Generally we think that the probability that a child will be born male or female is the same: the classic example of the coin, to understand each other. In fact, the data tell another story: every hundred girls, about 105 children are born. It means that the probability is not 50 and 50, but slightly unbalanced in favor of male biological sex (about 51.5% against 48.5% of females).
So, if you happen to bet on the biological sex of an incoming child, you know what to focus on yours chips. But why is there this imbalance? The causes are not completely clear: biological factors, evolutionary mechanisms and environmental conditions have to do with it. Certainly, however, they have nothing to do with the lunar calendar, the shape of the belly or other popular beliefs that science has repeatedly denied.
Because the births are not 50 and 50 and how much more likely a male child is
As mentioned, the explanation of the matter is not clear and univocal, scientists are still trying to unravel the question. Let’s start with conception, from the most recent studies (Orzack et al., 2015, Wang et al. 2025) It seems that males and females are conceived in similar quantities, but during pregnancy the probability of survival are not the same. In the early stages more males die, but in the following months the female losses are greater: the result is that at birth there is a slight male advantage, about 105 males per 100 females.
In a 1973 study, scientists report the evidence that in other animal species, health mothers tend to have more male puppies, while the worst health ones have more females. This, according to the Trivers-Willard theoryit happens because the reproductive success of the males is more variable and linked to their physical condition (a strong male will have many more likely to reproduce than a weak male), while that of females is more constant. In man this evidence is not so strong, but it must certainly be considered as one of the explanations of the male surplus at birth. For example, it was observed that stress during pregnancy more easily leads to the loss of a male child than a female.
Another evolutionary hypothesis, argues that this surplus compensates the greatest male mortality in childhood and adulthood, maintaining a balance between men and women of reproductive age. In fact, then at the most advanced ages, women represent the highest portion of the population.
What if it were just a coincidence? Spoiler: it is not, the data speak clearly
If we launch a coin ten times it can be, indeed it is very likely, that it does not happen exactly five times head and five times cross. It could also be a similar thing to determine the imbalance between the sexes. Well, that’s not the case. For a single population, perhaps in a single year, it could be that it is a coincidence, but when we put many years and many countries together, we must discard this option. For example, we look at the relationship between male born and females in Italy and in a series of other countries:
In Italy, yellow row, the ratio is permanently around 105 male births per 100 females in each year. So also for the United States, for France and Spain (albeit with some wider variations). Even outside the West, the imbalance is real, look for example Brazil or Chile. And in China what happened?
In some cases the relationship between male and female children changes a lot
The relationship between male born and born females in China is incredibly in favor of males. This is due to the famous policy of the only child introduced in the country in 1979, with which the government hoped to contrast the extreme increase in the population by limiting families to have only one child. This law, combined with traditional cultural preferences for males, has led to practices such as prenatal selection and targeted abortions, artificially increasing the number of male births compared to female ones. In recent years, with the loosening of the only child’s policy, the values are returning to the natural threshold of 105 males per 100 females, but you can still notice an evident preference in the selection of male births.
It is not the only case in which the value of the relationship between the born of the two sexes is far from what we are used to observe on average. A similar phenomenon of selection of births, for the benefit of males, also occurred in other countries such as India, Azerbaijan or Armenia.
However, there are also cases in which the opposite happened and the causes are more natural than artificial: in January 1995 there was a terrible earthquake in Kobe in Japan, starting from nine months later and for a few months later it was noted that the percentage of males and females that were born was strangely equal. The births in general had reduced by 6% and the ratio between the two biological sexes at birth had lowered to 100 males for every 100 females, a parity probably due to the reduction of male births due to the strong stress brought by the earthquake.
