Do brothers have more brothers?
March 23, 2011 12:14 AM   Subscribe

StumpedDadFilter: Do boys have more brothers or more sisters?

Asking for a curious 7 year old boy who has brothers. Is it more common for boys to have brothers, sisters, both or neither?

I'd imagine this varies by location, we are in Australia but I would be happy to have any pointers to an answer anywhere in the world.
posted by paulash to Human Relations (31 answers total) 1 user marked this as a favorite
 
I do not think there is any correlation at the biological level: each conception is an independent event, so my first approximation answer would be that for any child, it is slightly more probable to have a sister than to have a brother. There might be sex-related factors playing a role in the time between conception and birth, but I'm guessing any effect would be small. I am also not explicitly not considering cultural factors (e.g. state-mandated limits on number of children), which I think are well-known to skew the boy/girl ratio, and any effect from different infant mortality rates.

Note that talking about such questions from a statistical/probability viewpoint is a classic slippery slope, mostly due to difficulties with the phrasing and interpretation. Watching people with a poor grasp of formal probability theory and statistics argue with people who like to prove they are smart on counterintuitive technicalities is a classic internet trainwreck.
posted by Dr Dracator at 12:38 AM on March 23, 2011


More sisters, due to an essentially trivial cause.

For all families in which there is at least one boy yet more girls than boys, every boy in that family will have more sisters than brothers.

However, for all families with an equal number of boys and girls, any boy will also have more sisters than brothers because those boys do not count themselves as brothers.
posted by jamjam at 12:53 AM on March 23, 2011 [8 favorites]


Half remembered facts (there are probably askme rules against such things - but I'm new!)
Diet (maternal mainly I think because of it's ability to change vaginal ph) can (some believe?) affect the likelyhood of conceiving a boy or a girl.
And I remember something about how soon after ovulation conception takes place having an effect (not sure which way - I think it's if the sperm get there quickly, presumably more likely in people who have sex often, it's more likely to be a girl. But I might be biased, see below).
So presumably if a couple eat roughly the same and have the same pattern of making love throughout the years they are more likely to have a same sex family.
How you explain that to a 7 year old is your next question...
(I have one sister, I have three daughters, my sister has one daughter. But now I have a grandson!!)
posted by sianifach at 1:14 AM on March 23, 2011


I think that it is neither more brothers or more sisters.

The odds of having a subsequent brother or a sister with each new sibling is 1:2. Each new sibling represents an independent biological event, like each flipping of a coin is independent of the result of the previous x number of throws.

With two additional siblings, the odds of having:

2 brothers = 1:4
1 brother, 1 Sister = 2:4
2 sisters = 1:4

With three additional siblings

3 brothers = 1:8
2 brothers, 1 sister = 3/8
1 brother, 2 sisters = 3/8
3 sisters = 1:8

You might be thinking that this essentially says that there would be more boys than girls in the world, but taking into the account the 1:2 odds of having a girl initially, it ends up being even.
posted by clearly at 1:15 AM on March 23, 2011 [1 favorite]


I remember reading (I think it was in the book 'Adam's Curse') something about the fact that there seems to be a genetic disposition in some families for there to be more boys or more girls. Sounds silly - if there are more boys on the father's side, and then you add in the mother, doesn't that cancel it out - but it seems that it doesn't work that way, and that it is about more than just statistical odds in many cases.
posted by Megami at 1:40 AM on March 23, 2011


Both, because the majority of people on Earth live in the developing world, where it is de rigeur to have huge numbers of children, increasing the likelihood of any child having both brothers & sisters.
posted by UbuRoivas at 1:40 AM on March 23, 2011


Looking at it generically, boys have more sisters.

1. In the overall population, there are more females than males. So, in families with more than one child, the likelihood is greater that a boy will have more sisters than brothers (especially since he himself is not counted as his own brother).

2. In cultures where males are more valued than females, even discounting infanticide and selective abortion, a family that has multiple girls would "keep trying" for boys, so those boys would have more sisters, whereas a family with multiple boys would not be likely to purposefully add girls to the mix.
posted by amyms at 2:02 AM on March 23, 2011


jamjam nailed it. Assuming 1:1 chance of gender, 50% of a couple's children will be male, and 50% female.

Let's say that's 2 boys and 2 girls. Each boy then has 1 brother and 2 sisters, ipso facto more sisters than brothers. Q.E.D.

Now, of course, that's just basic math, and doesn't take into account biological factors like hormones and whatnot, which are probably what you're really asking about.

Still, if the population is stable at around 50/50, and there are families with, like, 8 boys and 1 girl, there would likely be corresponding families with 1 boy and 8 girls, or else the stats would skew dramatically in places like Hilldale, Utah.
posted by Sys Rq at 2:48 AM on March 23, 2011


I don't think you can discount the cultural factor. There are many places on earth where it is preferred to have a son, and many individuals even in more "progressive" societies who still place importance on having at least one son. I myself know a few families where the first child(ren) were girls, and they "kept trying" so they could have at least one boy.

So, globally, I would imagine there are a non-trivial number of parents out there who, if the first child is a son, will stop reproducing (boy with no sibs), while if the first child is a daughter, will keep trying for the son (boy with only sisters, perhaps multiple ones).
posted by drlith at 4:05 AM on March 23, 2011 [2 favorites]


For all families in which there is at least one boy yet more girls than boys, every boy in that family will have more sisters than brothers.

jamjam nailed it. Assuming 1:1 chance of gender, 50% of a couple's children will be male, and 50% female.

However, for all families with an equal number of boys and girls, any boy will also have more sisters than brothers because those boys do not count themselves as brothers.
-- jamjam


Let's say that's 2 boys and 2 girls. Each boy then has 1 brother and 2 sisters, ipso facto more sisters than brothers. Q.E.D. -- Sys Rq
FYI: this is totally wrong. If gender rates were 50/50 and random each boy would be equally likely to have a brother or sister follow him or precede him. So families with an older brother and a younger brother are just as common as families with an older brother and a younger sister.

Think about it this way: imagine all the two-child families out there. There are 4 different possibilities: bb, bs, sb and ss, each would be equally likely. There are 4 bothers, 2 of which have a sister, and 2 of which have a brother.

The mistake people are making is not isolating the independent variables. For families with n siblings, it's the same as having a random bit string of length n, or looking at coin flips. If you flip a coin once, the next flip is totally independent of the prior filp.


---
Anyway, the actual gender ratio isn't 50/50, in fact girls are slightly more common, so a boy is slightly more likely to have female siblings then male siblings. And the same is true for girls: girls are slightly more likely to have a sister then a brother.
posted by delmoi at 4:15 AM on March 23, 2011 [2 favorites]


In other words, everyone, regardless of their own gender, is (slightly) more likely to have a female sibling. Their own gender has no baring on that probability.
posted by delmoi at 4:18 AM on March 23, 2011


delmoi, you seem to have missed the point: You are not your own brother. While children may be distributed (roughly) 50/50, siblings are not.
posted by Sys Rq at 4:26 AM on March 23, 2011 [1 favorite]


Ooops, I double checked and actuall more boys are born then girls, so everyone is more likely to have a brother then a sister, at least in early childhood.
posted by delmoi at 4:27 AM on March 23, 2011


(Putting on my genetics hat):

Technically more boys are born than girls.

This is according to the CDC (Centers for Disease Control, Atlanta GA).

Why? Because the Y chromosome is much smaller than the X chromosome, and sperm with the lighter genetic load actually swim faster.

Other nations noted in the report have a larger number of boys born (China, Phillippines) likley due to sex-selection before birth.

And others have noted there are more women than men alive in the US, due to the higher mortality rate.

But for the OP, there does seem to be (this is anecdata) that there is some kind of trend that couples with multiple boys will continue to have boys until they have a girl, and then stop, or vice-versa. Or simply give up after having 3 (or 4 or 5) of the same sex. Will have to do more research on that one...
posted by scooterdog at 4:31 AM on March 23, 2011


delmoi, you seem to have missed the point: You are not your own brother. While children may be distributed (roughly) 50/50, siblings are not.
Nope. Look at the list again, I'll give them names to make it clearer

BB: Robert and George
BS: Steven and Samatha
SB: Amber and Jaden
SS: Katrina and Olga


So there are 2 boys with 1 brother each: Robert and George. There are two boys with zero brothers: Steven and Gaden. So the probability for a boy of a boy having a brother is 50%.

While children may be distributed (roughly) 50/50, siblings are not.

Totally incorrect. Siblings are distributed with the same 51.2/48.8 ratio as people in general
posted by delmoi at 4:32 AM on March 23, 2011 [2 favorites]


delmoi, you seem to have missed the point: You are not your own brother. While children may be distributed (roughly) 50/50, siblings are not.

Think about it this way: Of all the families with two kids, half of the older male siblings have a brother and half of the younger male siblings have a sister. Similarly, half of the younger male siblings have a brother and half have a sister. So out of the total male population, half of the sons have a brother, and half have a sister.
posted by Johnny Assay at 4:35 AM on March 23, 2011


Okay, a bit more searching came up with this link, looking at the odds of having a girl after the first, second or third boy, and the converse, and there is a slight bias away from a strict 50/50 chance.
posted by scooterdog at 4:43 AM on March 23, 2011 [1 favorite]


delmoi is right and I am an idiot
posted by Sys Rq at 5:00 AM on March 23, 2011 [1 favorite]


I may also be an idiot: Figure 2 here shows that the average number of children in households in the developing world hovers between 2 & 3: not unlike the cliched "2.5 kids" of the West.

This would not, however, account for deceased siblings, eg in cases of infant mortality. Does the question mean specifically living brothers &/or sisters, or any siblings born?
posted by UbuRoivas at 5:21 AM on March 23, 2011


Is it more common for boys to have brothers, sisters, both or neither?

In 2006-07, 10.1 million or 65% of the adult population in Australia (in private dwellings, excluding very remote parts of Australia) reported that they have had natural children. Of these people, 41% reported that they have had two children, and 39% reported that they have had three or more ( table 17).

Based on that, it looks like it's most common for Australian boys to have:

1. brothers only/sisters only (although this tie could probably be broken by finding out Australian gender frequencies)
2. both brother(s) and sister(s)
3. no other siblings
posted by 23skidoo at 5:28 AM on March 23, 2011


Let's not forget that the probabilities all change if you know on which day one sibling was born. I kid you not.
posted by KirkpatrickMac at 6:30 AM on March 23, 2011


My experience suggests the answer is "this question is wrong." Grandpa Parker had 6 brothers and no sisters, Grandpa Allen had 2 brothers and 11 sisters. And, thanks to divorce and remarriage, I can truthfully say that neither of my brothers has anything other than sisters (one has two sisters, one has three; I have two brothers and three sisters, and my mother used us as word problems with her math students.)

Also: one has to take into account the odds of boys, specifically, having no siblings, which thank-you, China, messes up the statistics quite a bit.

(Assuming we mean "in the world today, if I chose a random 18-year old male, is he more likely to have brothers, sisters, both, or be an only child?")
posted by SMPA at 6:38 AM on March 23, 2011


This question reminds me of the boy or girl paradox.
posted by selton at 6:53 AM on March 23, 2011 [1 favorite]


While children may be distributed (roughly) 50/50, siblings are not.

Totally incorrect. Siblings are distributed with the same 51.2/48.8 ratio as people in general


drlith brings up a good point above about gender of existing children affecting the choice of having more children. Think about a society in which each family may only have one boy, by law, and after that they have to stop producing. The overall population would still be 50/50, but no boy would have a brother:

Random set of bits (0=boy, 1=girl): 0, 1, 1, 0, 1, 1, 0, 0, 1, 1

Family 1: Adam (stop)
Family 2: Beth, Cathy, Dan (stop)
Family 3: Ellen, Felicia, Gary (stop)
Family 4: Hank (stop)
Family 5: Isabelle, Jen...

So all girls have between 0 and 1 brothers, and no boys have brothers, even though the girl/boy ratio is close to even (6/4).
posted by burnmp3s at 7:05 AM on March 23, 2011 [1 favorite]


"At 30 June 2010, the sex ratio of the total population for Australia was 99.2 males per 100 females. At age 0, the sex ratio for Australia in 2010 was 105.3 males per 100 females."
posted by DevilsAdvocate at 7:25 AM on March 23, 2011


Megami is right that there's a chapter about this in "Adam's Curse" by Brian Sykes. He has some good ideas as to the cause of there being non-50/50 ratios in some families. I don't know if the ideas have been empirically tested.
posted by Knowyournuts at 8:24 AM on March 23, 2011


What a good question.

After you are done teaching your child college-level statistics and population distribution, I hope you take this wonderful opportunity to talk about how there are lots of different combinations of families in this world, and there is no "right" answer. Boys can have brothers, sisters, step- or half-brothers and sisters, or none. They can also have a dad and mom, just a dad or just a mom, 2 of each, or none. (etc, ad infinitum). Doesn't matter, they are still families and (we hope) they all love each other, just like you love your son.

On another track, if you think it would be fun you could use it as a research project - have you son make a chart of all the families that he knows (relatives, friends, schoolmates), write out the data, analyse it on a big chart and come to his own conclusions. (Being careful to talk about sample size and cultural bias).
posted by CathyG at 8:27 AM on March 23, 2011 [4 favorites]


To those saying that it is more sisters because the boys themselves are not counted, take the case of three child homes:

ggb = 2 sisters, 0 brothers
gbg = 2 sisters, 0 brothers
gbb = 2 sisters, 2 brothers (one each per boy)
bgg = 2 sisters, 0 brothers
bgb = 2 sisters, 2 brothers (one each per boy)
bbg = 2 sisters, 2 brothers (one each per boy)
bbb = 0 sisters, 6 brothers (two each per boy)

Out of the seven possible combinations of three child families with at lest one boy, there are 12 boys and 9 girls (because we are excluding ggg families) and the boys have, cumulatively, 12 sisters and 12 brothers. The bbb family makes up for the difference.

But if the question was "do boys have more brothers or more sisters in families with at least one boy and one girl" then you would be right, boys would have more sisters in families with more than two children even though the chance that a given sibling was a sister would remain 50%.
posted by Nothing at 9:04 AM on March 23, 2011


I’ve done a lot of looking into this, because I did a few projects on the sex ratios of beetles, and of course people always wanted to talk about the sex ratios of humans instead.
1) Boys are more common at both conception and birth – the ratio at birth is about 51%. Male fetuses are more likely to spontaneously abort, and male children are more likely to die before reaching adulthood, which skews the ratio back the other way by adulthood.

2) The data are robust across time and culture that there is a predisposition for a given pair of parents to produce boys or girls. The two datasets which come to mind are one from 19th century Europe, and another from the US 1970 Census. The older dataset (Geissler 1889) looked at hospital records in Saxony for all families with 12 children (several thousand data points), and demonstrated that there were too many families with all boys or all girls, and too few families with an equal ratio, to be explained by each birth being an independent event. (The data are in the Sokal and Rolf statistics text if you happen to have one.) The US Census dataset showed that with each additional son in a family, the probability of the next child being a son went up. For a US family with 4 sons in 1970, the odds that the next child would be a son was 57.3%.

3) Why? Because the Y chromosome is much smaller than the X chromosome, and sperm with the lighter genetic load actually swim faster. This is not entirely true. While Y-bearing sperm swim faster, X-bearing sperm survive longer in the female reproductive tract. Y-bearing sperm are only advantaged if conception is close to ovulation.

4) There are a LOT of factors that go into human sex ratios, but they may eventually be overwhelmed by deliberate selection for male or female babies.
posted by endless_forms at 4:09 PM on March 23, 2011 [3 favorites]


All good answers - I'm learning a lot. As for how much the 7yro boy is learning though... ?
posted by doub1ejack at 5:27 PM on March 23, 2011


Lots of good answers, thank you all. Doubly appreciated as the question was vague, although not deliberately so.

I will report back to my son (but may skip the discussion of vaginal pH at this stage) and will suggest a class project.
posted by paulash at 8:37 PM on March 23, 2011 [1 favorite]


« Older Academic Librarian or Digital Librarian?   |   In an iSCSI context, what do LUNs mean? Newer »
This thread is closed to new comments.