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Missing data are common in social and clinical sciences and understanding the causes and patterns of missing data is important for selecting analysis approach and for the interpretation of the remaining data. Yet, knowledge about the factors influencing data loss is limited. Here, we assessed the contribution of genes and environments to data missingness across three experiments of infant brain and behavioural development. The sample consisted of 594 infant twins (330 monozygotic, 152 female, 178 male infants; 264 dizygotic, 132 female, 132 male infants) who were assessed with electroencephalography (EEG), pupillometry, and gaze tracking technologies at 5 months of age. Substantial familial factors (additive genetics and/or shared environment) for data missingness were found across all experiments. The amount of missing data showed only a low correlation across the experiments, suggesting a high degree of specificity in the factors contributing to missingness. The results underscore the need to adopt and improve procedural and analytical strategies that minimise data loss and its negative impacts on study conclusions.

Background: Sleep and behavioral regulation are both vital for early healthy development. Yet, little is known about the relative contribution of genetic and environmental factors to early sleep and regulatory behaviors, or how these etiological influences may change during the first months of life.

Methods: Genetic and environmental influences on sleep, settle, and crying behaviors at 2 and 5 months were examined in 998 twins, using a classical twin design. In addition, polygenic scores were derived for a range of sleep behaviors, as well as psychiatric and neurodevelopmental conditions.

Results: Genetic influences (A) explained a large part of the variation in duration of crying at both 2 and 5 months (A = 0.29–0.70) and in settle ability at 5 months (A = 0.51–0.67). Shared environment (C) primarily influenced number of wakeups per night at both ages (C = 0.61–0.90) and settle ability at 2 months (C = 0.36–0.65). Longitudinal analyses suggested modest shared genetic influence on settle ability in the daytime across the ages (24%), and non-significant shared genetic estimates for ability to settle in the evening and at nighttime. There was moderate shared influence of shared environmental factors on number of wakeups per night (56%) and modest but significant shared genetic influence on crying duration in the evening and nighttime (17%–33%). Unique environmental effects were mostly specific to each age. Finally, autism polygenic score associated with longer crying duration in the evening at 2 months (β = 0.16, p = .002).

Conclusions: Etiological influences tended to change from 2 to 5 months, reflecting a highly plastic period in infant brain development and in child-environment interactions.

Sleep, settle, and crying behaviors represent basic regulatory functions in early infancy, yet little is known about the factors that influence these behaviors and their relationship to later development in the general population. In this study, we assessed a sample of 362 infants, measuring parent-rated number of wakeups per night, time to settle, and crying duration at 2 months of corrected age (range 27–99 days), along with various background variables. We also measured several aspects of the infants? later development at 14 and 24 months. Age (corrected) at the first assessment showed a significant association with number of wakeups per night (β = -0.212, p < 0.001) and crying duration (β = –0.154, p = 0.012). Family income was a significant predictor of crying duration (β = –0.128, p = 0.018) and time to settle (β = –0.147, p = 0.011). Sleep, settle, and crying behaviors at 2 months were not significantly associated with parent-rated language comprehension or socio-communicative abilities at 14 months, nor with vocabulary, autistic traits, or hyperactivity at 24 months. While some sleep and settle behaviors in the first few months can be challenging for caregivers, our findings suggest that they are not indicators of atypical development in early childhood in the general population.

Summary

• In a sample of more than 350 infants, we found that higher family income was associated with shorter time until settled and shorter crying duration at 2 months of age.
• Sleep, settle, and crying behaviors at 2 months did not show statistically significant associations with language development, hyperactivity, or autistic traits in toddlerhood.

Infants' tendency to focus on socially relevant cues is commonly referred to as 'social attention' and is often conceptualized as a unified construct. However, few studies have examined the concordance among measures that fall under this label. In this study, we assessed 50 ten-month-old infants using three common measures of social visual attention: eye (versus mouth) preference, gaze following, and face preference (versus non-social objects). We also examined their association with concurrent parent-rated socio-communicative abilities. No significant associations were found among the gaze measures. Notably, only eye preference was uniquely associated with communicative abilities (β = 0.364, p = 0.018), while the other gaze measure showed no such relationship. These findings suggest that what is commonly referred to as 'social attention' reflects a set of functionally distinct and seemingly unrelated behaviors, highlighting the need to re-evaluate its conceptualization as a unified construct in early development.

It has been suggested that bilinguals take greater advantage of visual speech cues than monolinguals. Therefore, in a sample of 474 (47.3% females) monolingual and 101 (48.5% females) bilingual infants at 5 months of age, we examined the tendency to look at the eyes versus the mouth of dynamic faces, as well as the latency and ratio of looking at a static face interspersed with non-social objects. No significant differences were found for these measures, suggesting that monolingual and bilingual infants orient to and scan faces in a similar way. Although no association was found between the tendency to look at eyes versus mouth at 5 months and vocabulary at 24 and 36 months, a higher tendency to look at the eyes was related to a larger receptive vocabulary at 14 months, but only in the monolingual group (β = 0.15, 95% CI: 0.04; 0.27, p = 0.011). However, the difference in beta values of this association between mono- and bilinguals was not statistically significant. In conclusion, we did not find support for the hypothesis that bilingual infants rely on visual speech cues from the mouth more than monolinguals do, and there was no association between the tendency to look at eyes versus mouth and later language development in the bilingual group.

Summary

• It has been suggested that bilinguals take greater advantage of visual speech cues than monolinguals.
• Here, no differences between bilingual and monolingual 5-month-olds were found regarding any measures of face scanning.
• The findings suggest similar visual attention patterns in mono- and bilingual infants, with no impact on bilingual language development.

Twin studies are important for research on genetic and environmental influences on child development, but it is imperative to test whether findings can be generalised from twins to singletons. Since the first months of life are defined by the emergence of important sleep behaviours, we compared 451 (54.8% females) twins and 77 singletons (48.1% females) on a range of sleep, settle and crying behaviours at 5 months of age. No significant differences were found regarding duration of crying or time until settled. However, singletons were reported to wake up more frequently during nighttime than twins (F(1, 509) = 35.10, p < 0.001, η_p² = 0.065), suggesting that, when reported in twin studies, this measure might be slightly underestimated at a mean level in relation to singletons. In conclusion, despite the unique challenges and additional caregiver load of infant twins, there seem to be few differences between twins and singletons regarding parent-reported settle and crying behaviours in early infancy.

Eye tracking is a widely used tool to study infant development, but creating diverse stimuli while maintaining high control over confounding variables can be challenging. In this proof-of-concept study, we examined an innovative way to generate ecologically valid stimuli using AI technology, in order to create videos that can be used in culturally diverse settings. Using the eye-mouth-index (EMI), a commonly used paradigm in infant eye tracking, we examined the consistency of eye tracking measures across original videos and two types of AI-manipulated videos in a sample of 46 infants aged 12–14 months. We found a very strong correlation of the EMI across original and AI videos (r = 0.873–0.874), and there were no statistically significant differences between mean EMI in the original and AI conditions. Additionally, we created culturally diverse videos to measure gaze following, and found that children followed the gaze of the people in the AI-manipulated videos in an expected manner. In conclusion, AI technology provides promising tools to create ecologically valid and culturally diverse stimuli, that can be used to conduct studies in a wide range of settings and to examine the generalizability of earlier findings in the field of developmental psychology.

Autism is a neurodevelopmental condition with early onset, usually entailing language differences compared to neurotypical peers. Females are four times less likely than males to be diagnosed with autism, and the language features associated with this condition are less frequent in females than in males. However, the developmental mechanisms underlying these sex differences remain unclear. In neurotypical populations, sex differences in language development are also observable from early on, with females outperforming males. One mechanism underlying these sex differences may be early differences in selective attention to talking faces. During the first year, more mouth-looking generally predicts better language development, but sex differences exist. Female infants look at the mouth of a talking face more than males without penalizing looking to the eyes, and reduced mouth-looking in early infancy relates to better vocabulary in toddlerhood only in females. In this hypothesis and theory article, we propose that unique female gaze patterns to the mouth may constitute an early female-specific candidate marker that acts as a protective marker for language development also in autism. Since autism is highly heritable, investigating infants at elevated likelihood for autism offers the opportunity to search for sex-specific markers operating early in life before autistic features and language differences emerge. We argue that, as in neurotypical female infants, mouth-looking may also protect female infants-at-elevated-likelihood-for-autism population from potential later differences in language skills. If so, then sex-specific early behavioral markers, potentially acting as protective markers of language, may compensate for some genetic risk markers affecting this population. Here we gather evidence from neurotypical infants and those with elevated likelihood of autism to uncover why biological sex, the development of selective attention to the mouth, and language acquisition could be intimately related in both populations. We also propose hypotheses regarding potential sex-differentiated neurodevelopmental pathways. We end discussing future research challenges: how generalizable mouth-looking could be as a potential female-specific early language marker across contexts (experimental vs. real life), countries, and developmental time. Ultimately, we aim to target a novel protective candidate of language acquisition, informing tailored interventions that consider sex as an important source of individual variability.

Adaptive behaviour refers to the everyday skills that individuals are expected to have to function independently, based on their age and societal norms. Currently, we know little about the role of genetic and environmental factors in parent-rated adaptive behaviours in early infancy. The aim of this study was to investigate the aetiological factors that influence individual variability in different adaptive behaviour domains at 5 months, and the degree of genetic and environmental influences that are unique and shared across these domains. We analysed data from the Vineland Adaptive Behaviour Scale (VABS-II) motor domain and combined domain of socialization and communication (social-communication) using a multivariate twin modelling approach. Participants were a community sample of monozygotic and dizygotic twins assessed at 5 months of age (n = 594). The results show high shared environmental influence on both motor (0.67) and social-communication (0.78) domains with 45% shared variance. Both had low, but significant heritability estimates (0.21 and 0.12, respectively) but did not share genetic variance. No statistically significant associations were found between polygenic scores for autism, ADHD, schizophrenia, depression, and bipolar disorder, and either of the adaptive behaviours measured here. Our results highlight the importance of shared environmental factors in the development of social-communication and motor skills in infancy, whether it is through social interaction with caregivers, or the stimuli and opportunities presented at home. Summary During development structural arm length representation is underestimated, while the functional arm length representation is overestimated. Underestimation of structural arm length is driven by an underestimation of hand length, as forearm length is accurate. Structural hand length is underestimated, supporting that underestimation of hand length is a characteristic of human body representation. The opposite pattern of results between structural and functional arm representation suggests the existence of multiple independent representations of the body.

Computer-vision-based gaze estimation refers to techniques that estimate gaze direction directly from video recordings of the eyes or face without the need for an eye tracker. Although many such methods exist, their validation is often found in the technical literature (e.g., computer science conference papers). We aimed to (1) identify which computer-vision-based gaze estimation methods are usable by the average researcher in fields such as psychology or education, and (2) evaluate these methods. We searched for methods that do not require calibration and have clear documentation. Two toolkits, OpenFace and OpenGaze, were found to fulfill these criteria. First, we present an experiment where adult participants fixated on nine stimulus points on a computer screen. We filmed their face with a camera and processed the recorded videos with OpenFace and OpenGaze. We conclude that OpenGaze is accurate and precise enough to be used in screen-based experiments with stimuli separated by at least 11 degrees of gaze angle. OpenFace was not sufficiently accurate for such situations but can potentially be used in sparser environments. We then examined whether OpenFace could be used with horizontally separated stimuli in a sparse environment with infant participants. We compared dwell measures based on OpenFace estimates to the same measures based on manual coding. We conclude that OpenFace gaze estimates may potentially be used with measures such as relative total dwell time to sparse, horizontally separated areas of interest, but should not be used to draw conclusions about measures such as dwell duration.