Many of these theories of parenting styles are almost entirely based on evidence from high income countries, especially the USA. However, there are many fundamental differences in child development between high and low income countries, due to differences in parenting styles and practices. For instance, in sub-Saharan Africa children are likely to have more than one main care giver, to acquire language in a bilingual environment, and to play in mixed aged peer groups. However, when comparing African American caregiving among lower, middle, and upper socioeconomic families, the number of non-parental caregivers decreases as economic resources increase. In addition, international studies have found Chinese parents to be more concerned with impulse control, which may explain the greater use of authoritarian style as compared to U.S. parents. Thus, social values and norms within a culture influence the choice of parenting style that will help the child conform to cultural expectations.