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The Odds of Having a Boy or a Girl

You may have heard it from your know-it-all in-law, or a snidely friend who happens to have one-of-each, or maybe even your doctor: after having 2 or 3 boys, you might as well forget about ever having a daughter, because the odds are you're going to keep having boys. Or if you've only had girls, that the odds are stacked against you for having a boy.

Well, my dear, we are about to take a look at data from over 6,000 families in the US, and you are about to become better informed than your MIL on this issue!

A Lot of Babies

The data used here is from "NLSY", the National Longitudinal Study of Youth conducted by the US Dept. of Labor. The NLSY is a nationally representative sample of 12,686 men and women who were aged 14 to 22 in 1979. These people were interviewed annually on all sorts of topics through 1994, and are currently interviewed biannually. (Data is taken from the Chance Magazine article, Does Having Boys or Girls Run in the Family?)

NLSY Families by Number of Children
1 Child1,881 31%
2 Children2,444 40%
3 Children1,301 21%
4+ Children463 8%
Total Families6,089 
Note: There weren't enough families with more than 4 children for statistical analysis. In these families, only the first 4 children were counted.

Total Boys/Girls in NLSY Families
Boys6,389 51%
Girls6,135 49%
Total12,524 

The 51/49 Boy/Girl Ratio

The ratio of 51% boys to 49% girls seen here is representative of overall US birth rates. There are many hypotheses about why there are slightly more boys born each year than girls, but no one knows for sure why this is so. Here are a couple of the theories:
  • Males are more fragile (male infants are less likely to survive their first year, and a man's expected lifespan is less than a woman's). The slightly higher conception rate of males is nature's way of evening out the balance.
  • As Shettles contents, Y-bearing male-producing sperm may have a speed advantage over X-bearing female-producing sperm, more often winning the race to fertilize the egg and resulting in more male conceptions.
Regardless of the reason, the 51/49 ratio remains constant year to year throughout the US population.

Are Boys or Girls Preferred?

The Chance study describes "stopping behavior" -- the tendency of parents to stop having children after having the desired gender or genders. Parents in the US may have a "balance preference", a desire to have one of each gender. Here's what the data shows:

Did the first child's gender influence whether to have a second child?

  • Parents are slightly more likely to stop having children if the first child is a girl.
First ChildParents Having 2nd Child
Boy 70%
Girl 68%

Did previous children's gender influence whether to have a third child?

  • Parents are 6% more likely to stop having children if the first two children are a boy and a girl.
  • Of parents with two same-gender children, parents of two girls are 3% more likely to stop having children than parents of two boys.
Previous ChildrenParents Having 3rd Child
Boy/Boy 46%
Boy/Girl 39%
Girl/Girl 43%
This table shows families with two same-gender children combined.
Previous ChildrenParents Having 3rd Child
Same Gender 45%
Mixed Gender 39%

What about the fourth child?

  • Parents of 3 same-gender children are only 2% more likely to have a 4th child than parents with mixed-gender children.
  • Parents of 2 boy, 1 girl families are the most likely to have a 4th child.
Previous ChildrenParents Having 4th Child
3 Boys 29%
2 Boys, 1 Girl 35%
1 Boy, 2 Girls 20%
3 Girls 26%
This table shows families with same-gender children compared to families with mixed-gender children.
Previous ChildrenParents Having 4th Child
Same Gender 28%
Mixed Gender 26%

Odds of Having a Boy or a Girl

Okay, finally! Let's look at the odds of having a boy or a girl, given that previous children are all of the opposite gender.

Odds of Having a Girl

The odds of having a girl seem decrease after having each boy, but only very slightly. Even after 3 boys, you are only 6.4% more likely to have a 4th boy than a girl.

Previous Children% Girl Births
None 49% Girls
1 Boy 50% Girls
2 Boys 47.7% Girls
3 Boys 43.6% Girls

Odds of Having a Boy

The odds of having a boy seem to increase after having girls, except after 2 girls, when a 3rd girl is more likely.

Previous Children% Boy Births
None 51% Boys
1 Girl 54.5% Boys
2 Girls 46.0% Boys
3 Girls 52.7% Boys

Gender and Birth Order

Given the charts above, it looks like you are slightly more likely to have a boy, regardless of previous children. This is probably due to the overall 51/49 boy/girl birth ratio. This ratio, interestingly, varies slightly with birth order; it isn't consistent among first-borns, second-borns, etc.
1st Born 51.0% Boys
2nd Born 52.2% Boys
3rd Born 48.6% Boys
4th Born 50.8% Boys

Odds of Having an All Same-Gender Family

If there are roughly even odds of having a boy or a girl with each baby, given the laws of chance we should still expect to see some all same-gender families, even in large families. Here is the number of all same-gender families we would expect to see, purely by chance:
Family SizeSame-GenderMixed-Gender
2 Children50%50%
3 Children25%75%
4 Children12.5%87.5%
5 Children6%94%
6 Children3%97%
7 Children1.6%98.4%
Now let's take a look at how this compares with the actual data.

2-Children Families
All Boys 25.8% Actual
25.0% Predicted
Mixed 52.2% Actual
50.0% Predicted
All Girls 22.0% Actual
25.0% Predicted
3-Children Families
All Boys 14.9% Actual
12.5% Predicted
Mixed 73.0% A
75.0% P
All Girls 12.1% Actual
12.5% Predicted
4-Children Families
All Boys 9.1% Actual
6.3% Predicted
Mixed 85.4% A
87.4% P
All Girls 5.5% Actual
6.3% Predicted

Conclusion

Although we often hear the "statistic" that you are 30% or even 70% more likely to keep having the same gender, this is just an old wives tale. It is NOT a fact. The truth is, your odds stay pretty close to 50% for each child and only vary slightly. If you have had 2 or 3 boys, you are only about 2% to 6% more likely to have another boy. If you have had girls, you are slightly more likely to have a boy next.