Introduction: Although the seasonality of human births has been known for nearly 175 years, it has not been adequately explained: Societal and environmental reasons have often been cited as the reasons for seasonality. Since increased light is known to initiate reproduction in many animals, photoperiod, light intensity and melatonin are likely variables related to this phenomenon. This retrospective study demonstrates a logical connection between these factors and human conceptions. Methodology: Seasonality data were extracted from previous studies. Daily cloud cover amounts were utilized as indices for environmental light intensity. Variables were evaluated by Pearsonian r. Discussion: Results were evaluated and discussed in a series of 11 sub-studies involving environmental light intensity and the seasonality of human births. Conclusion: Human birth seasonality may be primarily influenced by environmental light intensity with photoperiod in a secondary role. This result is supported by significant r-values for US and Europe, a logical link between US, Canadian, Indian, European, and Hong Kong conceptions as well as the concurrent transition of conceptions and cloud cover. Israeli conceptions, however, contradict US and European patterns.
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