The sex ratio at birth (2).

This second of two Best Practise Guidelines will continue to explore facets pertaining to factors that influence the sex ratio at birth (M/F). The papers include a quantitative study in the United States, investigating the effects of Thanksgiving Day, Christmas Day and Valentine's Day on M/F (which rises). A second paper is devoted to the factor that most influences M/F worldwide: male offspring preference. The third paper focusses on radiation which appears to be the only toxin/stressor that increases M/F by culling female in excess of male foetuses. The paper investigates the effects of the Mainz research nuclear reactor in Germany. Yet another paper reviews seasonality of M/F. Two more theoretical papers discuss the significance of low M/F of offspring and of siblings of probands with systemic lupus erythematosus or rheumatoid arthritis, implicating a common role for testosterone. In yet another paper, high levels of maternal first-trimester intrauterine testosterone are also implicated in male sex, autism, gender dysphoria and non-right-handedness. Finally, a paper reviews the effects of toxoplasmosis on M/F since may be one of the most important environmental factors on this ratio. It is hoped that this second set of papers will further provide stimulation and provocation for research on this intriguing topic.

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