This study aimed to investigate whether paranoid experiences and levels of self-esteem fluctuate over the menstrual cycle and whether levels of self-esteem are lower when perceived persecution is felt to be deserved.Measures of anxiety, depression, persecution, deservedness and self-esteem were completed on-line by 278 women over their menstrual cycle. Responses were compared at the paramenstrual (three days before and after menses onset) and mid-cycle phase.At the paramenstrual phase persecution, negative self-esteem, anxiety and depression were higher and positive self-esteem was lower than at mid-cycle. A greater proportion of women experienced persecution as deserved at the paramenstrual phase. This was associated with higher depression and negative self-esteem scores.Increased levels of deservedness significantly strengthened the relationship between persecution and negative, but not positive, self-esteem.These findings suggest that the paramenstrual phase is a time of vulnerability to increased paranoid experiences, an increased likelihood that feelings of persecution will feel deserved and lowered self-esteem. The findings support the view that interpersonal sensitivities may be key to menstrual cycle symptoms and have an impact on relationships. Further, the study illustrated that ideas developed for psychosis could make a valuable contribution to understanding and managing this aspect of menstruation-related distress. KeywordsParanoia; Self-esteem; Menstruation; Premenstrual; Deservedness PARANOIA, SELF-ESTEEM AND THE MENSTRUAL CYCLE 3 Relationships between Paranoia, Self-Esteem and the Menstrual CycleThe menstrual cycle is characterised by hormonal fluctuations which accompany the processes of ovulation and menses in female human reproduction. The phases of the cycle have been linked to changes in both physical and psychological phenomena, the presentation and intensity of which varies considerably from woman to woman (Gonda et al. 2008). Typically, negative effects occur in the few days preceding and during menstruation. Increased interpersonal oversensitivity is commonly reported within the PMS literature (Pinkerton et al. 2010). Such sensitivities are likely to have major impacts upon relationships, and other commonly reported symptoms such as anxiety and depression. However, these sensitivities are rarely specifically investigated.! Paranoia and low self-esteem both reflect oversensitivity to interactions with others.Links between menstrual cycle phase and psychosis have long been postulated (see Brockington 2005 for a review). The literature also includes examples of exacerbation of bipolar disorder (Hendrick et al. 1996), exacerbation of schizophrenic symptoms (Harris 1997;Levitte 1997;Riecher-Rossler 2002;Hsiao et al. 2004;Bergemann et al. 2007) and the presentation of psychotic symptoms in major depression during the premenstrual and menstrual phases. Due to the physiological changes underpinning the menstrual cycle, research has focused upon physiological reasons for these phenomena, s...