Since the beginning of time, couples have been trying to figure out how to influence the sex of their babies. A quick google search and you’ll find plenty of weird superstitions that people used to try to have either a boy or a girl. Still, the question has remained for generations: Why do some women seem to give birth to girl after girl after girl, whereas others have only boys? This is what science knows so far.
Why Some Women Give Birth To All Girls or All Boys
Long have we assumed that when women give birth, the chances of having a girl or a boy are 50/50. Yet, it doesn’t quite seem to shake out that way. Though many of us grew up with brothers and sisters, many still have grown up in either all-girl or all-boy homes. It also often seems to be a family thing, therefore influenced by genetics. There are families that have hardly any girls and those that seem to be male dominant. Finally, billionaires and presidents, while they don’t necessarily have 100% boy broods, do tend to have a higher son-to-daughter ratio. (1)
In reality, the ratio of boy babies born to girl babies born is about 109 boys for every 100 females. Despite this, the male-to-female population is still roughly around 50/50. Many scientists say this is an evolutionary thing. Boys and men tend to have weaker immune systems, higher cholesterol, are more prone to diabetes, heart disease, and dying from cancer. They’re also two-thirds of murder victims and are the majority when it comes to traffic fatalities. Having a higher birth rate of males means that the ratio of men to women when these babies reach the age of reproduction will be more or less equal.
Another unavoidable statistic is that women tend to give birth to more female babies in times of stress, such as famine or a natural disaster. This can be explained similarly as to why wealthy men and powerful women tend to have more boys: Boys are bigger and consume more than girls. Therefore, traditionally, it is more expensive to have boys. When times are tough, when there isn’t much food available, having girls is a more viable option. Also, in times of high stress, such as war or famine, women are more likely to suffer miscarriages. Women are also more likely to miscarry male fetuses than female ones. (2)
Similarly, if you’re a billionaire or have a lot of money, providing for a brood of boys is less likely to be a problem than if you’re on the opposite side of the socioeconomic spectrum. Naturally, women who have lots of money, either by their own accord or that of their partner, have more comfortable, less stressful lives on average. At the very least, they are not facing stress of whether or not they will have food or a foreign army will attack their town, village, or city during the night. Less stress means less likely to have a miscarriage. (3)
Read: This Image Shows Why Women Need Lots Of Time To Recover After Childbirth
Who Influences The Sex of A Baby?
In the past people assumed that it was the mother who had the most influence over whether or not her baby was male or female. This is because she was the one carrying and creating that child. However, science now shows that it is actually the man.
Humans have a pair of chromosomes, X and Y. They have inherited them from their parents, and, amongst other things, they determine biological sex. Men are XY and women are XX (to keep this the most basic and simple as possible). Women carry only two x chromosomes. Men’s sperm, however, is a mixed bag. They can have sperm with the Y chromosome, but also can have sperm with the X chromosome.
This means that it is actually the man’s sperm that influences the sex of the child. It depends which sperm is successful at fertilizing the egg released by the woman’s ovaries. If it is an X chromosome sperm, the baby will be a girl. If it is a Y chromosome sperm, the baby will be a boy.
Purely Down To Chance
Despite all of the theories and tendencies that we see, science still says that whether or not you have a boy or a girl is down to chance. There is still no solid scientific evidence that girls or boys can run in a family. We don’t currently have any proof to say that certain genetics cause one sex to be more likely than the other. Scientists came to this conclusion after they studied the entire Swedish population since 1932.
“We found individuals don’t have an innate tendency to have offspring of one or the other gender — instead, the sex of their offspring is essentially random,” said Dr. Brendan Zietsch, a fellow at the University of Queensland’s School of Psychology and the lead author of the study. “If you have a lot of boys or girls in your family, it’s just a lucky coincidence,” (4)
This study was the largest ever done, having followed 4.7 million births – every single birth in Sweden since 1932. Other studies that have been done on the topic had much smaller sample sizes, which means that they were more likely to find false positives. There have been studies that showed extreme conditions, such as famine or extreme heat, can influence whether a woman gives birth to a boy or a girl. Still, the Swedish scientists say that, at least based on their study, it still seems rather like a game of chance.