Abbott at the Wisconsin National Primate Research Center found that among many primate species, less egalitarian social structures correlated with higher levels of stress hormones among socially subordinate individuals. Research by Robert Sapolsky of Stanford University provides similar findings. Geographic Inequality There is well-documented variation in health outcomes by geographic variation in many countries around the globe. This includes the U.S., with the addition of health care utilization & costs geographic variation, down to the level of Hospital Referral Regions (defined as a regional health care market, which may cross state boundaries, of which there are 306 in the U.S.). However, data availability of health indicators for sub-national geographies is limited in both number, data source and geographic scale. Across the 38 OECD countries, region, or equivalent large subnational entities, is the predominant geographic level for both mortality and morbidity indicators. Health indicator availability at smaller geographies was sparse, and varied considerably by geographic definition, health indicator, age range of population and years available. In all cases, geographic boundaries used only administrative definitions. There is ongoing debate as to the relative contributions of race, gender, poverty, education level and place to these variations. The Office of Epidemiology of the Maternal and Child Health Bureau recommends using an analytic approach (Fixed Effects or hybrid Fixed Effects) to research on health disparities to reduce the confounding effects of neighborhood (geographic) variables on the outcomes. Critiques Population health has been subject to ongoing critiques and its assumptions Subfields Family planning Family planning programs (including contraceptives, sexuality education, and promotion of safe sex) play a major role in population health. Family planning is one of the most highly cost-effective interventions in medicine. Family planning saves lives and money by reducing unintended pregnancy and the transmission of sexually transmitted infections. For example, the United States Agency for International Development lists as benefits of its international family planning program: "Protecting the health of women by reducing high-risk pregnancies" "Protecting the health of children by allowing sufficient time between pregnancies" "Fighting HIV/AIDS through providing information, counseling, and access to male and female condoms" "Reducing abortions" "Supporting women's rights and opportunities for education, employment, and full participation in society" "Protecting the environment by stabilizing population growth" Mental health There are three main kinds of population-based approaches to mental health: health care system interventions; public health practice interventions; and social, economic, and environmental policy interventions. Health care system interventions