The claim is that human sperm has different physical characteristics (i.e. phenotype) depending on whether it is carrying an X or Y chromosome.

  • One theory is that sperm that carry the male chromosome are tiny and fragile but very quick, whereas the female sperm are larger and hardier, but rather sluggish. Source

  • The male sperm are FAST... but die faster also. The female sperm are slow and steady and have a longer life span. Source

  • The X sperm (which are the sperm that make females) live longer than the Y sperm (which are the sperm that make males). However, the Y sperm swim faster, so the chances are equally good that you will get a girl or a boy. Source

None of these sources provide references.

Do "male" sperm swim faster than "female" sperm?

  • 1
    Note: this question was asked as part of the "topic of the week" initiative to raise our questions per day stat. Please contribute some great questions!
    – Oddthinking
    Commented Feb 12, 2012 at 15:40
  • I read this question title several times because I was wondering what you meant by swimming chromosomes that are carrying sperm. the title is actually fine (leave it!) but it still tripped me up. Commented Feb 12, 2012 at 18:41
  • @Konrad, I changed the title, partly because I remember as a kid being bewildered by "cat-chasing dogs" from a children's book I owned.
    – Oddthinking
    Commented Feb 15, 2012 at 6:37

1 Answer 1


XY Source

A famous method for gender selection is the Shettles Method, which makes use of the

"male (Y) sperm are faster than female (X) sperm "


But according to Entrenched misinformation about X and Y sperm (2006):

The widely held idea that spermatozoa bearing the Y chromosome (Y sperm) swim faster than those bearing the X chromosome (X sperm) seems to have originated from Shettles's work in 1960, using phase-contrast microscopy.

Reading Shettles's reports in Nature and other peer reviewed journals, many researchers thereafter believed that Y sperm swim faster than X sperm. The finding particularly influenced research on sperm separation.

Although several attempts have been made to correct this impression, it was not until the development of computer assisted sperm analysis (CASA) that reliable observations could be made.

So far, researchers have found no morphological differences between human X sperm and Y sperm.

Neither mature sperm nor their precursors possess significant morphological differences between X and Y genotypes;


  • 1
    Nice! I was half-expecting an answer that said "Yes, they are slightly faster, but not enough to make the Shettles(*) method effective." (*I didn't know its name.) This is even better!
    – Oddthinking
    Commented Feb 12, 2012 at 17:16
  • I feel like the study you heavily quoted doesn't really refute the claim. The sperm could be morphologically the same, but if the Y chromosomes had less mass, it stands to reason to me that given the same morphological structures, the Y sperm could achieve greater speed, because it has less mass to move, and the same structures to move itself. I would think focusing on the study "Y bull sperm do not swim faster than X sperm" would make more sense.
    – JMac
    Commented Aug 7, 2020 at 17:40

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