Will the baby be male or female? If it was a random biological switch then, with a huge sample number, the sex ratio at birth would be exactly 50-50. But it isn't; the ratio is about 51-49 in favour of males. So how does this inequality arise?

Further to @Remib's comments. I was specifically thinking of the sex ratio in humans. Would it be true to say that the genetic determinant of the sex ratio would be exactly 50:50 M/F? If the disparity is environmental, then how does it work, and wouldn't an environmental effect be more variable than the 51:49% that we see?

  • 2
    $\begingroup$ Are you talking about humans only? Are you more interested into a mechanistic explanation or rather an evolutionary explanation? $\endgroup$
    – Remi.b
    Commented Aug 19, 2016 at 1:00
  • 1
    $\begingroup$ If your interest goes beyond humans, then note that the sex-determination system varies (a lot) from species to species. The post Do males of all sexual species have Y chromosomes? gives an overview of this variation and while the answer does not specifically talk about the sex-ratio, I think you can easily make your way to the understanding of why the sex-ratio isn't always 50:50. $\endgroup$
    – Remi.b
    Commented Aug 19, 2016 at 1:02
  • $\begingroup$ youtube.com/watch?v=3IaYhG11ckA This is a lovely video explaining it all. There are a ton of references down there in the description $\endgroup$
    – Polisetty
    Commented Aug 19, 2016 at 11:45

1 Answer 1


There are a number of ways in which the true sex-ratio at birth (let's assume the 51:49 is well estimated and is the true ratio) could deviate from 50:50. This list is just an brief overview.

  1. Sex, in humans, is determined by the sex chromosome present in the haploid sperm cell which fertilises the egg. If males produced more or less Y-bearing sperm than X-bearing sperm, then the birth sex ratio would be expected to deviate from 50:50. However, studies suggest that sperm production is not biased towards X or Y bearing in humans.

  2. The probability of fertilisation may differ between X- and Y-bearing sperm. If Y-bearing sperm were more likely to succeed in fertilising the egg then there would be a male-bias in the birth sex ratio. I've seen it suggested that the small Y chromosome makes the sperm slightly smaller and therefore faster, but can't find those claims right now. Regardless, it appears that sex ratio at fertilisation is not sex-biased.

  3. Bias in the birth sex ratio can come through sex-biases in the failure rate in fetuses. If female fetuses are more likely to miscarry than male fetuses, over the course of the gestation, then the sex ratio at birth will be male-biased. It appears that there is sex-bias in the failure rate of fetuses which can be male or female-biased throughout gestation, but the tendency is for female-biased failure rate during the second trimester to generate an overall male-bias birth sex ratio.

  • 1
    $\begingroup$ But would you really consider the 1.5% as a significant deviation? There are of course demographic differences which are in many cases affected by "cultural" practices. In many places the sex ratio is skewed toward male because of female infanticide (preference for a male child). $\endgroup$
    Commented Aug 19, 2016 at 8:31
  • 4
    $\begingroup$ birth sex ratio won't be affected by infanticide - I've also not said that 1.5% is significant, I'm just operating under the assumption that the quoted statistic of 51:49 at birth in the question is correct and pointing out how it could be $\endgroup$
    – rg255
    Commented Aug 19, 2016 at 9:17

Not the answer you're looking for? Browse other questions tagged .