Positive youth development programs typically recognize contextual variability in youths' experience and in what is considered "healthy" or "optimal" development for youth in different settings or cultures.[9] This cultural sensitivity reflects the influence of Bronfenbrenner's ecological systems theory. The influence of ecological systems theory is also seen on the emphasis many youth development programs place on the interrelationship of different social contexts through which the development person moves (e.g. family, peers, school, work, and leisure).