A hypothesis based on the correlations found between month or season of birth and psychological tendencies

 There are some interesting studies that have found positive, though relatively modest, correlations between the month or season of birth and certain psychological tendencies: those born in the winter, especially late winter and early spring, at least in the Northern Hemisphere, are at greater risk of developing mental disorders, especially schizophrenia. The most common explanation is that pregnant women who conceive during this period are more exposed to viral infections, and that, if infected, their children could experience long-term effects later in life, including mental disorders. Another possible factor is that cold weather induces a vitamin D deficiency, which could complicate the normal development of pregnancies. A correlation between prematurity (or low birth weight) and schizophrenia (and psychopathology in general) has also been noted. Perhaps normal brain development that is abruptly interrupted by external factors could also be a long-term trigger for the expression of a mental disorder. Or it may be that genetic, and therefore hereditary, factors explain a good portion of psychiatric disorders. First, a couple whose two partners have the same disorder are at greater risk of passing it on to their natural descendants than a couple with only one carrier, and especially if there is no explicit sign of mental disorder between a pair of humans biologically capable of procreating or who have already done so. It is precisely this last possibility that seems most likely to me, perhaps because it is the least extraordinary. First, because assortative relationships, in which there is a mutual and active interest in mating based on the perception of phenotypic similarities—usually a combination of biotype, personality, and social circumstances—increase the likelihood that individuals of opposite sexes with the same health risks, including psychiatric ones, will enter into a relationship that will result in natural offspring. Second, the proposed mechanism to explain the correlation between being born in winter (cold, in the Northern Hemisphere) and developing certain mental disorders requires empirical proof, and to my knowledge, I don't know if it exists in humans. Third, most individuals born in winter do not become schizophrenic, although it would be interesting to see if a higher proportion of individuals born in this period (especially in temperate climates) develop other psychiatric disorders, or at least a greater expression of associated personality and cognitive traits. Fourth, human individuals with different personality profiles (which are predominantly stable patterns of behavior) tend to exhibit different habits, likely also in mating. Even more directly related to this correlation between month or season of birth and psychological tendencies, one of the behavioral or personality differences that may explain it is climate preference. Those who prefer cold weather (or who don't exhibit this type of preference) feel more motivated during this time of year, even to have sex/procreate (or who don't exhibit any differences in motivation throughout the year), while the opposite pattern occurs for those who prefer warm weather. For example, more emotionally unstable individuals tend to feel more "depressed" or less motivated in cold weather, especially in environments with harsh winters, including for sexual intercourse. The opposite pattern occurs during the warmer months. And since human pregnancy lasts, on average, nine months, a pregnancy that began in summer or spring will complete its cycle in fall or winter. So perhaps the most important element in this situation is the influence of personality type, which expresses various biological aspects: hormonal, mental, genetic... and not necessarily the influence of the environment acting directly on the human organism, even if it influences it indirectly, through behavior.

Okay, but then how can we explain this pattern in tropical and equatorial climates, if these climates are characterized by less significant temperature variations?

Even if there is less temperature variation in these regions, the underlying factor is the same: there are differences in mental traits among human beings that result in differences in habits. In addition, these mental traits are not just abstractions, as they also express more organic and variably hereditary aspects of the body. So, even if this pattern isn't confirmed in all regions, its possible absence of correlative universality does not invalidate its evidence of specific correlation. But perhaps this correlation is nothing more than "genetic confounding," in which a correlation that suggests environmental causality for a given behavioral tendency actually has as its underlying, primary, and most influential factors: biological characteristics, such as personality traits, which are also transmitted from parents to children. And, before concluding this text, I wonder if the risk of premature birth (or even miscarriage) could also be an intrinsic and variably heritable predisposition, not just a completely random incident—that is, one that doesn't present the same probability of occurring in any pregnant woman. I know there are studies showing differences in this risk between ethnic or racial groups (during a quick search on this topic, I found studies showing that, in fact, there is indirect evidence of a genetic influence on this risk)...

Sources:

Season of birth in schizophrenia: a maternal-fetal chronobiological hypothesis

https://pubmed.ncbi.nlm.nih.gov/21382670/;

https://www.uol.com.br/vivabem/noticias/redacao/2017/10/06/maioria-dos-casos-de-esquizofrenia-e-genetico.htm

Heritability of Schizophrenia and Schizophrenia Spectrum Based on the Nationwide Danish Twin Register

https://www.biologicalpsychiatryjournal.com/article/S0006-3223(17)31905-4/abstract;

Family history is a predictor of current preterm birth

https://www.ajogmfm.org/article/S2589-9333(20)30270-6/pdf;

The Role of Genetics in Preterm Birth

https://link.springer.com/article/10.1007/s43032-023-01287-9;

Levels of the stress hormone cortisol are higher in women who give birth in the autumn and winter than those who give birth in the spring or summer, finds a new study by researchers at Cardiff University.

https://www.cardiff.ac.uk/news/view/1547555-new-research-could-explain-why-babies-born-during-winter-are-at-higher-risk-of-developing-mental-health-disorders