twin community, and promote events that foster a closer relationship between the public and scientific discovery.

We organize annual Twin Festivals to celebrate twinship and engage the public in interactive educational activities, with the 10th edition currently in preparation. We also provide psychological support programs for mothers of twins to help them navigate the unique challenges of raising twins through evidence-based guidance and community support. In partnership with the Sedes Sapientiae Institute, we have offered support groups for parents. Additionally, the pediatric dentistry section of USP’s Faculdade de Odontologia provides free dental treatment for children of participating families. These twins also take part in a research project under the supervision of Professor Fausto Medeiros Mendes.

The Painel USP de Gêmeos effectively integrates research, training, and outreach. A notable study, co-led by Professor Emma Otta and Professor Nancy L. Segal, focused on Brazilian twins separated at birth and reunited in adulthood. The study’s findings highlighted both convergences in vocational interests and personality traits, and notable differences in gender identity. To maximize impact, the research was disseminated across three formats: a scientific publication (Fernandes et al., Reference Fernandes, de Felipe, Ferreira, Segal, Otta, Crosato and Brizot2025), a journalistic narrative for broad public engagement (Martinez & Otta, Reference Martinez and Otta2024), and a graphic novel adaptation created by a graphic artist to reach younger and nonspecialist audiences (Monticelli, Reference Monticelli2024). For additional research on reared-apart twins, also see Segal (Reference Segal2012).

The story of the Brazilian twins reared apart made a remarkable impact, as illustrated by a message from an educator:

This example perfectly illustrates how scientific research, dissemination, and university outreach are complementary activities that strengthen the social role of the university. The research we conduct generates knowledge, scientific dissemination makes it accessible to the public, and outreach enables its practical application through interaction with society. This integration amplifies the social impact of science, reaffirming the university’s commitment to social transformation and the promotion of knowledge.