II. METHODS AND PROCEDURES FOR THE FAMILY TRANSITIONS STUDY

Developmental trajectories of children’s sibling jealousy after the birth of a sibling: Strict parental control, parenting stress and parental depression as pre-birth predictors

This longitudinal research identified the developmental trajectories of sibling jealousy in firstborn children (M age = 49.9 months; 55% boys) among 107 Chinese families from 1 to 12 months after the birth of a younger sibling. Four sibling jealousy trajectories were identified: (a) low-increasing, (b) middle-stable, (c) high-stable, and (d) sharp-increasing. Trajectory group membership varied as a function of pre-birth parental factors including strict parental control (i.e., tiger parenting), parenting stress, and parental depression. These results highlight the benefit of examining the development of sibling jealousy over the transition to siblinghood and the importance to recognize pre-birth parental characteristics, especially paternal factors, as determinants when identifying children who may be at greater risk of adjustment difficulties following the birth of a younger sibling.

Transition to siblinghood causes a substantial and long-lasting increase in urinary cortisol levels in wild bonobos

In animals with slow ontogeny and long-term maternal investment, immatures are likely to experience the birth of a younger sibling before reaching maturity. In these species, the birth of a sibling marks a major event in an offspring’s early life as the older siblings experience a decrease in maternal support. The transition to siblinghood (TTS) is often considered to be stressful for the older offspring, but physiological evidence is lacking. To explore the TTS in wild bonobos, we investigated physiological changes in urinary cortisol (stress response), neopterin (cell-mediated immunity), and total triiodothyronine (T3, metabolic rate), as well as changes in behaviors that reflect the mother–offspring relationship. Following a sibling’s birth, urinary cortisol levels of the older offspring increased fivefold, independent of their age, and remained elevated for 7 months. The cortisol level increase was associated with declining neopterin levels; however, T3 levels and behavioral measures did not change. Our results indicate that the TTS is accompanied by elevated cortisol levels and that this change does not coincide with nutritional weaning and attainment of physical independence. Our results suggest that bonobos and humans experience TTS in similar ways and that this developmental event may have emerged in the last common ancestor.

Development of externalizing symptoms across the toddler period: The critical role of older siblings

We examined child and family risk factors that were associated with the development of individual differences in externalizing problems across the toddler years. Our central hypothesis was that toddlers with more aggressive and disruptive older siblings would be at elevated risk for heightened levels of externalizing behavior. We tested this hypothesis in the context of other theoretically relevant risk factors: toddlers' inhibitory and internalized control, experiences of coercive parental discipline, and gender. Participants were 167 toddlers, their older siblings, and parents in a longitudinal study following younger siblings across the first 3 years of life. Mothers and fathers contributed ratings of externalizing symptoms between 1 and 18 months across the transition to siblinghood for older siblings and 18 to 36 months for younger siblings. Toddlers' inhibitory and internalized control were assessed using behavioral and parent report measures, and parents completed questionnaires concerning their use of coercive discipline. Individual differences in toddler externalizing symptoms were highly stable between 18 and 36 months of age for a sample of second-born children with older siblings, even though there was evidence of significant decline in externalizing symptoms from 18 to 36 months. As predicted, toddlers with elevated levels of externalizing symptoms tended to have older siblings with higher externalizing difficulties across the first 18 months of the younger sibling's life. Thus, these findings highlighted the need for further research on older siblings' contributions to the behavioral development of their toddler-age younger siblings. (PsycINFO Database Record (c) 2020 APA, all rights reserved).