September 24, 2013

The odds that you will give birth to a boy or girl depend on where in the world you live

Research over hundreds of years has consistently found that boys naturally outnumber girls at birth. The speculation is that this is nature’s way of countering the relatively high mortality rates of males, and creating more of a gender balance in the population. While historically, there have been about 105 boys born for every 100 girls worldwide — which creates a “sex ratio at birth” of 1.05 — the share of boy babies has increased in recent decades. 2011 data from the World Bank show the global sex ratio at birth is now 1.07, or 107 boys born for every 100 girls.

FT_sex-ratios-at-birthThis increase in the sex ratio is driven largely by births in China, where sex ratios have declined slightly in recent years but remain the highest in the world. The world’s most populous country has 118 boys for every 100 girls, and accounts for 12% of births worldwide. However, disproportionately large shares of baby boys are found in other countries scattered throughout Asia and the Caucasus, as well. Azerbaijan, Armenia, Georgia, South Korea, and the Solomon Islands round out the list of places with the highest sex ratios. India is tied with Macedonia, Montenegro, Papua New Guinea, Samoa, Serbia and Suriname for 7th place, with a ratio of 108 boys born for every 100 girls.

Even while some countries seem to have a disproportionate share of boys, others have particularly high shares of baby girls. While there are still slightly more boys born than girls in these places — which are centered in sub-Saharan Africa — the sex ratios are nonetheless much lower than average. The six countries with the lowest sex ratios include: Rwanda, Sierra Leone, Togo, Zimbabwe, Ivory Coast, and Madagascar.

So what explains these differences in the shares of baby boys and girls?

Perhaps the best-known reason relates to the practice of sex-selective abortion, which has been identified in Asia, and in the Caucasus, as well. The ability to determine fetal sex, along with strong son preferences, accounts in large part for the high shares of boys in many countries in these regions. The desire to limit family size, either due to government regulations as in China, or due to global social and economic changes that have reduced the need for large families, seems to further contribute to sex-selective abortion and a dearth of baby girls.

But this is only one of myriad factors that may be affecting the sex ratio at birth. Some research suggests that the share of newborn boys declines with older parents, and that the high share of girls in Sub-Saharan Africa may be linked to the practice of polygamy (multiple wives). What do these two phenomena have in common? Researchers hypothesize that both situations are associated with less frequent intercourse. (For possible explanations of this association, see this article from the academic journal Human Reproduction.)

On the opposite end of the spectrum, most research shows that the share of baby boys increases during and after wartime. And once again, frequency of intercourse is cited as the likely reason, at least in the case of post-war reunions.

In some cases, though, wartime is associated with a higher share of girl babies. This may relate to the fact that heightened maternal stress, such as would occur during wartime, can lead to an increased likelihood of a miscarriage which is more likely to occur among male fetuses.

Further evidence of this association between maternal stress and the increased share of girl babies emerges from an analysis of births in the vicinity of the Kobe earthquake in Japan, which showed that the share of females born nine months after that event was significantly higher than would have been expected under normal circumstances.

Topics: Gender, Birth Rate and Fertility

  1. Photo of Gretchen Livingston

    is a senior researcher focusing on fertility and family demographics at Pew Research Center.


  1. John Tallamy2 years ago

    Throughout the world and all down the ages, the proportion of male to female births remains the same. The odds of such statistics happening by chance must be billions to 1 against. This makes nonsense of the evolution theory and points to intelligent design.

    1. Ariel2 years ago

      I fail to understand that logic. How exactly does that disprove natural selection? If you take biology into effect, males have the same number of x chromosome and y chromosome carrying sperm cells. So statistically speaking for each birth there should be exactly 50% chance for either sex.

  2. Joanne M2 years ago

    I dont think having a female is related to polygamy. Think female foetus are stronger than male, and so in Africa, the world poorest countries, they would survive in the womb longer.

  3. Dennis Blewett3 years ago

    This is a great article that argues about the statistics of having a boy or girl. Even though there have been statistical arguments based on biology, this article shows that biology is not the only predicting factor prior to birth: Social and economic reasons play in. It can make a person question the reality of statistics and predictive factors.

    – Dennis F. Blewett Jr. of Rockford, Illinois

  4. warwick4 years ago

    this is very elementary arithmetic and you have got it wrong.
    Imagine 100 couples each have a child one year.
    There will then be 50 male children and 50 females.
    The next year the parents of the female babies try again.
    There will then be another 25 male and 25 female children.
    As long as the birth rates of females and males re the same, the balance of male and female births will remain constant.

    1. tatiyanna2 years ago

      god made more boys than girls

  5. bluebeard4 years ago

    There is an obvious reason for skewed ratios in government and economically caused small families, that never seems to get mentioned. Your decision to have a second child (there really is no universal single child policy in China anymore) is dependent on what sex your first child is. Let pretend that every couple that has a male as a first child stops having children. Lets also assume that every person that has a female first child goes on to try for a male second child. Lets also assume the chance of having each sex is 50%, for simplicity’s sake. Within these assumptions, you’d end up with 2/3’s males and 1/3 females. Just one more contributing factor.

    1. Shirley Adams2 years ago

      That’s no true. It would still be 50/50. One-half boys and one-half girls.

      1. jay2 years ago

        Yeah probability doesn’t change. Still 50/50. You are assuming that having a girl the first time increases the probability of having a boy the second time to 100%

  6. tater tot4 years ago

    Very nice piece. It’s fascinating to learn some explanations for high girl ratios since the attention is so often focused on the opposite issue (for good reason, of course). The relationship between high coital frequency and a tendency toward boy births makes me wonder if non-union or unplanned births then have a greater tilt toward female, since sexual frequency would be presumed to be lesser. And during periods of high fertility booms, such as the American 1950s, is the male SRB higher?