Question: From a physiological point of view, when sex is determined in a human fetus, is it equally likely to be male or female?

Studies in this area typically measure age at birth, where the data would have already been biased. There's a slight imbalance in the human sex ratio, in that there's an overall tendency towards males. However, this could be accounted for in numerous ways, such as abortion.

Fisher's principle suggests there would be an evolutionary tendency towards 50:50 chance.

  • $\begingroup$ @Dane Why are you marking the older question as a duplicate though? $\endgroup$
    – March Ho
    Feb 27, 2015 at 22:10
  • $\begingroup$ @MarchHo Is jan23 earlier than jan11? $\endgroup$
    – user14640
    Feb 28, 2015 at 17:19
  • $\begingroup$ @Dane Jan 23, 2012 vs Jan 11 2015? $\endgroup$
    – March Ho
    Feb 28, 2015 at 17:40
  • 1
    $\begingroup$ @MarchHo A great mistake of us. $\endgroup$
    – user14640
    Feb 28, 2015 at 17:42
  • 2
    $\begingroup$ Hi, as long as the newer question has an answer, it is accepted to close the older question. There are many such instances in meta. $\endgroup$
    – One Face
    Mar 17, 2015 at 14:15

3 Answers 3


The Fisher's principle is not applicable to the fetuses because it has been formulated for parental expenditure and basically states that the ratio of male to female parents (implying that both parties have reached the age of fertility) will tend to 1:1.

There are several mechanisms that we can use and that are mentioned in the canonical paper by James (2007):

(a) there are equal numbers of X and Y chromosomes in mammalian sperms
(b) X and Y stand equal chance of achieving conception
(c) therefore equal number of male and female zygotes are formed, and that
(d) therefore any variation of sex ratio at birth is due to sex selection between conception and birth.

James brings much evidence that none of these conventional beliefs is true. Rather, they are dependent upon many factors: exposure to stress during pregnancy, glucose level etc. He reports that there is an excess of males at birth in almost all human populations, and the natural sex ratio at birth is usually between 1.02 and 1.08. However, the ratio may deviate significantly from this range for natural reasons. (I really recommend reading the paper I linked, it is available for free).

Branum et al (2009) analyze birth statistics in the US taking into account many factors like ethnicity, gestational age and plurality and show that the ratio can increase even more with growing gestational age and has different values among different races.

So, taking everything together, I can say thay YES, the chances for human fetuses to be male are indeed higher.

  • 4
    $\begingroup$ Note there is a slightly higher mortality rate for males, which tends to bring the ratio towards 50:50 at reproduction age. $\endgroup$
    – mgkrebbs
    Jan 23, 2012 at 21:54
  • $\begingroup$ "the ratio of male to female parents (implying that both parties have reached the age of fertility) will tend to 1:1" Does this equal to that female parents and male parents would have equal mean amount of offsprings? But this is obvious claim! $\endgroup$
    – rus9384
    Nov 1, 2018 at 17:08

The following paper(1) studies the trends in male to female ratios among newborns over the period 1950-1994 in 29 countries (20 major European countries, USA, Canada, Japan, and others). In all countries at any time point, the male to female ratio was always higher than one. In some countries (Mexico and a few northern eastern European countries) the male proportion has been decreasing over the 1950-1994 period, whereas in southern Europe and Australia it has been increasing. Overall, the ratio has remained constant.

The authors mention several factors that could contribute to the decrease of males. Among those factors are:

  • hormonally induced ovulation
  • maternal nutrition
  • environmental factors (like alcohol exposure of the fathers)

On another note, the increased male to female ratio in newborns could be explained with the size of the sex chromosomes. The X chromosome contains ~155 million base pairs, whereas the Y chromosome contains ~58 million base pairs. Thus, the X chromosome is heavier than the Y chromosome, and a sperm cell carrying the 22 autosomal chromosomes +Y chromosome will swim faster and have a slightly higher chance of fertilizing the egg. This explanation is purely my speculation.

  1. Trends in male:female ratio among newborn infants in 29 countries from five continents
  • $\begingroup$ Could you provide some reference for the last paragraph, ie. that the ratio might be dependent on the weight of Y & X chromosomes? $\endgroup$
    – kasia
    Feb 16, 2012 at 17:46
  • $\begingroup$ @kate The last paragraph is my speculation. I can only support it my the reference I gave for the length of the sex chromosomes. $\endgroup$ Feb 16, 2012 at 18:15
  • $\begingroup$ I see; thank you. It sounds really interesting, though. $\endgroup$
    – kasia
    Feb 16, 2012 at 18:30
  • $\begingroup$ @Gergana Vandova: then you should probably state it is just your guess in the answer. Right now it looks like a very (unsubstantiated) bold claim. $\endgroup$
    – nico
    Sep 14, 2012 at 6:34

there is some evidence that the female reproductive system may also do some gender based selection and skew the chances of one or another outcome depending upon environment, if I recall the snake work correctly.

I found a recent (2002?) review: West SA, Reece SE, Sheldon BC. 2002. Sex ratios. Heredity 88: 117–24.

Furthermore, in some of these cases individuals appear to show extremely precise control of their offspring sex ratio. For example: (a) in the Seychelles warbler (described above), females vary the sex ratio of their offspring from 90% female to 80% male depending upon environmental conditions.

It isn't so easily predictable what the outcome is for a given environmental set of conditions. To me it is not surprising since sperm competition and selection in the oviducts is common to nearly all animals.


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