Sibling gender (biological sex at birth)?

[elam84]

I hope this isn’t a stupid place to ask this question..
Is there any evidence of patterns for sibling gender (biological sex at birth)? E.g. likelihood of having a boy after a girl, girl after boy, two of the same, and any further patterns with more than two siblings?

Note: I put sibling gender as saying “sibling sex” sounded … not ok. Even though it’s the correct term. Please don’t crucify me.

No specific reason for the question, just curious if there’s any actual evidence that isn’t purely anecdotal. Thanks in advance!

 

[comicbookguy]

@elam84 I looked this up as I always thought I saw more girl /girl boy/boy families than mixed.

You are as likely to have a second of either gender immaterial of your first.

However it APPEARS that you are more likely to have the same.
Because in your mind there are 3 options for 2 kids
G/G
G/B
B/B

2 of these scenarios involve 2 kids of the same gender which makes you think this is more common.
But the statistics say not. If I can find the link I’ll add it.

 

[alvinou]

@comicbookguy This can't be right, you've missed off an option

G/G B/B G/B B/G

So there are 2 options the same and 2 options different - still appears 50/50

 

[testoffaith]

@alvinou In this instance, whether the boy or girl is born first is irrelevant; both B/G and G/B count as mixed gender siblings.

 

[cqw]

@testoffaith Yes, but since there are two different ways to get mixed sex siblings, it is twice as likely. About half of sibling sets will be male/female, a quarter female/female and another quarter male/male.

Same with flipping a coin, H and T for heads and tails. You can get H/H, H/T, T/T, and T/H.

It is twice as likely to get a mixture (edit: twice as likely as either both girls or both boys).

 

[deafmommy]

@cqw It's not twice as likely because there are also two different ways to get same sex siblings. Just look at the coin flip scenario in your comment. There are four possible outcomes, each having equal probability. Two result in a mixture and two do not. Therefore you are just as likely to get a mixture as not.

 

[cqw]

@deafmommy Yep yep, sry I think I was missing your intention/meaning above.

What I meant is that a mixed set is twice as likely as just girls, and a mixed set is twice as likely as just boys.

 

[cutin]

@cqw What? No it isn't. It's only twice as likely to get a mixture as it is to get both boys OR both girls, individually.

It is just as likely to get a matched pair as a mixture.

 

[cqw]

@cutin Yep, that's what I was trying to say, apparently not very clearly! Half mixed, quarter all girls, quarter all boys is the breakdown. I added in an edit above, probably still misleading but that's all the time I have between diapers!

 

[alvinou]

@testoffaith Then B/B and G/G both count as same gender siblings so we're back to 50:50

 

[deepblues36]

@alvinou Is this the Monty Hall problem?

 

[cutin]

@deepblues36 Kind of. If you already have a girl, then G/B and G/G are the possibilities. B/B is not a possibility.

It's always 50/50 (well 51/49 as somebody pointed out).

 

[deepblues36]

@alvinou Is this the Monty Hall problem?

 

[comicbookguy]

@alvinou No because there would also in that case be
G/g g/g b/b b/b g/b b/g

So out of the 6 options for a 2 kid family 4 involve 2 of the same sex.

 

[alvinou]

@comicbookguy What are you talking about

Simplify to you have a boy first
Your options for number 2 are girl or boy

If you have a girl first your options for number 2 are still girl or boy

There are 4 possible options not 6

 

[cutin]

@comicbookguy Why have you included g/g and b/b twice each?

 

[christian99]

@comicbookguy It’s better to look at it like “SAME” or “DIFFERENT” which is two options instead of 3. Whatever sex the first child is, there’s a 50/50 chance the second will be the same or different.

 

[katrina2017]

@comicbookguy I think we need a statistician.

 

[crocodilehuricane]

@elam84 My understanding is until recently studies generally found no pattern, but more recently when focussing on the male line the evidence has emerged that there is some correlation for sex being passed down along the male line- men with more brothers will tend to have more sons and visa versa.

So the only pattern I’m aware of would be that if you have two girls already, and there’s lots of sisters along the male line, it’s more likely you’ll have another girl rather than boy (and visa versa).

https://www.sciencedaily.com/releases/2008/12/081211121835.htm

 

[gizmorazaar]

@crocodilehuricane I was the only girl in a male dominated family. Until our generation started having kids. My brothers all had girls, and the eldest brother’s girls have only had girls. I’m the only one with boys. Which I adopted, so … yeah, anecdata strikes again.