“…In line with Grey et al (2013), Glynn et al (2007), and Hinde et al (2014), we controlled for maternal educational level and parity. Though infants with a birth weight lower than 2,500 g were excluded, birth weight was also included as a control variable because it can reflect infant vulnerability and predict both infant crying behavior (Milidou, Søndergaard, Jensen, Olsen, & Henriksen, 2014) and maternal stress (Halpern, Brand, & Malone, 2001). Furthermore, taking cortisol circadian variations into account and in line with previous research (Glynn et al, 2007;Grey et al, 2013;Nolvi et al, 2017), we controlled for collection time of breast milk sampling.…”