The University of São Paulo Twin Panel (Painel USP de Gêmeos), based at the Institute of Psychology of the University of São Paulo, started formally in 2017. Our registry is new, but in only two years of formal existence, it comprises a volunteer sample of 4826 registered individuals (98% twins and 2% higher-order multiples), recruited at the University of São Paulo and by social media campaigns. Our main aim is to conduct and promote research with twins on psychological processes and behavior. The University of São Paulo is the largest higher education and research institution in South America, and the Painel USP de Gêmeos has great potential for fostering research on twin-related issues from a psychological perspective in Brazil and South America.

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Agradeço a minha mãe, Andrea Cristina Souza França, minha fortaleza e porto seguro, pelo amor, cuidado e apoio imensuráveis, pelo coração incansável, por sempre acreditar e incentivar os meus sonhos e por ser o exemplo da mulher que busco ser. Agradeço meu pai, Fabio da Silva Ferreira, por todo o amor e suporte, por ser exemplo de determinação e por me inspirar a ser uma profissional competente, sem perder de vista as minhas origens. Vocês são a melhor parte de mim, sou eternamente grata a vocês! Aos meus irmãos, Daniel, Luana e Manuela, meus grandes amores, que iluminam minha vida e me ensinam tanto desde que nasceram.Às minhas avós, Idalina e Nilza, e meu avô, Nelson (in memoriam), por serem exemplos de pessoas fortes, guerreiras e integras, que eu amo demais. Aos grandes presentes que a vida me deu: meu "paidrasto", Gilberto, por ser essa pessoa tão benevolente, incrível e presente e Bruna, minha irmã de coração, por toda a cumplicidade e por compartilhar comigo o sonho da vida acadêmica. A todos os meus tios, tias, primos e primas que sempre acreditaram em mim.Aos meus sogros, Vanessa e todos os familiares que estiveram ao meu lado.Ao meu namorado, Rodolfo, que acompanhou de perto todo o processo de construção e realização desse trabalho, por discutir horas e horas sobre minhas ideias, por revisar minha Palavras-chave: gêmeos na escola, separação de gêmeos, relacionamento entre gêmeos, criação de gêmeos, educação This dissertation aimed to verify the causes and consequences of the decision of keeping or separating twins in different classrooms, according to their mother's perspective. Three studies were carried out using quantitative and qualitative approaches and the results were interpreted according to the psychoethological perspective. The First Study focused to obtain an overview of the twins' classroom placement and the maternal's practices and beliefs about twins at school. two hundred and eighty-seven mothers of twins, from 2 to 10 years old, participated in this study and answered the Zygosity and Twins at School Questionnaire. We found that 65% of twins studied in the same classroom and the majority of mothers reported to have participated in the decision of separating or not their children, especially in private schools. Focusing on parental practices, mothers that lived in big cities chose more often for the separation, as well as mothers with high income. Besides that, younger twins were less separated than older ones. Regarding maternal beliefs, the relationship and age of the children were considered important factor for the decision of separating or not. Individuality and personality's development, school imposition, dominance, conflict and dependency between twins were reported as being the main reasons for separation. The Second Study investigated the influence of zygosity, age and classroom placement on the twins' relationship and their reaction towards separation. The answers of 422 mothers of twins, from 4 to 12 years old, to the Twin Relationship Questionnaire were analyzed. Monozygotic ...

Our aim was to adapt and validate a Brazilian Portuguese version of the Twin Relationship Questionnaire developed by Fortuna et al. (2010) and validated by H. Segal and Knafo-Noam (2019) in Israel. The respondents were 862 Brazilian mothers of twins ( N = 1724 twins) with mean age of 35 years (SD = 6.1). The majority of the sample lived in the Southeast (61.8%) or in the South (24.5%) of Brazil. We conducted a Multilevel Confirmatory Factor Analysis with the pair of twins as second level variable, and the five-factor structure (closeness, dependence, conflict, dominance, and rivalry) proposed by the original validation study of H. Segal and Knafo-Noam (2019) was confirmed. The final model retained 15 items out of 22 proposed in the original version of the questionnaire. Although the TRQ-BR has fewer items, the accuracy compared to the original questionnaire was maintained. Mixed Model Analysis (LMM) of TRQ scores were used to investigate twins’ relationships as a function of zygosity, age groups, and sex in order to provide evidence of convergent validity of the instrument. As expected, mothers perceived monozygotic twins (MZ) as more depedent than dizygotic twins (DZ). Furthermore, male twin pairs were considered more conflictive when compared to female twins. The present study showed that TRQ-BR is an adequate instrument for research purposes in the Brazilian population. It can also be useful for applied areas such as clinical and educational fields.