Why do we have more male infants born?

Question

For every 104 male babies, we have 100 female babies and there are statistics which are more reliable. (Those tell us that we have 997 female infants born per 1000 male infants born.) What is the reason for that? Why do we have more male babies than female babies?

Answer 1

Note: This is based on literature searches I've done a while ago out of general curiosity. I'm in no way an expert on human reproduction.

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First, I'm not sure if you are asking about evolutionary reasons or the developmental causes for for a difference in sex ratio. Here, I will focus on the developmental causes.

There is much evidence for a male bias in human sex ratio at birth (secondary sex ratio), and the ratio have been shown to change over time, e.g. as in 20th centrury Europe after WWI and WWII (see data in Gellatly, 2009),in relation to environmental or parental factors (see e.g. Jacobsen et al, 1999), or between ethnic groups.

A bias in sex ratio can be due to (at least) three different mechanisms (Irving et al (1999)):

1. deviations in X/Y-ratio of sperm
2. selection of sperm within the female reproductive tract
3. biases in implantation success and/or survival for embryos of different sex

I guess that success of fertilization/zygote formation could maybe be considered a separate step (between 2-3). From what I've seen, there is little evidence for explanation one, see tests in e.g. Irving et al (1999) and Graffelman et al (1999). Graffelman et al (1999) also tests for an effect of male age, but do not find support for this idea.

A problem is that it is extremely difficult to study these factors under natural conditions (practically and ethically). Interestingely, there is also evidence for an excess loss of male fetuses in recognized pregnancies (Ingmarsson, 2003, Boklage, 2005), and this would indicate that the sex ratio close to conception is even more skewed towards males than the sex ratio at birth. This is even more curious with the background of an unbiased sex ratio in sperm (explanation 1 above). However, it is very problematic to study what happens in the first part of pregnancies, since a number of weeks have usually passed before the pregnancy is even recognized. I haven't found any papers reporting the human primary sex ratio (sex ratio at conception), which could be affected by factor 1 & 2 above.

Even so, the most likely explanation seems to be a strong bias during embryogenesis, as mentioned in Boklage (2005):

The usual excess of males is present through pregnancy beyond clinical recognition, but not present at fertilization. The difference, therefore, must arise between fertilization and clinical recognition, through a preferential loss of females during embryogenesis.

The same paper also points out that fewer than 25% of natural human fertilizations go full term (probably even less), and 2/3 of these fail before the pregnancies have been clinically recognized. By back-calculating from the secondary sex ratio and the excess of failed male pregnancies, the sex ratio close to conception has been estimated to 125-135 male:100 female (Pergament et al, 2002))

The exact causes for such a bias during is not entirely known from what I've seen. However, Boklage (2005) mentions a couple of different explanations. For instance, among mammals in general, the earliest stages of embryogenesis proceeds more rapidly in male embryos:

Since some of the products of these earliest stages are signals from one part of the embryo to another, or from the embryo to the maternal physiology, signals which are required for continuation and maintenance of the pregnancy, the establishment of viable, clinically recognizable pregnancy is correspondingly more efficient in general for male embryos (Krackow, 1995; Kochhar et al., 2003).

The same issue is also mentioned in Ingvarsson (2003):

Studies in recent years indicate that sex differentiation begins at conception. The SRY gene on the Y-chromosome is already transcribed at the 2-cell stage and triggers growth acceleration in the XY embryos. This accelerated growth is believed to be important for the male embryo as it allows complete testicular differentiation before the levels of oestrogenic hormones become too high as pregnancy progresses.

Boklage (2005) also mentions genomic imprinting and chromosome X–Y segregation as possible explanations for a bias (see references in paper for more).

So, in summary; there is evidence for an male bias in secondary sex ratio (sex ratio at birth), even though there is usually a excess of failed male fetuses in recognized pregnancies, which means that the male bias must be even higher very early in pregnancies. There is some evidence that this might be due to a higher success of embryogenesis of male fetuses, but the evidence isn't strong and the issue is difficult to study.

The evidence provided by these results is indirect. That is unlikely to change. Ethical, technical and financial considerations argue against the destruction for karyotyping of statistically sufficient numbers of products of natural human fertilizations. Granted, any bias of the sort we seek to discover and understand might, after all, operate differently, or not at all, in sham fertilizations of hamster oocytes, versus oocytes from artificially induced human ovulations, versus naturally fertilized human oocytes. The best evidence we have, or are likely to have in the foreseeable future, indicates that the consistent excess of males observed in human births does not originate in a consistent bias in spermatogenesis or in fertilization. (Boklage, 2005)

Finally, it should also be mentioned that the bias in sex ratio at birth can be partially due to sex-selective abortion and infanticide, but to what extent is not entirely known (see Wikipedia: Sex-selective abortion).

You might also find this related question on the heritability of sex ratio interesting: The gender of offspring of Twins?.

Answer 2

There is an article in the Journal of Popular Science from 1885. However, I do know there is at least one more recent article floating around some where since I read it. At the moment, I can't find it but will update if or when I do.

The article goes on to state that during times of scarcity the number of male births outweighs the number of females whereas in times of plenty, the number of female birth outweighs the number of males. I will let you read linked article but give you the information I recall from the more recent article I can't locate at the moment.

The reason for the differences in births has to do with the lifestyles men and women lead. Men tend to do more dangerous activities at younger ages which causes unnatural deaths early on. Due to these unnatural deaths, in times of scarcity, the reproducing population converges to 1:1 (not exactly) male to females due to the lifestyle choices of men. It was also hypothesized that women are almost guaranteed to find a mate so having less females during rough times wont hurt the population since they should all find a mate.

Additionally, the phenomenon of environmental factors influencing male and female births is well documented in alligators. The first linked article also goes on to examine this occurring with other species as well.

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Thanks to @Anne, who sent me a discover article, we have the name of two sets of researchers on this topic. Here is an exert from the article Famine Can Tilt the Sex Ratio of Future Generations. But Why? (Discover, Nov. 2013). Additionally, we can look up the articles by the respected researchers now. I believe the research by Robert Trivers and mathematician Dan Willard will be the most applicable.

While demographers were struggling to understand sex ratio anomalies in the context of culture, evolutionary biologists had largely embraced an idea put forth in 1973 by biologist Robert Trivers and mathematician Dan Willard. The Harvard-based pair theorized that as the physical condition of a female declines — if she’s nutritionally deprived, for example — she’ll tend to produce a lower ratio of male to female offspring. Evidence of the theory came from red deer and humans; in both species, adverse conditions in the mother’s environment during pregnancy are correlated with a shift toward female births.

Although natural selection ideally favors a 50/50, or .500, sex ratio in a population, mammals typically produce slightly more males than females. Because sex ratio is biased toward males, the figure is expressed by dividing male births by total births. It’s estimated that women give birth to 3 percent more boys, for a standard .515 sex ratio (with 48.5 percent female births). When fewer boys and more girls are born than that, it’s described as a sex ratio decline.

Evolutionary biologists say male mortality, which is overall higher than that of females, explains the male bias in sex ratio: A slightly skewed sex ratio at birth that favors males ensures that there are roughly an equal number of males and females of reproductive age. (Theoretically, a .500 sex ratio at birth may be possible if the gender difference in mortality is eliminated.)

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Again thanks to @Anne, who provide this link as well, we see that men do outpace female births. We can see the world data here.