Boy sperm and girl sperm swim at essentially the same speed. The widely repeated claim that Y-bearing sperm (which produce boys) are faster but more fragile, while X-bearing sperm (which produce girls) are slower but hardier, traces to a single researcher’s work in the 1960s that has not held up under modern scrutiny. Recent reviews of the evidence describe negligible or no differences between X and Y sperm across a range of physical traits, including motility and swimming pattern. The reality is more interesting than the myth, though, because there are genuine molecular differences between the two types of sperm that have nothing to do with speed.
Where the Speed Myth Began
The idea that Y sperm are speedier originated with Landrum Shettles, a physician who claimed in 1960 that X- and Y-bearing sperm differed enough in size, shape, and motility to be visually distinguished and physically separated. Shettles asserted that Y sperm were smaller, lighter, and faster, while X sperm were larger, heavier, and slower.1Europe PMC / BMJ. Entrenched misinformation about X and Y sperm From those claims grew an entire cottage industry of sex-selection advice. The “Shettles Method” told couples hoping for a boy to time intercourse close to ovulation so the supposedly faster Y sperm could reach the egg first, while couples wanting a girl were told to have sex a few days before ovulation and let the supposedly longer-lived X sperm outlast the Y sperm.
The problem is that Shettles was working with the microscopy tools available in the late 1950s, and his observations about size and speed differences were never reliably replicated. As the evidence piled up over the following decades, the speed difference he described essentially evaporated. A 2006 letter in the BMJ called the persistence of these claims “entrenched misinformation,” noting that the original assertions about X and Y sperm have been carried forward in textbooks and popular media despite the lack of supporting evidence.1Europe PMC / BMJ. Entrenched misinformation about X and Y sperm
What Direct Measurements Actually Show
When researchers used computer-assisted sperm analysis to compare X and Y sperm head-to-head, the speed difference that Shettles described failed to appear. A study using bovine sperm separated by flow sorting found that Y sperm did not swim faster than X sperm in a simple salt solution. The two types did differ in measures of how straight their paths were, but the actual forward velocity was the same.2PubMed. Comparative motility of X and Y chromosome-bearing bovine sperm separated on the basis of DNA content by flow sorting That distinction matters: swimming in a straighter line versus a more curved path is not the same thing as swimming faster, even though it might look that way at a glance under a microscope.
A broad review of the literature on X and Y sperm published in Frontiers in Cell and Developmental Biology reached a similar conclusion, finding that while earlier work proposed substantial differences between the two types, recent studies suggest negligible or no differences in motility, swimming pattern, shape, size, strength, electric charge, pH response, and stress response.3PubMed Central. New Biological Insights on X and Y Chromosome-Bearing Spermatozoa The picture that emerges from modern research is not one of two dramatically different cells racing each other. It is one of two cells that are, for most practical purposes, remarkably similar.
The Timing-of-Intercourse Test
If Y sperm really were faster, a logical prediction follows: having sex right at ovulation should produce more boys (the fast Y sperm would get to the egg first), while having sex days before ovulation should produce more girls (the slower but longer-lived X sperm would still be alive when the egg arrived). This is the core logic of the Shettles Method. Multiple well-designed studies tested exactly this prediction, and it failed.
A study published in the New England Journal of Medicine tracked hundreds of cycles and found that cycles producing boys and cycles producing girls had similar patterns of intercourse relative to ovulation. The researchers concluded that for practical purposes, the timing of sex in relation to ovulation has no influence on the baby’s sex.4PubMed. Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby A separate study looking at both planned and unplanned pregnancies among women using natural family planning found the same thing: the sex ratio did not vary with the estimated timing of insemination relative to ovulation.5PubMed. Sex ratio associated with timing of insemination and length of the follicular phase in planned and unplanned pregnancies during use of natural family planning
These results are about as direct a test of the speed theory as you can get. If one type of sperm consistently outran the other, timing should shift the odds. It does not.
What About pH and Acidity?
Another popular claim holds that vaginal pH can tip the balance toward one sex, with more acidic conditions favoring girls and more alkaline conditions favoring boys. This idea also flows from the Shettles framework, which suggested that Y sperm were more vulnerable to acidic environments. Some sex-selection guides still recommend douching with baking soda or vinegar to shift the odds.
But when researchers tested how X and Y sperm survived in different pH environments, they found no difference. Human X and Y sperm survived equally well in both acidic (pH 5.2) and alkaline (pH 8.0) buffer solutions, even after 11 hours of incubation. The study’s authors concluded that altering the ratio of living X and Y sperm through pH treatment is not an effective method of changing the sex ratio.6PubMed. Equality in survival of X and Y chromosome-bearing human spermatozoa So the diet-and-douche approach to sex selection rests on a premise that does not hold up in controlled conditions.
One Genuine Physical Difference
Not everything about X and Y sperm is identical. The X chromosome carries considerably more DNA than the much smaller Y chromosome, which means X-bearing sperm contain roughly 2.8% more genetic material. That difference is real and measurable; it is the basis for the flow cytometry technique used to sort sperm in agricultural settings. But the DNA content difference is tiny in absolute terms and does not translate into meaningful differences in cell size or swimming ability under normal conditions.
One measurable physical difference that has been confirmed is surface charge. Researchers who separated X and Y human sperm and measured their zeta potential found that X sperm carry a higher net negative charge on their cell surface (about −20 millivolts) compared to Y sperm (about −16 millivolts).7PubMed. Zeta potential of human X- and Y-bearing sperm This difference is enough to separate the two types using electrophoresis in a lab, but there is no evidence that it changes how sperm behave during natural conception. A sperm cell navigating the reproductive tract is responding to chemical gradients, muscle contractions, and fluid currents, not sorting itself by electrical charge.
A Real Molecular Difference That Affects Motility
Here is where the science gets genuinely interesting. While X and Y sperm are essentially identical in their baseline swimming speed, researchers have discovered a molecular lever that can selectively slow down one type. The X chromosome carries genes for immune receptors called TLR7 and TLR8. Because the Y chromosome does not carry these genes, only X-bearing sperm express them on their surface. When a chemical activates these receptors, it suppresses the motility of X sperm without affecting Y sperm at all, and without killing either type.8PLoS Biology. Activation of Toll-like receptor 7/8 encoded by the X chromosome alters sperm motility and provides a novel simple technology for sexing sperm
Follow-up work in dairy goats confirmed the mechanism: activating TLR7/8 triggers a signaling cascade inside X sperm that reduces their mitochondrial activity and ATP levels, essentially cutting their energy supply. In a test tube, this causes X sperm to sink to the bottom while the still-active Y sperm concentrate in the upper layer, allowing the two populations to be separated without expensive equipment.9PubMed Central. TLR7/8 signalling affects X-sperm motility via the GSK3 α/β-hexokinase pathway for the efficient production of sexed dairy goat embryos
This is not evidence that X sperm are naturally slower. Under normal conditions, TLR7/8 receptors are not being activated by anything in the reproductive tract. But it reveals that the two types of sperm are not genetically identical in every functional sense, and it provides a mechanism that can be exploited in the lab. The irony is worth noting: when you can artificially create a speed difference, it is the girl sperm that slow down, not the boy sperm, which is the opposite of what the Shettles myth would predict.
Do Some Fathers Naturally Produce More of One Type?
Families with four or five children of the same sex are common enough to make people wonder whether some men just produce more Y sperm or more X sperm. Researchers tested this directly by analyzing the ejaculates of men who had three or more children of the same sex. Men with only sons had about 49.7% Y-bearing sperm, and men with only daughters had about 44.8% X-bearing sperm. Neither number was significantly different from what you would expect by chance.10PubMed Central. The ratio of X- and Y-bearing sperm in ejaculates of men with three or more children of the same sex A preponderance of sons or daughters in a family cannot be explained by an altered ratio of X and Y sperm in the father’s semen. Whatever causes some families to run heavily toward one sex, it is happening somewhere downstream of sperm production.
Y Sperm May Be More Vulnerable to Certain Toxins
One area where X and Y sperm do appear to behave differently involves exposure to endocrine-disrupting chemicals. A study exposing sperm to several environmental pollutants, including dioxin, a pesticide called diazinon, and the soil fumigant DBCP, found that the ratio of live Y sperm to live X sperm dropped significantly after exposure to three of the five chemicals tested. The researchers suggested that some endocrine disruptors have a larger effect on the viability of Y sperm than X sperm, which could theoretically shift the sex ratio at birth toward more girls.11PubMed. Effect of endocrine disruptors on the ratio of X and Y chromosome-bearing live spermatozoa
This finding resonates with larger population-level observations. Some researchers have noticed slight shifts in the birth sex ratio in regions with heavy industrial pollution or after environmental disasters, and differential sperm vulnerability is one proposed explanation. The effect is small and the evidence at the population level is mixed, but at the cellular level, Y sperm do seem to be the more fragile of the two when hit with certain toxic chemicals. That fragility has nothing to do with speed, but it is one of the few confirmed ways the two types of sperm respond differently to their environment.
Maternal Stress and the Sex Ratio
Another factor that shows up in population-level data is maternal stress. A large study examining the relationship between a mother’s occupation and the sex of her children found that women in job types categorized as high stress were more likely to give birth to daughters, while women in low-stress jobs had roughly equal sex ratios or a slight male bias.12PubMed Central. Mother’s occupation and sex ratio at birth The mechanism behind this is not well understood. Stress hormones could affect the uterine environment in ways that differentially affect implantation or early survival of male versus female embryos. Or the effect could operate even earlier, at the level of sperm selection. But the point is that whatever subtle forces push the sex ratio around at a population level, they are not operating through the crude mechanism of one sperm type swimming faster than the other.
How Actual Sex Selection Works in Livestock
If the speed difference were real, the livestock industry would have been exploiting it for decades. Dairy farmers have an enormous economic incentive to produce female calves, since males have low value in a dairy operation.13PubMed Central. Bovine semen sexing: Sperm membrane proteomics as candidates for immunological selection of X‐ and Y‐chromosome‐bearing sperm The technology that actually works for predetermining the sex of offspring is flow cytometry, which stains sperm DNA with a fluorescent dye and then uses a laser to measure how much dye each cell absorbed. Because X sperm contain slightly more DNA, they glow a little brighter, and a machine can sort individual cells into X and Y populations at high speed.14Methods in Molecular Biology. Sex-Sorting Sperm Using Flow Cytometry/Cell Sorting
This technology is expensive, slow relative to the number of sperm processed, and slightly damages some of the cells during sorting, which is why sexed semen costs more and produces lower conception rates than conventional insemination. If anyone could just let Y sperm swim ahead and collect the front-runners, the entire flow-sorting industry would be unnecessary. The fact that the agricultural world invested billions in laser-based DNA measurement rather than a simple swim-up race tells you everything about how little the speed difference matters.
When Y Chromosome Damage Does Create a Real Motility Gap
There is one scenario where Y-bearing sperm genuinely do swim worse than X-bearing sperm, and it involves damage to the Y chromosome itself. Researchers studying mice with a large deletion on the long arm of the Y chromosome found that Y-bearing sperm from these animals had impaired motility compared to X-bearing sperm from the same males. The Y-bearing sperm also showed morphological abnormalities, including changes in head shape and reduced overall size. The result was a skewed sex ratio favoring female offspring.15Cell Press (Current Biology). Y Chromosome Long Arm Deletion Leads to Differential Sperm Motility and Morphological Defects Causing Sex Ratio Skewing
This finding is specific to a genetic mutation, not a feature of normal Y sperm. But it illustrates an important principle: the Y chromosome carries genes that matter for sperm function. When those genes are disrupted, Y-bearing sperm suffer more than X-bearing sperm, because X sperm have a whole second set of genetic resources to fall back on. Under normal circumstances, though, the Y chromosome’s genes work fine, and the two sperm types are functionally equivalent swimmers.
The Ethics of Sex Selection in Humans
Given that folk methods based on the speed myth do not work, couples who genuinely want to choose their child’s sex must turn to assisted reproduction technologies. Preimplantation genetic testing during IVF can identify the sex of embryos before transfer with near-perfect accuracy. Sperm sorting using flow cytometry has also been offered at some fertility clinics, though its availability varies by country. Sex selection for non-medical reasons is forbidden in many countries, while others allow it only when there is a medical justification, such as avoiding sex-linked genetic diseases like hemophilia or Duchenne muscular dystrophy.16PubMed Central. Preconception sex selection for non-medical and intermediate reasons: ethical reflections
The ethical debates around non-medical sex selection are active and unresolved. In some cultures, strong son preference has created measurable demographic imbalances where sex-selective technologies or practices are available. Other ethicists argue that in the context of family balancing, where a couple with three children of one sex wants to try for the other, the stakes are low enough that parental autonomy should take priority. Wherever you fall on that spectrum, the technologies that actually work for sex selection rely on identifying genetic material, not on giving one type of sperm a head start.