Why Are More Boys Born Than Girls?

Thanks to both Justin Stoler and Debbie Fugate for linking me to a Christmas Day story from BBC News on the sex ratio at birth. I'm not sure if there was any significance to its publication yesterday, but the headline is a question: Why are more boys than girls born every single year?  This happens all over the world and has been going on for as long as people have been collecting such information. It is well accepted that the "normal" sex ratio at birth is 105 boys born for every 100 girls born. Why?The short answer is that we really don't know for sure. The first theory discussed in the article is the evolutionary theory, although here we can only describe what we see, without knowing for sure why the pattern has evolved as it has.

The article doesn't really articulate the evolutionary theory very well, so let me quote from the 12th edition of my text, first from Chapter 5 (p. 158):

The most basic health difference between males and females is that males have higher death rates than females from conception to the very oldest ages. Seemingly to compensate for this, more males are conceived than females. Fetal mortality is higher for boys than girls, but there are still typically more males born than females. Infant and childhood mortality rates are higher for males, with a roughly equal number of males and females being reached, quite conveniently from an evolutionary perspective, in the prime reproductive ages of the late teens and early twenties. After that, the only bump in the road for females compared to males is high maternal mortality, and by the older ages we can almost always expect to find more women than men.

And then from Chapter 8 (p. 305):

Despite the concern about the high and even increasing sex ratio in some countries, there is still the underlying question of why the “normal” sex ratio is not simply 100. The answer is that no one really knows (Clarke 2000). This is perhaps a biological adaptation to compensate partially for higher male death rates (or vice versa, since we also are not sure why death rates are higher for males, as I mentioned in Chapter 5). In fact, data on miscarriages and fetal deaths suggest that more males are conceived than females, and that death rates are higher for males from the very moment of conception. Thus, some of the variability in the sex ratio at birth could be due to differences in fetal mortality. But we aren’t sure why those differences exist, either. Research done as part of the human genome project suggests a role played by the X chromosome, but that is still just a guess (Gunter 2005).

That pretty well sums up what we know. The patterns are clear--the reasons for those patterns are not.