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Young learners would seem to face a daunting challenge in selecting to what they should attend, a problem that may have been exacerbated in human infants by changes in carrying practices during human evolution. A novel theory proposes that human infant cognition has an altercentric bias whereby early in life, infants prioritize encoding events that are the targets of others’ attention. We tested for this bias by asking whether, when the infant and an observing agent have a conflicting perspective on an object's location, the co-witnessed location is better remembered. We found that 8- but not 12-month-olds expected the object to be at the location where the agent had seen it. These findings suggest that in the first year of life, infants may prioritize the encoding of events to which others attend, even though it may sometimes result in memory errors. However, the disappearance of this bias by 12 months suggests that altercentricism is a feature of very early cognition. We propose that it facilitates learning at a unique stage in the life history when motoric immaturity limits infants' interaction with the environment; at this stage, observing others could maximally leverage the information selection process.

Details on software, materials, analyses (frequentist analyses) and other decisions during the study. Extra figures, tables, decisions. Replicated conditions discussed separately for the frequentist reporting.

Based on a recent study identifying distinct and independent cortical brain regions associated with the early development of implicit and explicit Theory of Mind (ToM), respectively (Grosse Wiesmann et al., 2020), we ask whether the structural connectivity of these regions is relevant for the emergence of ToM. We hypothesize a dual pathway model of ToM: a dorsal connection supporting explicit ToM and a ventral connection supporting implicit ToM development in preschool-aged children.

In this study, we aim to investigate the changes in grey matter (GM) and white matter (WM) structures associated with early language and syntactic development between the ages of 3 and 4.

Social interactions are of particular importance for social individuals because they contain highly relevant information that individuals can use to learn about their (social) environment. Active engagement as well as attending towards and observing social interactions can positively influence individuals' own social behavior. In this project, we want to get a better understanding of the developmental trajectory of social interaction preferences in great apes. Subjects will be confronted with picture pairs showing two individuals socially interacting versus the same individuals acting individually. The picture showing individuals acting individually will be an exact copy of the interaction picture, but the individuals are mirrored in a way that they appear to not interact with each other. We will use eye-tracking to measure the looking time towards each of the pictures. Simultaneously, we will take proximity data outside of the experimental environment to calculate a sociality index for the subjects, to see whether their preference for interaction pictures correlates with their real-life sociality. In this project, we will take data from great apes of all age groups living in three different sites: one zoo population and two sanctuary groups.

In this study, we aim to investigate changes in grey matter and white matter structures associated with increasing working memory capacity between the ages of 3 and 4 years.

Social interactions require their participants to represent each other’s beliefs. This is especially challenging when the individuals' beliefs contradict each other. Humans struggle with the task of holding two different, contradictory views about their environment in their minds. Interestingly, not only does inhibiting their own beliefs appear to be difficult, but in some situations, the beliefs of others interfere with their own behavior. It seems that humans automatically represent their partner’s view about the environment, even in situations where this point of view is irrelevant or even adverse. A research team around Kovács (2010) showed that even infants showed this interference towards the other person's belief (so-called altercentric interference/bias). In their study, the infants’ looking time was influenced by the false belief of an observing agent, even in situations where the infant was holding a true belief of the hiding location of a ball. Interestingly, these false beliefs not only modulated the looking time when the agent was present but also later when the agent was not present anymore. So far, most studies examining the altercentric interference were tested in WEIRD (Western, educated, industrialized, rich, and democratic) populations, which is a known problem specifically for psychology studies (Nielsen et al., 2017). To claim that altercentric cognition is a human-specific “social sense” (Kovács et al., 2010), we need to study this trait cross-culturally. Therefore, one aim of this project is to study whether non-westernized populations (i.e., BaYaka and Bandongo, Likouala Region, Republic of the Congo) also show the altercentric interference effects.

A traditional view of understanding other’s mental states is that early in ontogeny infants start from a me-first position, and through themselves learn to understand others. Here we propose an opposite developmental trajectory, where infants might start out highly altercentric (Southgate, 2020) and through development increasingly rely on their own point of view. In a pre-registered cross-sectional study we present 1-6-year-old children with a task where altercentric modulation has been found with 14-month-old infants (Kampis & Kovács, 2020). There, infants tended to search longer in a box when another person believed an object to be present, than when she believed it was empty. We predicted this tendency to decrease with age. Preliminary results with n=191 children included based on their search in baseline trials, show a decrease in altercentric modulation with age (r=-.174, p= .016), which will be discussed together with the relationship between altercentrism and the development of self-concept.

We test for infants’ proposed altercentric learning bias (Southgate, 2020). By tracking others’ attention, infants build their own models of the environment based on the well developed models of the adults. If true, an event will be better remembered if witnessed together with someone else than if attended alone. Eight-month-old-infants (6 conditions, n = 32/group) saw an object being hidden first in one, then in another location. At the end of each trial, one of the locations was revealed, always empty. Participants correctly remembered the location and looked longer if it was empty (conditions 1-2). When an agent attends the first hiding but not the second, infants misremember the object in the first location (condition 3), providing initial evidence for the altercentric bias. Contrary to our predictions, when the agent only attends to the last hiding location, or both, infants have no expectation of the object’s whereabouts (conditions 4-6).

As adults, we do not only choose what we prefer, but we also tend to adapt our preferences post-hoc to fit our previous choices, even when these were blind. This is thought to result from cognitive dissonance as an effort to reconcile our choices and preferences. It has been argued to rely on the self-concept, but has also been found in preschoolers and monkeys. In a preregistered study, we therefore investigated when blind choice-induced preferences emerge and whether they are related to self-concept development in the second year of life. Results in N=200 children aged 16-36 months provide strong evidence that blind-choice induced preferences develop between 1 and 2 years (BF10=12.5). Two-year-olds avoided an object that they had previously discarded in a blind choice, whereas 1-year-olds did not show any such preferences yet. Further, we found substantial evidence against a relation with measures of self-concept development (BF10=0.2).

Human social interaction crucially depends on the ability to attribute thoughts and beliefs to other individuals. This ability is referred to as Theory of Mind (ToM), and understanding that other people can have false beliefs about the world is considered to be a critical test of ToM. In childhood, a developmental breakthrough is achieved around the age of 4 years, when children start explicitly reasoning about others’ false beliefs. The cognitive and neural developments that lead to this milestone of human cognition, however, are currently unknown. Moreover, recently, novel im- plicit paradigms have shown that, already before the age of 2 years, infants display correct expectations of the actions of an agent with a false belief. The processes that underlie these expectations and their relation to the later-developing explicit false belief reasoning, however, are unclear. The current thesis addresses these open issues in three studies. The first study investigates the developmental trajectory and robustness of an implicit false belief task longitudinally from the age of 2 to 4 years. We find that children only perform above chance by the age of 4 years, but not at 2 and 3 years. This indicates that early success on implicit false belief tasks is fragile. The second study examines the correlation of implicit and explicit false belief tasks with each other and with co-developing cognitive abilities. This shows a dissociation of implicit and explicit false belief tasks in that performance on the two task types does not correlate, and that explicit false belief tasks correlate with syntactic and executive functions, whereas implicit false be- lief tasks do not. Finally, the third study shows that the maturation of white matter in brain regions that support false belief reasoning in adultsand of their dorsal connectivity to the inferior frontal gyrus, suggested to support hierarchical processing, is associated with the emergence of explicit false belief reasoning in 3- and 4-year-old children. These associations are independent of implicit false belief-related action anticipation and of developments in other cognitive domains. Taken together, our results speak for a dissociation of the processes underlying implicit and explicit false belief tasks. We suggest that the developmental breakthrough in explicit false belief reasoning around the age of 4 years might result from improved belief processing, emerging hierarchical processing abilities, and the maturation of the connection between the relevant brain regions. Furthermore, I speculate on processes that might underlie early success on implicit false belief tasks in infancy.:Acknowledgements iii Summary xi Deutsche Zusammenfassung xvii 1 General Introduction 1.1 Theory of Mind . . . . . . . . . . . . . . . . . . . . . . . . . 1 1.1.1 Precursors of ToM in Infancy . . . . . . . . . . . . . 3 1.1.2 False Belief Understanding . . . . . . . . . . . . . . . 4 1.1.3 Relation between Implicit and Explicit False Belief Tasks . . . . . . . 8 1.1.4 Theoretical Accounts of the Emergence of ToM . . . 14 1.2 Relation to Other Cognitive Domains . . . . . . . . . . . . . 20 1.2.1 Executive Function . . . . . . . . . . . . . . . . . . . 25 1.2.2 Language . . . . . . . . . . . . . . . . . . . . . . . . 26 1.2.3 Correlations with Implicit False Belief Tasks . . . . . 29 1.3 Neural Basis of ToM . . . . . . . . . . . . . . . . . . . . . . 31 1.3.1 Neural Basis of ToM in Adults . . . . . . . . . . . . . 31 1.3.2 Neural Basis of ToM in Development . . . . . . . . . 35 1.3.3 Structural Brain Development in Early Childhood . . 36 1.4 Research Questions and Hypotheses . . . . . . . . . . . . . . 38 2 Study 1: Longitudinal evidence for 4-year-olds’ but not 2- and 3-year-olds’ false belief-related action anticipation . . . . . . . . . . . .45 3 Study 2: Implicit and explicit false belief development in preschool children . . . . . . . . . . . .73 4 Study 3: White matter maturation is associated with the emergence of Theory of Mind in early childhood . . . . . . . . . . . .91 5 General Discussion 5.1 Is there a continuity from early-developing to later explicit false belief abilities? . . . . . . . . . . . . . . . . . . . . . . . 106 5.2 What is the relation of implicit and explicit false belief tasks to other cognitive domains? . . . . . . . . . . . . . . . . . . 109 5.3 What is the neural basis of the emergence of ToM? And what does this tell us about the underlying cognitive processes? . . . . 114 5.4 What processes underlie implicit false belief tasks?. . . . . 118 5.5 Future Research and Limitations . . . . . . . . . . . . . . . 120 5.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 References . . . . . . . 128 A Supplements Study 1 . . . . . . . 161 B Supplements Study 2 . . . . . . . 163 C Supplements Study 3 . . . . . . . .181 Abbreviations . . . . . . . . 187 List of Figures . . . . . . . . 191 List of Tables . . . . . . . . 193