Preclinical studies demonstrate oxytocin's crucial role in bonding. Oxytocin receptors are richer in the monogamous prairie voles than in mountain voles, which do not form stable monogamous relationships (
+101). Oxytocin receptor levels in the central nucleus of the amygdala are significantly higher in rats exhibiting maternal behaviors, including licking, grooming, and nursing of pups, than in those with low levels of these behaviors (
+91). In humans, oxytocin is a critical mediator of social connectedness and bonding in the context of romantic attachments, enduring monogamous partnerships, and maternal behavior. For example, in romantically unattached young adults, oxytocin is associated with self-report measures of bonding to parents and inversely related to psychological distress (
+92). Since receptors for oxytocin are more abundant in the reward centers of prairie voles, which form enduring monogamous relationships, it has been hypothesized that oxytocin plays a similar role in adult human romantic attachments (
+102,
+103). While studies of oxytocin and monogamous behavior in humans have not been conducted to date, one study found that in cohabiting couples, greater partner support is associated with higher plasma oxytocin in both men and women before and after a period of warm partner contact (
+93). In humans, oxytocin concentrations during early pregnancy and during the immediate postpartum period are associated with maternal bonding behaviors, including gaze, vocalizations, positive affect, and affectionate touch, as well as thoughts related to attachment and frequent monitoring of the infant (
+104). Furthermore, oxytocin mediates attachment behavior over the course of development, with lower urinary concentrations of oxytocin found in maltreated children (
+105) and in adult males with a history of early separation (
+106), as well as in the CSF of adult females with a history of childhood abuse (
+107). However, these few studies are based on retrospective data, such as history of trauma, and the validity of one of the studies (
+105) has been questioned (
+108). Therefore, additional studies, particularly with prospective designs, are needed to establish the role of oxytocin in a developmental context.