Parental investment observations

Focal follows were conducted on 35 children [17 infants aged 0–1.9 years (average 0.87 ± 0.55) and 18 toddlers aged 2–5.9 years (average 3.9 ± 1.2)] for 9 h per child to assess parental investment. These intensive observations were conducted using focal sampling techniques detailed by Meehan (2005) and further details and descriptive statistics can be found in the Supporting Information (see Table S1). The following parental activities were documented and classified into either ‘high’ or ‘low’ investment: high – carrying, feeding, grooming, medical attention, teaching, playing; low – talking to, watching, being in proximity. For the 35 children (13 = female) observed this produced 124,701 dyadic child–carer data points, of which 24,949 were between mothers (n = 23) or fathers (n = 22).