Introduction: The evolutionary and biocultural causes and consequences of rising cesarean birth rates

Cesarean section (the surgical removal of a neonate from the mother's womb through an incision in her abdominal and uterine walls) became a relatively common surgical procedure in many developed countries in the 20th century, with about 1.3 million performed each year in the United States (Martin, Hamilton, Osterman, Driscoll, & Drake, 2018). Cesarean birth rates are also rising in most parts of the world (Betrán et al., 2016). While they can be life-saving procedures for many mothers and their offspring, cesarean sections can also have negative consequences for both (Rosenberg & Trevathan, 2018). For this reason, the World Health Organization recommends an optimal cesarean section target rate of about 10-15% (WHO, 2015). For mothers, cesarean births are often associated with decreased fertility, poor breastfeeding outcomes, and increased morbidity (Keag, Norman, & Stock, 2018; Norberg & Pantano, 2016; Prior et al., 2012). For offspring, they are linked to increased postnatal morbidity and mortality and child and adulthood obesity, asthma, and allergic disease (Bager, Wohlfahrt, & Westergaard, 2008; Huang et al., 2015; Kuhle, Tong, & Woolcott, 2015; Kuhle & Woolcott, 2017; Villar et al., 2006). Despite widespread epidemiologic studies of the causes and consequences rising cesarean delivery rates, relatively few focus on low income, epidemiologically transitioning populations that experience the dual-burden of malnutrition (coexistent undernutrition and overweight/obesity) (Varela-Silva et al., 2012), often in conjunction with rising cesarean delivery rates (but see Veile & Kramer, 2016 and Wells, Wibaek, & Poullas, 2018). As such, most existing epidemiologic studies do not consider the range of cesareanassociated outcomes that are likely to be found across human populations experiencing vastly different biocultural environments. Furthermore, few studies invoke evolutionary theoretical perspectives, which are relevant to the problem of rising cesarean delivery rates. For this reason, we were motivated to organize an invited symposium session at the American Association of Physical Anthropologists meeting in Austin, Texas, in 2018. Our aim was to demonstrate that evolutionary, biocultural and cross-cultural perspectives are useful in understanding the causes and consequences of globally rising cesarean birth rates. The symposium papers built on a long tradition of birthfocused research in biological anthropology and related fields (Abrams & Rutherford, 2011; Berry, 2013; Cosminsky, 2016; Davis-Floyd & Cheyney, 2019; DeSilva, 2011; Dunsworth & Eccleston, 2015; Rosenberg & Trevathan, 1995 and 2002; Stone, 2016; Trevathan, 1987; Veile & Kramer, 2018; Wells, 2017; Wells, DeSilva, & Stock, 2012). The symposium drew together cultural and biological anthropologists, biologists, ecologists, and health care practitioners, all conducting research rooted in evolutionary theoretical perspectives. An evolutionary perspective into human childbirth involves comparing it with birth in our closest living primate relatives, and an examination of the human fossil record to determine when the changes affecting the birth process took place in our evolutionary history. This includes changes in our ancestors' pelvic morphology that occurred with the evolution of bipedalism (4-5 million years ago) and subsequent changes in maternalneonatal size relationships, particularly with respect to size of the neonatal head and brain. The legacy of these aspects of our biology are that: (a) humans birth large babies with large crania; (b) a bipedal pelvis makes it likely that infants will rotate as they DOI: 10.1002/ajhb.23230