Salib & Cortina-Borja ^{Reference Salib and Cortina-Borja1} describe a disproportional excess of people who kill themselves when born in early winter and between late spring and midsummer, and a disproportional deficit when born in late autumn. This month of birth effect can be interpreted in the context of another unexplained characteristic, namely the increasing south–north gradient (i.e. the geographical latitude effect, as shown in different countries).

Optimal maturation of the oocyte in animals and humans has been proposed to occur during the prime time of the seasonally-bound ovulatory seasons and to lead to optimal development of the zygote leading to less morbidity during pregnancy, birth and adulthood. In contrast, non-optimal maturation would occur during the inherent transitional stages leading to errant early neural migration and/or developmental differentiation. ^{Reference Jongbloet and Blandau2} This seasonally-bound month of birth effect is recognised in the presented data, particularly in females (violent and non-violent methods) and males (non-violent methods), and in anencephalia, schizophrenia and related diseases such as eating disorders. ^{Reference Jongbloet, Groenewoud and Roeleveld3} This concept also explains the shorter life expectancy for people born during the first part of the year v. the longer expectancy during the second part, and its mirror image on the southern hemispere. ^{Reference Doblhammer and Vaupel4}

Seasonality of the ovulatory pattern as cause of month of birth effect on suicide can easily be connected with the geographical latitude effect. In fact, the consistent relation between timing of mating seasons in different animals and humans causes stronger transitional stages the further distanced from the equator and, thus, higher frequency of non-optimal maturation of the oocytes. This biological phenomenon explains the mentioned geographical latitude effect on suicidality, schizophrenia and congenital anomalies of the nervous system, diverging between both hemispheres. The highly biased tertiary gender ratio in both suicidality and schizophrenia, and other high-risk factors such as teenage motherhood, multiparity and intrauterine growth retardation, ^{Reference Mittendorfer-Rutz, Rasmussen and Wasserman5} are quite compatible with this concept. This month of birth factor, therefore, does not need to be interpreted in terms of the ‘foetal origins’ hypothesis, nor the ‘maternal–foetal origins’ hypothesis, as suggested by the authors, but rather of the ‘oocyte origins’ hypothesis.