Organized activity participation is associated with a wide array of positive developmental outcomes. Latinos are one of the largest and fastest growing ethnic groups in the U.S., yet are less likely to participate in organized activities than their peers. Theoretically, the alignment or fit between adolescents' and their activities' characteristics is critical to support youths' use and engagement in organized activities. Using qualitative data in Study 1, I examined parents' and adolescents' perspectives and experiences related to several indicators of ethnicity and culture in their activities. Results suggested that alignment on Spanish-language use was critical for participation. However, some Latino families did not prefer aspects of ethnicity and culture in their activities because adolescents learned about their culture with family or because adolescents wanted to fit in with their majority White peers. Study 2 tested quantitatively whether features of ethnicity and culture in the activity mattered for Latino adolescents' experiences during activities. Ethnic and cultural features in activities, particularly respect for one's ethnicity and culture, fostered positive experiences during activities. Unexpectedly, some ethnic and cultural features were detrimental, such that overt teaching about ethnicity and culture was related to negative feelings during the activity. There was little evidence that the relation between ethnic and cultural features in activities and concurrent experiences varied by Latino cultural orientation. Integrating the findings across these two studies, there was mixed evidence for the traditional theoretical notions that optimal development occurs in environments that fit with individual's characteristics. Complementary fit was optimal when adolescents' needs were considered across the many contexts in which their lives are embedded, including their families and neighborhoods. I recommend that practitioners should take care in learning about the specific families and youth that their activity serves to best understand how to meet their needs. Some aspects of culture, such as Spanish-language use may be critical for participation; other aspects may require special attention from activity leaders, such as teaching about ethnicity and culture. This dissertation is an important step in understanding how to best design activities that promote the recruitment and retention of Latino youth in organized activities.

Growing concern about obesity prevalence among youth has prompted the examination of socio-environmental influences that shape the development of eating and activity behaviors believed to regulate weight. Given the presumed significance of close friendships during adolescence, the present investigation assessed longitudinal relations between friends' physical activity, sedentary activity, and healthy eating behaviors and explored whether friends' obesity-promoting behaviors are linked to heightened obesity risk among adolescents. This prospective study utilized two Waves of data from 862 reciprocal and 1908 nonreciprocal same-sex friend dyads participating in the National Longitudinal Study of Adolescent Health. To account for nonindependence tied to membership in a particular friendship dyad, multi-level models were estimated for indistinguishable (i.e., reciprocal) and distinguishable (i.e., nonreciprocal) friend pairs using the Actor Partner Interdependence Model. Adolescents' self-reported physical activity and healthy eating were significantly associated with their own and their friends' physical activity and healthy eating one year later; the strength of socialization across friend dyads did not vary with the frequency of interaction between friends or the stability of friendships over time. Limited support was found for a cumulative risk model of obesity-promoting behaviors as a predictor of increased obesity risk; heightened risk for weight gain was found only for adolescents whose reciprocal same-sex friends reported a higher number of obesity-promoting eating and activity behaviors. Overall, study findings highlight the role of close friends for adolescents' obesity risk and obesity-related behaviors. Stronger evidence of socialization resulted for adolescents that perceived their friends to be salient social models, as reflected by their acknowledgement of an existing friendship with such peers (i.e., reciprocal friends and nominators within nonreciprocal friend dyads).