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Although it occurs regularly, there is little research on tandem-breastfeeding and its psychological aspects. The present set of two studies fills this research gap. Study 1 follows a qualitative approach to generate relevant psychological topics related to tandem breastfeeding through semi-structured interviews with tandem breastfeeding mothers and mothers who breastfed two children but not in tandem. In the quantitative Study 2, an online questionnaire was generated based on the relevant factors for tandem-breastfeeding identified in Study 1. We found that older children who were not breastfed in tandem changed their focus from the mother to the partner after birth of the younger sibling and rejected their mother, which was not the case for children who were breastfed in tandem. Tandem-breastfeeding mothers showed more attachment parenting and less authoritarian parenting. However, they felt increased anger and aggression towards the older child. The current set of studies provides the first scientific basis for research in psychological aspects of tandem nursing. 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