When the children come depends on a few factors. During holiday periods, for example, people are on average more sexually active than during working weeks, and this is reflected in the birth statistics. Newly married couples also tend to produce offspring; This ensures that the number of births increases ten months after popular wedding months. Biologically, the fertility of women and men is influenced by various environmental factors such as temperature and light, and these also have an effect. And finally, it plays a role whether and when people consciously decide to have a child. But another factor has apparently been overlooked so far.
Because the month in which parents have their children is also determined in their cradle. Men and especially women who were born in September, for example, are more likely to have their own children in September. The same applies to other months. This emerges from an analysis by researchers led by the Spanish mathematician and health scientist Adela Recio Alcaide from the University of Alcalá now in the trade journal Population Studies has appeared. The scientists evaluated more than ten million birth records from Spain and France and from various periods in the 1980s, 2000s and 2010s. They found noticeable clusters in both countries and in all periods examined. On average, 4.6 percent more children were born in the mother’s month of birth than would have been expected based on random distribution.
The month of birth is not in the genes
This finding was not to be expected: one Previous studies with a similar focus had not yet been able to identify such an accumulation – However, the authors evaluated fewer than 7,000 birth dates. The team led by Recio Alcaide suspects that the effect can only be seen reliably in large amounts of data.
And the researchers discovered further abnormalities. Fathers also more often had their children in the month in which they were born; Here the accumulation is only half as pronounced as in mothers. Successive siblings, on the other hand, statistically had their birthdays in the same month 12.1 percent more often than would actually be expected. And there was also an increase among parent couples, at least in Spain: fathers and mothers of children together statistically had birthdays in the same month 4.4 percent more often than would be expected with a random distribution. This data was not consistently included in the files from France.
The month of birth is not in the genes. Rather, the researchers explain these abnormalities with social and economic factors. The phenomenon probably stems from the fact that related people have similar social backgrounds. It is known that people are more likely to get involved with partners who are socialized similarly to themselves. In addition, the social background influences the time of year in which people have their children. In Spain, for example, women with higher education are more likely to have their children in spring than in autumn. These children, in turn, are more likely to achieve a higher level of education than others – and if they are female, they are more likely to have their own children in the spring, the researchers argue. Apparently not only the season of birth, but even the month of birth is passed on from generation to generation.
The researchers believe that these results provide a further explanation for why the birth distribution in a society is often very constant. But they would also urge caution. There are various studies that determine the month of birth of a person for example, associate it with risk, later developing asthma, ADHD, heart disease or another illness. In the future, such analyzes would also have to ask about parents. Otherwise the image might be distorted.