Mothers and children have birthdays in the same month more often than you might think

Celebrate birthday in the same month as your mother? You're not the only ones. A study of millions of families shows that the phenomenon is more common than expected. Not only that: brothers also tend to share the month of birth, as do sons and fathers. Previous research has shown that women's season of birth influences that of their children. The new research, published in the journal Population Studies
, goes further, showing that women are more likely to have children in the same month they are born. The researchers, Spanish and American, analyzed the data of over 10 million people: all births in Spain from 1980 to 1983 and from 2016 to 2019 and in France from 2000 to 2003 and from 2010 to 2013. They then compared the month of birth of the child with those of the parents and any sibling closest in age . Births tend to follow a pattern, with more babies being born at certain times of the year. A phenomenon known as «seasonality of births».

Same month for mothers and children

But when researchers formed groups based on the mothers' birth month, the data did not follow the expected pattern. They observed a peak in births in January among mothers born in January, a peak in February among mothers born in February, and so on. Overall, there were 4.6% more births in which mother and child shared the same month of birth than expected. The phenomenon occurred in both countries and for all four time periods studied. The same goes for brothers (12.1% more births in which adjacent siblings had the same birth month) and for children with the same birth month as their father (2% more). A second analysis of all births in Spain from 1980 to 2019 and in France from 2000 to 2019 confirmed the result.

Socio-demographic characteristics

The phenomenon probably has its roots in sharing of socio-demographic characteristics between relatives: People with similar backgrounds mate and are more likely to give birth at certain times of the year, say the study's two lead authors, Adela Recio Alcaide, of the University of Alcalá in Spain, and Luisa Borrell, of the City University of New York. In Spain, for example, a woman with a high level of education is more likely to give birth in spring compared to a less educated woman. A possible daughter, in addition to being more likely to be born in the spring, could have a better chance of having a higher education. And again, if the daughter has children, they will be more likely to be born in the spring. In other words, the girl will be more likely to have children in the same season in which she was born because she has maintained the socio-demographic characteristics of the family which make her more likely to give birth in a certain period – spring – and, consequently, the birth season is transmitted between generations.

Social and biological factors

«What factors increase the probability that members of a family will be born in the same season? The explanations appear to be both social and biological – says Adela Recio Alcaide -. The high number of children with father and mother born in the same month appears to be due to social or behavioral causes prior to conception, which concern choosing a partner born in the same month, as we have observed in marriage statistics.” «This is not surprising, considering that couple unions tend to be formed by people with similar socio-demographic characteristics – adds the co-author, Luisa Borrell -. Furthermore biological factors that influence the seasonality of births, such as the duration of daylight, temperature, humidity and availability of food, also depend on socio-demographic characteristics, since different social groups are exposed to these factors to different extents.”

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