The Intervention Nurses Start Infants Growing on Healthy Trajectories (INSIGHT) study is an ongoing prospective, randomized, controlled trial evaluating the efficacy of a responsive parenting intervention designed to prevent rapid infant weight gain and childhood obesity among 276 first-born infants. Capitalizing on the infrastructure and extensive data collection occurring as part of INSIGHT, the proposed research adds two major pieces by enrolling second born siblings and collecting genetic specimens from both siblings and their parents. Specifically, this translational research will a) prospectively evaluate obesity-related parenting similarities and differences as well as weight-related outcomes between first and second-born siblings, b) explore how genetic differences among siblings that are associated with appetite, temperament, and obesity susceptibility affect parent-child interactions, degree of responsive parenting, and weight status, and c) determine whether INSIGHT study intervention carryover effects occur among families participating in the observation-only second-born child evaluation. National Health and Nutrition Examination Survey data demonstrate the need for early intervention to prevent obesity: 26.7% of children aged 2-5 years are already overweight or obese. These findings are concerning as overweight infants and toddlers are at increased risk of obesity and its co-morbidities later in life. Infancyis an opportune time to begin obesity prevention because it is a critical period of rapid growth and developmental plasticity with long-lasting metabolic and behavioral consequences. The promising pilot study results that informed INSIGHT demonstrated that an obesity prevention program focused on messages of responsive feeding, division of feeding responsibility, and healthy dietary choices can improve weight-related outcomes for first-born infants. Interestingly, data from diverse cultures have shown that first-born children have a higher risk for obesity despite the fact that pregnancy related risk factors for childhood obesity (high pre-pregnancy body mass index, high gestational weight gain, occurrence of gestational diabetes, high birth weight) are more common during pregnancies with second-born children. This suggests that postnatal factors related to parenting are the cause for the disparity between first and second born children's obesity risk. Using the conceptual framework of responsive parenting, it can be hypothesized that mothers have improved responsiveness and more appropriate caretaking behaviors due to the experience gained with their first child. This hypothesis will be tested by comparing the >100 second-born siblings that can be expected to be born during the proposed funding period with their older siblings with the additional goal to explore how differences in genetic susceptibility to obesity and observed differences in appetite and temperament moderate associations between responsive parenting and weight status at age one year.