The prevalence of overweight/obesity in youth has increased 3-fold, so it is important to understand the mechanisms of why so many adolescents are becoming overweight. Normal weight youth may either better regulate energy intake and expenditure in the face of obesigenic experiences or experience fewer obesigenic pressures from social (parent and peer) influences. The discordant sibling methodology controls for an average of 50 percent of genetic variability and is an ideal design for determining whether normal weight and overweight/obese siblings experience non-shared behavioral responsivity and/or exposures to obesigenic environments. If a putative non-shared experience is associated with discordance in obesity then variability in the experience within siblings will be large relative to between families. This research studies the energy balance behaviors (usual eating, physical activity, sedentary) of discordant siblings and key putative non-shared experiences contributing to sibling differences in weight behaviors and thus their discordance in adiposity. Non-shared eating behaviors are likely primary risk factors for the discordance in sibling adiposity. Thus, one set of risk factors to be studied is differential sibling responsiveness to eating paradigms that test increased sensitivity to external eating cues and reduced sensitivity to internal satiety cues. Non-shared experiences affecting energy balance behaviors occur within the context of social influences. Thus, the second set of risk factors to be tested is social influences on eating and activity that occur within (parent influences on eating and activity, via questionnaires) and outside (peer influences on eating via dietary recalls and on physical activity via ecological momentary analysis) the family. 48 same- sex biologic adolescent sibling pairs (24 pairs of boys and girls, age 13 to 17 years) will be studied with an equal number of pairs (n=12) within each sex consisting of a younger-normal weight (BMI <70th percentile) and an older-overweight/obese (BMI > 85th percentile) adolescent, or of a younger-overweight/obese and an older- normal weight adolescent. Given the unique developmental stage of adolescence, their exposure to obesigenic experiences and responsivity to those experiences are likely different than for children or adults, but there are few experimental studies of the mechanisms of obesity in adolescents. The specific aims are to use intraclass correlations to determine adolescent discordant sibling resemblances for energy balance behaviors (Aims: 1a. usual eating, 1b. physical activity, 1c. sedentary behaviors), non-shared eating experiences (Aims: 2a. increased eating cue responsivity, 2b. reduced internal satiety cue sensitivity), and non-shared social influences on eating and activity within (Aim 3a. parent influences) and outside (Aim 3b. peer influences) of the family. This research will provide important information on the mechanisms of adolescent obesity and why youth are becoming overweight in our obesigenic environment.