Be honest: have you ever made love to your partner in a specific position because you heard that it would result in a baby boy?
There are many intergenerational rumors about what to do if you want to give birth to a boy or a girl. Indulging in sugar is  to result in a girl, while drinking coffee before sex will somehow manifest a boy. Aristotle thought that  in the bedroom was tied to the sex of the baby: the more intense the passion, the warmer the seed, the more likely a boy would emerge nine months later. Oh, and if you’re into astrology, making love on a full moon is bound to result in a baby girl! Or maybe a werewolf.
Scientifically, none of this is even remotely true. The sex of the baby is random. It’s determined by which spermatozoon makes it to the egg first. If the spermatozoon carries a Y chromosome, you’ll get a boy; if it’s an X chromosome, you’ll get a girl.
The odds are 50:50, which is why as many boys as girls are born each year. Right?
Surprisingly, no.
Typically, more boys than girls are born. This is  of firstborns, or when the parents are young, or when parents are of a higher social class. Scientists have explored a number of possible explanations, from sugar molecules at the surface of the egg to Quebec’s own sovereignty referendum. Indeed, asking Quebecers to separate from the rest of Canada may have skewed the sex of the babies being born that year.
A weight difference in the race for the egg?
It’s not just us: the animal kingdom itself is skewed—sex-wise—in many ways.
´ˇ»ĺłÜ±ôłŮĚý±á˛â±č´Ç±ôľ±łľ˛Ô˛ą˛őĚýbutterflies are , as almost all males are killed by a sex-specific infection. A small marsupial mouse native to Australia is also quite Amazonian in its sex distribution: all the males die after a two-week breeding period which drives them to . These two critters are good examples of a heavily skewed adult sex ratio.
But sex can be skewed at conception and at birth, too. In the , it was discovered that the sex of a tiny lizard named Agama agama was influenced by temperature. When its eggs were kept in a slightly warmer environment, its offspring were all male. I guess there was something to Aristotle’s “hot seed” hypothesis after all.
We now have  that invertebrates manipulate the sex ratio of their offspring. Mammals apparently didn’t want to get left behind. Mounting evidence shows that mammals, including us, have some “control” over the sex of their progeny. For every 100 human girls born, somewhere  are born, too. Clearly, there are social factors at play, particularly in some countries. The term “gendercide” is used to describe the deliberate killing or lethal neglect of a particular sex (often female) because the other sex is seen as more desirable. But even when we remove these grim social reasons, a biological skew remains, which varies slightly from one population to the next.
This has puzzled biologists, statisticians, social scientists, even theologians, who have all chimed in with their hypotheses for why we humans birth a bit more boys than girls. The difference is indeed small, but it has been noted so many times in so many populations that it is indeed significant and real. We know that are lost before birth through miscarriages, but that of course doesn’t explain the surplus of boys. Many of our assumptions about reproduction have thus been re-examined over the years. Maybe men produce slightly more Y-chromosome-carrying spermatozoa than X-carrying ones. Maybe X-carrying sperm is slightly less viable. I was surprised to see weight being mentioned. Sure enough, the Y chromosome is tiny compared to the X chromosome, which means that spermatozoa don’t all weigh the same. At their microscopic scale, this weight difference could mean that the flagellated messengers with the potential to make boys have a significant speed advantage in the race for the egg.
None of these putative mechanisms seems to be under the control of either parent. But the most famous hypothesis put forth to explain our human sex ratio actually claims that one of the parents does, in a manner of speaking, have some control over the sex of their child. It’s called the Trivers-Willard hypothesis.
Conditions affect the survival of each sexÂ
By control, we don’t mean that a parent’s wish comes true. We have to see the word “control” from an evolutionary perspective.
You see, there are benefits for species to control how likely they are to give birth to boys versus girls. The two sexes do not fare equally in the same circumstances, which affects the chances that a species has to perpetuate itself. Males tend to grow faster and need more food than females: when food is thin on the ground, they are thus more likely to die. But when conditions are good, males outreproduce females. It’s a lot easier and less laborious for a male to spread his genes than for a female. And so, for a species to have survived for thousands of generations in an ever-changing environment, skewing its progeny toward male or female depending on the circumstances would be beneficial to its survival.
This is the argument at the core of the Trivers-Willard hypothesis. In good conditions, males outreproduce females. In bad conditions, females outreproduce males. Might there not be a way for future parents to favour the sex better adapted to the overall conditions? If this mechanism exists, it probably lies with the mother, who bears the brunt of reproduction. It would likely be something that takes place around the time of conception, as constantly investing energy into this sex-skewing mechanism would be a waste. And this mechanism would have to pick up on how good or bad the environment is in order to correctly skew the sex of the offspring.
Hence, it’s not the mother ł¦´Ç˛Ô˛őł¦ľ±´ÇłÜ˛ő±ô˛âĚýchoosing the sex of her progeny; but something ľ±˛Ô˛őľ±»ĺ±đĚýthe mother would be scanning the environment and influencing, in a tiny way, the sex of the child.
This hypothesis,  in 1973 by Robert Trivers and Dan Willard of Harvard, is missing two important pieces of information: what exactly is this mechanism and what does it pick up in the environment to determine if the conditions are good or bad? Much ink has been spilled over the decades by scientists proposing a variety of potential solutions to these two important questions and we are still waiting for clear answers.
When it comes to the mechanism, an interesting idea has been put forward: the egg decides. When the winning spermatozoon touches the egg, it actually binds to one of the many proteins at the surface of the egg, in what is akin to a molecular handshake. This handshake triggers a quick series of events. Enzymes inside the egg migrate to the surface, causing all of the other proteins sticking out of the egg to stick to each other. This way, no other sperm can enter the egg. It’s the equivalent of the Starship ·ˇ˛ÔłŮ±đ°ů±č°ůľ±˛ő±đĚýturning on its shields.
These proteins are glycosylated, meaning that they have sugars attached to them. Scientists have thus  if the kind of sugar attached to these proteins might be influenced by this sex-skewing mechanism. Maybe when the conditions are good, the egg’s glycosylated proteins are such that they bind a little bit better to sperm that is carrying a Y chromosome. It’s a plausible mechanism but for now it remains hypothetical.
Having a biological mechanism that essentially scans the environment and alters a species’ sex ratio implies that that sex ratio will change when the environment does. And we have indeed seen examples of that.
Conflicting dataÂ
If you were living in Quebec in the 1990s and were old enough to have some political awareness, you remember the 1995 sovereignty referendum. The Quebec government asked us if we wanted to split from Canada and form our own country. Voter turnout was an astonishing 93.5%, and the pro-sovereignty contingent  (49.4% against 50.6%). It’s fair to say this event stressed our province.
In 2014, a researcher compared the sex distribution of babies born in Quebec in 1995 with the preceding and following five-year periods. Wouldn’t you know it, there was  right in the aftermath of the referendum: instead of seeing more boys born than girls, we almost got to parity (50:50) before bouncing back. No change in Canada as a whole, only in Quebec.
When nations are stressed, the sex ratio often dips down, allowing more girls to be born than normal. That, however, is not always true, and Ěý´Ç´ÚĚý done on natural disasters and on terrorist attacks often lead to inconsistent results. The same can be said of exposure to toxic chemicals.
In the 1980s, men in Washington State who listed “carbon setter” as their occupation had  than expected. Carbon setters worked at a type of aluminum reduction plant, where they are thought to have been on the receiving end of a lot of heat, magnetic fields, and air pollutants compared to their fellow workers. This is a strange anomaly, since the sex ratio mechanism is hypothesized to reside within the mother. But there is a chance the father has something to say about it as well, although the evidence is not yet convincing.
Looking at exposure to environmental and occupational toxins, like endocrine disruptors, the data we have is . Taiwanese fathers exposed to dioxin, a persistent environmental pollutant, had fewer boys than expected, but there was no effect in Japanese fathers exposed to the same pollutant. Many of these studies are  for scientists to be able to conclude anything: which parent was exposed, when, for how long, to what dose? And obviously, none of these studies are experimental: we can’t forcefully expose future parents to large amounts of pollutants to see how many girls they will give birth to.
At the end of the day, we do know this: we humans (and many other animal species) are birthing more boys than girls, and this skewed ratio can change in certain circumstances. Beyond that, we lack certainty. As a  put it, the sex ratio at birth “remains one of the most longstanding unresolved problems in the biological sciences.” And it’s not just an intellectual exercise: if we understood the precise mechanism at play, tweaking it could benefit both the livestock industry and animal conservation, for example.
In the meantime, if you are looking to get pregnant and are wondering about the sex of your future child, do know that it is a coin toss. Only the coin is not perfectly fair.
Take-home message:
- Overall, slightly more baby boys are born than baby girls, and part of the reason for this seems to be biological
- It has been hypothesized that the mother’s body can sense when the conditions around her are good and will increase the probability of conceiving a boy because they tend to do better in good conditions, and vice versa for girls
- There is inconsistent evidence that societal stress (coming from terrorist attacks or natural disasters) and the exposure of a parent to contaminants like endocrine disruptors will favour the birth of girls a bit more than normal
Note: Intersex people do exist, and some people have atypical sex chromosomes (such as XXY or a single X chromosome on its own). The literature on human sex ratios rarely mentions them.
