Does your family have a “birthday season?” That’s no fluke, scientists suggest.
Women are more likely than not to give birth to children in the same month as their own birthday, a new study published in the peer-reviewed journal Population Studies reveals — with researchers claiming to have figured out why.
A team from Spain and the United States analyzed official data on more than 10 million births in Spain and France over a period of years.
It has been known that births in particular countries follow certain patterns that align with cultural and psychological trends — a phenomenon known as birth seasonality.
But when the research team categorized the birth data into groups based on the birth month of the mothers, they saw a spike in correlation.
“Although research on birth seasonality has shown that women’s season of birth somehow influences that of their children, the mechanisms of these relationships have not [yet] been identified,” wrote the study authors.
“Not only did we find that women’s own season of birth influenced the seasonal patterns in the births of their children, but we have clarified how this influence works: women are more likely to have children in the same month as their own birth (with a 4.6 percent excess of births where birth months coincided, on average across periods and countries studied),” they continued.
For example, in Spain women with a higher education are more likely to plan to give birth in the spring than a woman without. Being raised by a woman with a higher education increases the chances that a child will themselves seek a degree and therefore be more likely to give birth in the spring.
This pattern then trickles down to siblings, fathers and parents.
Siblings who were closest in age where found to be 12.1 percent more likely to share a birth month and children and their fathers were found to be 2 percent more likely to be born in the same month.
Even parents were found to have a higher chance of having similar birthdays.
Couples who had a child were found to be 4.4 percent more likely to celebrate birthdays in the same month.
“The excess of children with a father and mother born in the same month seems to be due to social or behavioral causes prior to conception that relate to the choice of a partner born in the same month, as we have observed this excess with marriage statistics, with spouses being more likely to mate with someone from the same month,” lead researcher Dr. Adela Recio Alcaide explained in a statement.
The researchers noted that this does not come as a total surprise as people tend to choose partners with similar socio-demographic characteristics, which correlates with different seasonal birth patterns.
People with similar backgrounds seem to have children around the same time of the year.
“Rather than being a random variable, birth season seems to be related to family characteristics, which should be controlled for when assessing birth-month effects on subsequent outcome,” the study concluded.
This study better explains situations like when parents in Ohio and Alabama made headlines for welcoming babies on their shared birthdays.