Which Parent Determines the Gender of a Baby?

The father determines the biological sex of a baby. Every egg a mother produces carries an X chromosome, while each sperm from the father carries either an X or a Y. When a Y-bearing sperm fertilizes the egg, the result is a boy (XY); when an X-bearing sperm wins the race, the result is a girl (XX). That basic coin flip happens inside the father’s body before conception even takes place, but the story gets considerably more interesting once you look at what nudges the odds.

What the Y Chromosome Actually Does

Carrying a Y chromosome would not matter much on its own if it did not contain a specific stretch of DNA called the SRY gene. This gene acts as a master switch for male development. It produces a protein that binds to DNA and bends it, which in turn activates a cascade of other genes that steer an embryo toward forming testes rather than ovaries.¹ In the simplest terms, SRY suppresses a set of signals that would otherwise push development in the female direction, clearing the path for male sex determination.²

Without SRY, an embryo with XY chromosomes can develop female anatomy. And in rare cases, the SRY gene can accidentally end up on an X chromosome during sperm production, meaning a person with two X chromosomes develops male characteristics. Researchers have documented individuals with a 46,XX karyotype who nonetheless have testes because SRY was transferred to one of their X chromosomes.³ These unusual cases underline that it is not the Y chromosome itself that makes someone male; it is the SRY gene the Y chromosome normally carries.

Are X and Y Sperm Really Different?

A widespread folk belief holds that Y-bearing sperm swim faster but die sooner, while X-bearing sperm are slower but hardier. This idea has been used to justify all kinds of timing-based methods for choosing a baby’s sex. The evidence, though, does not back it up. A large analysis of nearly 300,000 sperm found the ratio of Y-bearing to X-bearing cells was almost exactly one to one, unaffected by the donor’s age.⁴

What about speed? A study using flow-sorted bovine sperm found that Y sperm did not actually swim faster than X sperm in a simple solution, though they differed in a couple of measures related to path straightness.⁵ A broad review of the research reached a similar conclusion: there are negligible or no differences between X-bearing and Y-bearing sperm in size, shape, motility, swimming pattern, pH response, or stress tolerance.⁶ At the molecular level, some recent work has found subtle differences in gene activity related to energy metabolism and structural proteins in the flagellum, which may open the door to future sex-sorting techniques.⁷ But those differences are far too small to explain the folk wisdom about fast boy sperm and slow girl sperm.

Do Some Fathers Naturally Have More Boys or Girls?

If the sperm ratio is essentially fifty-fifty and neither type swims meaningfully faster, you might expect every father to have equal odds of producing a son or a daughter. Yet anyone who knows a family with five boys and no girls might wonder whether something genetic tilts the balance. A researcher at Newcastle University analyzed family trees spanning several generations and concluded that men do appear to inherit a tendency to sire more sons or more daughters from their parents. A man with many brothers was more likely to have sons, and a man with many sisters was more likely to have daughters. The proposed mechanism involves a gene (not yet identified in humans) with variants that subtly bias the ratio of X- to Y-bearing sperm a man produces.

When researchers looked directly at the sperm of men who had three or more children of the same sex, however, they found no detectable skew in the X-to-Y ratio. Men with only sons did not produce more Y-bearing sperm, and men with only daughters did not produce more X-bearing sperm.⁸ That does not necessarily disprove the family-tree observation. It could mean the bias operates through something other than raw sperm counts, such as differences in how each sperm type interacts with the female reproductive tract, or through selective embryo survival. But it does mean the mechanism is still an open question.

How the Mother’s Body Responds

Calling the father the sole “determiner” is an oversimplification. The mother’s body is not a passive bystander. In a striking experiment using pigs, researchers introduced X-bearing sperm into one oviduct and Y-bearing sperm into the other oviduct of the same female. The oviductal tissue responded differently to each type: over 500 genes changed their activity depending on which sperm were present. The differences involved local immune responses and cell-signaling pathways specific to each sperm type.⁹

This raises the possibility that the female reproductive tract could, in some circumstances, favor one sperm type over another. If the oviduct’s immune or chemical environment shifts due to stress, hormonal changes, or other factors, it might create conditions that differentially help or hinder X- or Y-bearing sperm. Research in this area is still early, but it complicates the simple “dad decides” narrative. The father provides the genetic lottery ticket, but the mother’s body may influence which ticket gets cashed.

Stress, War, and Shifts in the Birth Sex Ratio

Globally, roughly 105 boys are born for every 100 girls. That slight male surplus has been documented for centuries and is considered the baseline. But it shifts under certain conditions, and stress is one of the most studied influences.

Wars provide a grim natural experiment. A study of 19th-century Basque Country found that all major conflicts in the region reduced the proportion of boys born, with prolonged battles having the largest effect. In areas with sieges and heavy fighting, the sex ratio dropped by about three percent, and the shift was especially visible in births occurring within the first three months after the fighting.¹⁰ A similar pattern shows up in modern disasters: researchers in Japan documented drops in the proportion of male births nine to ten months after major earthquakes, including the Kobe and Tohoku events.¹¹

The mechanism is not fully settled. One possibility involves the hormonal environment around conception: stress hormones like cortisol may selectively affect the viability of male embryos, which tend to be slightly more fragile in early development. Another possibility is that stress shifts the timing or chemistry of the reproductive tract in ways that favor X-bearing sperm. Whatever the pathway, the pattern is consistent enough across centuries and continents that population-level stress clearly tilts the ratio toward girls.

Chemical Exposures and Paternal Occupation

Some workplace and environmental exposures also skew the sex ratio, and the father’s exposure often matters more than the mother’s. A systematic review of environmental and occupational hazards found that paternal exposure to dioxins was consistently linked to fewer boys, while exposure to PCBs was linked to more boys.¹² The dioxin effect is one of the most dramatic in the literature. After an industrial accident in Seveso, Italy, that released large amounts of the dioxin TCDD, fathers with high blood levels of the chemical fathered significantly more girls than boys. Men who were exposed before age 19 had an especially lopsided ratio, with only about 38 percent of their children being boys.¹³

A large birth-cohort study from Japan found that fathers regularly exposed to insecticides at work were also more likely to have daughters, even after accounting for other factors.¹⁴ The leading explanation is that these chemicals act as endocrine disruptors, interfering with the father’s hormonal environment in ways that either damage Y-bearing sperm preferentially or alter the hormonal signals that influence embryo survival. The fact that paternal hormone levels at the time of conception appear linked to offspring sex ratios supports this idea.

Wealth, Resources, and Adaptive Sex Ratios

One of the more provocative findings in this field comes from evolutionary biology. The Trivers-Willard hypothesis, proposed in 1973, predicts that parents in good condition should produce more sons (because a well-resourced son can outcompete others for mates), while parents in poor condition should produce more daughters (who have a more reliable chance of reproducing regardless of resources). This was originally developed to explain animal behavior, but researchers have tested it in humans as well.

A study of billionaires found that they had about 60 percent sons, well above the population average of 51 percent.¹⁵ A systematic review of the broader Trivers-Willard literature, covering studies across disciplines from anthropology to economics, found that the overall pattern of results supports the hypothesis, with no evidence that the findings were driven by selective reporting.¹⁶ The effect sizes are small in most studies, and the mechanism is unclear. It could work through maternal condition at conception, through selective implantation or embryo loss, or through some combination. But the pattern is real enough to show up across many different populations and measurement approaches.

Embryo Survival and the Slight Male Surplus at Birth

Male embryos develop slightly faster and more efficiently than female embryos in the earliest stages of pregnancy. This means that male embryos are more likely to survive the transition from embryo to established clinical pregnancy, contributing to the roughly 105-to-100 male surplus at birth. But male embryos and fetuses are also more vulnerable to loss later in pregnancy, so the male advantage at the embryo stage has to be large enough to offset higher male losses during the fetal period.¹⁷

Epigenetic differences, meaning differences in how genes are read and expressed rather than differences in the DNA sequence itself, play a role in this unequal survival. Genomic imprinting and other epigenetic marks behave differently in male and female embryos, and changes in the biochemical environment around the time of conception can shift these marks. This is one of the most plausible explanations for why the birth sex ratio fluctuates in response to environmental conditions like stress and nutrition. The father supplies the chromosome, but the epigenetic environment of early embryogenesis, shaped by both parents and external conditions, helps determine which embryos survive to birth.

Centuries of Getting It Wrong

For most of human history, people had no idea that sperm and eggs even existed, let alone that the father’s contribution decided the baby’s sex. The most persistent pre-scientific theory held that males came from the right side of the body and females from the left. Aristotle argued that the heat of the parents during intercourse mattered. By the late 1800s, the dominant theory was that nutrition determined sex. Mothers were sometimes blamed or credited for the sex of their children, a cultural attitude that persists in some communities today despite having no biological basis.¹⁸

The irony is that while the father’s sperm does carry the decisive chromosome, the simplistic modern version of “dad decides” misses how many other factors modulate the outcome. The research reviewed above suggests a more nuanced picture: the father provides the initial binary input, but maternal physiology, parental hormones, environmental stress, chemical exposures, and epigenetic processes all influence which input ultimately produces a live birth.

Can You Actually Choose? Sex Selection Technology

Given all these subtle influences, plenty of people want to know whether they can reliably choose the sex of their baby. The short answer is that natural methods and most sperm-sorting techniques do not work well enough to count on. The Ericsson albumin method, which attempts to separate X and Y sperm by filtering them through protein columns, was tested alongside standard IVF and showed no advantage: the proportion of male embryos in the sex-selection group was not meaningfully different from the control group.¹⁹

More recent approaches look more promising. A clinical trial of a newer sperm sex-selection technique reported that the method successfully enriched sperm samples with the desired chromosome type, yielding a higher proportion of embryos of the chosen sex when used with IVF. The technique did not appear to harm fertilization rates or embryo development.²⁰ The most reliable method currently available, though, is preimplantation genetic testing during IVF, which examines the chromosomes of embryos before they are transferred. This is essentially 100 percent accurate but requires a full IVF cycle, which is expensive and invasive. It is banned for non-medical sex selection in many countries.

Finding Out Early with Cell-Free DNA

If you are already pregnant and want to know the sex, blood tests that detect fragments of fetal DNA circulating in the mother’s bloodstream have become highly accurate. A meta-analysis found that these tests correctly identified the presence of Y-chromosome sequences with about 95 percent sensitivity and 99 percent specificity after seven weeks of pregnancy, improving further as the pregnancy progresses.²¹ A separate meta-analysis put the overall sensitivity at about 97 percent and specificity at 99 percent, with performance remaining reliable across trimesters.²²

These tests were originally developed for medical reasons, particularly to identify pregnancies at risk for sex-linked genetic disorders like hemophilia or Duchenne muscular dystrophy, where knowing the fetal sex early allows doctors to plan further testing. The test works by looking for Y-chromosome DNA fragments in the mother’s blood; if they are present, the fetus is male. If absent, the fetus is female. Before seven weeks of gestation, accuracy dips because there is not yet enough fetal DNA circulating in the mother’s bloodstream to detect reliably.²¹

How Other Animals Handle Sex Determination

Humans are far from the only way nature handles this question, and looking at other species puts the role of the father in perspective. In birds, the system is essentially reversed: males carry two identical sex chromosomes (ZZ) and females carry two different ones (ZW). That means in birds, the mother’s egg determines the offspring’s sex, not the father’s sperm.²³

Many reptiles skip chromosomal sex determination entirely. In species like painted turtles and many crocodilians, the temperature at which the egg is incubated during a critical developmental window determines whether the hatchling is male or female.²⁴ Neither parent “decides” anything; the environment does. This system makes these species especially vulnerable to climate change. Monitoring of painted turtle nests showed that offspring sex ratio was tightly correlated with summer air temperature, and even a modest temperature increase could dramatically skew the ratio. A rise of just four degrees Celsius would effectively eliminate the production of male offspring in that population.²⁵ In the broader sweep of animal life, having the father’s sperm determine sex through chromosomes is just one of many strategies, and not necessarily the most common one.