The Rhythm of Conceptions. Seasonality of Births in Taiwan and the Netherlands, 1800–1940

The seasonality of births provides us with insights into the economic, cultural, and biological effects on the private lives of our predecessors in time. This comparison between two culturally very different societies results in the conclusion that there is an amazing geographic and historical stability in the patterns of seasonality in conceptions, both in Taiwan and in the Netherlands. Economic and cultural variables only have a minor influence. The fact that a biological variable like temperature is so dominant in the Taiwanese case suggests that future studies may benefit from focusing more on the biological determinants of human behaviour.

The seasonality of 65 million births in Brazil – 1997-2018

The seasonality of births has been a universal phenomenon worthy of attention since the XVIII century. Exhibiting a variety of forms, many times showing similar patterns in dissimilar societies, and dissimilar patterns in societies alike, there has never been a plausible unifying explanation for their behavior. Finding reasonable causes to understand birth seasonality worldwide is still a challenge. This paper intends to contribute to the discussion on birth seasonality in light of the Brazilian reality, analyzing data about 65,764,313 births from 1997 to 2018 in Brazil. This study investigates monthly birth seasonality, according to social-demographic characteristics of mothers (region, age, race/color, schooling, fertility, and marital status), adjusted to consider the same length for each month of the year. Brazil has two clear periods of birth peaks: the main one is observed in March, April, and May, due to winter conception. A second important peak is observed in September, related to conceptions in December. A clear valley is observed in November and December, due to summertime conceptions. In spite of the continental dimensions of Brazil, its climatic diversity and broad social differences, the Brazilian population presents the same temporal birth distribution, according to geographic spaces and population segments, due mainly to conceptions happening in the Brazilian winter season.

Analysing the seasonality of births in mainland China

The seasonality of human births varies in different countries and regions. Explanations for this variation have been divided into biological and behavioural factors. This paper documents birth seasonality in mainland China using data for a large sample from Chinaʼs Fifth National Population Census (FNPC) conducted in 2000. The main method used was the decomposition of monthly time series birth data into annual, seasonal and random trends. The results show large seasonal birth fluctuations, with a salient peak of October births. The study hypothesis is that this seasonal birth pattern is partially due to a home-bound wave of movement of people after the annual Spring Festival. Subsequent analysis of the calculated de-trended monthly births provided supportive evidence for this hypothesis. Further in-depth analysis showed that the magnitude of births varied with location and family characteristics. This result should inform researchers in the field of economics, where seasonality of births has been previously regarded as exogenous.

Seasonality of births in horizontal strabismus: comparison with birth seasonality in schizophrenia and other disease conditions

Recent work has implicated one type of horizontal strabismus (exotropia) as a risk factor for schizophrenia. This new insight raises questions about a potential common developmental origin of the two diseases. Seasonality of births is well established for schizophrenia. Seasonal factors such as light exposure affect eye growth and can cause vision abnormalities, but little is known about seasonality of births in strabismus. We examined birth seasonality in people with horizontal strabismus in a retrospective study in Washoe County, Nevada, and re-examined similar previously obtained data from Osaka, Japan. We then compared seasonal patterns of births between strabismus, refractive error, schizophrenia and congenital toxoplasmosis. Patients with esotropia had a significant seasonality of births, with a deficit in March, then increasing to an excess in September, while patients with exotropia had a distinctly different pattern, with an excess of births in July, gradually decreasing to a deficit in November. These seasonalities were statistically significant with either χ2 or Kolmogorov–Smirnov-type statistics. The birth seasonality of esotropia resembled that for hyperopia, with an increase in amplitude, while the seasonality for myopia involved a phase-shift. There was no correlation between seasonality of births between strabismus and congenital toxoplasmosis. The pattern of an excess of summer births for people with exotropia was remarkably similar to the well-established birth seasonality of one schizophrenia subtype, the deficit syndrome, but not schizophrenia as a whole. This suggests a testable hypothesis: that exotropia may be a risk factor primarily for the deficit type of schizophrenia.