Although early life experiences of language and parenting are critical for children's development, large home observation studies of both domains are scarce in the psychological literature, presumably because of their considerable costs to the participants and researchers. Overcoming some of these difficulties, we used here digital audio-recorders to unobtrusively observe 107 preschoolers, who were aged 2.03 to 3.99 years (M = 2.77, SD = 0.55), and their families over 3 days (M = 15.06 hours per day, SD = 1.87). The recording software estimated the total number of words that a child heard over the course of a day. In addition, we transcribed six 5-minute excerpts per family (i.e. 30 minutes overall) to extract estimates of children's and parents' lexical diversity, positive and critical parenting, and children's internalizing and externalizing behaviors. We found that home language input (i.e. number of words and lexical diversity) was positively associated with children's cognitive ability and lexical diversity but not with their behaviors. In addition, we observed that home language input varied as much within as between families across days (intra-class correlation = .48). By comparison, parenting predicted children's behavioral outcomes but was not related to their cognitive or lexical ability. Overall our findings suggest that home language input affects child development in cognition and language, while parenting informs their behavioral development. Furthermore, we demonstrated that digital audio-recordings are useful tools for home observation studies that seek to disentangle the complex relationships between early life home environments and child development.

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Key Points Question Is exposure to outdoor air pollution in childhood and adolescence associated with the development of psychopathology at the transition to adulthood? Findings In this cohort study of 2039 UK-born children followed up for 2 decades, early-life exposure to nitrogen oxides was significantly associated with general psychopathology at 18 years of age, representing greater internalizing, externalizing, and thought disorder symptoms. The associations were not attributable to individual or family-level factors or to disadvantageous neighborhood characteristics. Meaning These findings suggest that exposure to nitrogen oxides in early life may be a nonspecific risk factor for the development of psychopathology as young people begin the transition to adulthood.