The arrival of a newborn brings great joy to a mother's life, as happiness appears to be the predominant mood experienced by most mothers in early postpartum (Kendell et al., 1981). Paradoxically, early postpartum is also a tumultuous period when most new mothers encounter various degrees of mental problems, including increased anxiety, depression, amnesia, and psychotic symptoms. The actual estimates may differ in different studies, but the overall picture is discouraging. Approximately 50–85 % of women experience postpartum blues (i.e., mild and transient mood disturbances, such as temporary mood swing, anxiety, insomnia, poor appetite, and irritability) (Miller, 2002; O'Hara et al., 1991), 10 to 20 % of women develop postpartum depression (i.e., a major depressive episode with peripartum onset, with five or more depressive symptoms during the same two-week period, such as persistent depressed mood and sadness, diminished pleasure in nearly all activities, changes in sleep patterns, etc.)(Josefsson et al., 2001), and 0.1 % develop more severe postpartum psychosis (i.e., paranoid, grandiose, or bizarre delusions, mood swings, confused thinking, and grossly disorganized behavior, etc.) (Brockington, 2004; Kendell et al., 1987). Even in non-clinical mother samples, self-reported maternal depression is common and associated with all patterns of parenting (i.e., Supportive Parenting, Self-Enjoyment Parenting, Overwhelmed Parenting, and Affectionate Parenting) (Unternaehrer et al., 2019). One might wonder why early postpartum becomes one of the most vulnerable periods of a woman's life to experience mood disturbances. In principle, although it can be said that the postpartum mood problems stem from a host of changes in the nervous and endocrine systems in response to stress associated with pregnancy, parturition and maternal care (Sacher et al., 2020; Schiller et al., 2016, Schiller et al., 2015), the specific mechanisms remain poorly understood, partially due to the lack of deeper understanding of (a) the genetic, neurobiological, behavioral mechanisms of postpartum mood disturbances, and (b) the evolutionary significance of such changes.

In this article, I propose a proximate-ultimate framework and argue that increased incidence of mental disturbances in early postpartum could be understood from the evolutionary (its ultimate potential adaptive function) and neurobiological (its proximate mechanism) perspectives (Fig. 1). The central idea is that increased incidence of mental disturbances in early postpartum is the natural consequence of neuroadaptive changes in the maternal brain that are selected by the evolutionary pressure to enhance inclusive fitness of the mother. At the neurobiological level, pregnancy- and postpartum-induced alterations in the maternal brain are necessary to prepare a new mother for adequately mothering both physiologically (e.g., increasing prolactin and oxytocin production, etc.) and psychologically (e.g., enhanced cognitive processing of infant-related cues and maternal responsiveness, increased sensitivity to threatening and fearful stimuli, etc.). However, the same neuroadaptive changes also place a new mother in a chronically stressed state, thus increasing her vulnerability to experience mood problems. Evolutionarily, postpartum mild mood disturbances have the evolved functions of conserving resources and soliciting more social support to help raise a child when the unfavorable circumstances demand them (Hagen and Barrett, 2007). When the mother faces the difficulty in successfully rearing an infant due to lack of adequate time, energy, and resources or infant health problems, the psychic pain (also termed “emotional or psychological pain”, referring to the deep emotional distress arising from non-physical causes, such as social loss, threat to caregiving, perceived resource shortfall) signals to herself, family and community that costs of child rearing outweigh the benefits, which could lead to a reduction in maternal investment and an increase in social support. Evolutionary explanations would consider that mild mood disturbances as either an unfortunate “by-product” or a potentially adaptive behavioral strategy to conserve maternal resources; while severe clinical postpartum depression is considered an extreme form of a ‘defection strategy’ that solicits social investment or reduces maternal commitment when offspring survival is unlikely. Therefore, instead of viewing postpartum mood disturbances (not clinical depression) as abnormal psychological problems, this alternative theoretical framework casts postpartum mood disturbances as a normal and healthy function required by motherhood, while clinical postpartum mental disorders are only an extreme and unhealthy form of this function. This explains well why up to 85 % of women, a vast majority, experience “postpartum blues” (Henshaw, 2003) and about 18–20 % of them experience a severe enough mood change that merits a diagnosis of major or minor depression (Pawluski et al., 2019).

The article starts by briefly reviewing notable functional and structural changes in the maternal brain, focusing on changes occurring in dopamine and serotonin signaling in the mesolimbic and mesocortical systems, neuroplastic changes in the medial preoptic area (MPOA), hippocampus and the hypothalamic-pituitary-adrenal (HPA) stress response axis, as they are most intimately associated with mental disturbances. Next, I discuss how such normative changes are needed to prepare a female to become a competent mother physiologically (e.g., increasing prolactin and oxytocin production) and behaviorally, while in the meantime increasing the vulnerability of developing postpartum mood disturbances (e.g., anxiety and depression). After this, I argue that postpartum mental disorders tend to develop in certain individuals with increased genetic, environmental, and social risks, which may increase their vulnerability to experience extreme brain changes throughout the reproductive process (from pregnancy to parturition and postpartum) (the proximate mechanisms). On the other hand, the ultimate evolutionary perspective regards postpartum mental disorders as an evolved consequence when a mother attempts to successfully raise a newborn infant under adverse circumstances (Hagen, 2011). Postpartum mental disorders represent the extreme form of these behavioral strategies. Finally, I discuss the ethical and clinical implications of the proximate-ultimate framework and how it is useful for future research on behavioral and neurobiological processes involved in postpartum mental disorders, animal modeling and treatments for postpartum mental disorders. I should emphasize that this framework is used to generate testable hypotheses about both the immediate neurobiological correlates and the potential evolutionary adaptive function of postpartum mood disturbances, and that much of the evidence remains correlational. Future work should move even more beyond correlation studies and further increase our knowledge about causal pathways, thereby refining both clinical interventions and our understanding of the neurobiological and adaptive significance of postpartum mood disturbances and mental disorders.