

Subject:
probability of having a boy
Category: Science > Math Asked by: cdabga List Price: $10.00 
Posted:
09 Oct 2005 05:35 PDT
Expires: 08 Nov 2005 04:35 PST Question ID: 578137 
my sister has just got pregnant with twins, she already has a boy. My brother in law believes that the probability of having a girl is 50%. I believe it is higher because they already have a boy. My arguement is based on the fact that before my nephew was born, there was a 50% chance that if they had 3 children, they would have a boy. his arguement is that the caclualtion of the probability has now gone back to zero as my nephew being born is a historic event. where can I find a mathmateical arguement to back myself 

There is no answer at this time. 

Subject:
Re: probability of having a boy
From: politicalguruga on 09 Oct 2005 06:07 PDT 
You won't, because your brother is right. There are various factors that determine the baby's sex, so it is not exactly like throwing a dice, but it is still closer to that (in probablity) than to what you suggest: even if you've already had "3" in one throw, it wouldn't prevent you from having "3" in the next (or one of te next) throw. For the record, there are many families where all the siblings have the same sex. 
Subject:
Re: probability of having a boy
From: wwwsurferga on 09 Oct 2005 06:27 PDT 
The probability of the baby's gender depends on when it is conceived, so the percentage from last conception would not come in to play; you would need to recalculate the probability again: here is an article on the subject http://www.amazingpregnancy.com/pregnancyarticles/74.html Also, Here's a form that will predict the baby's gender: http://www.boyorgirl.com/predict3.htm 
Subject:
Re: probability of having a boy
From: myoaringa on 09 Oct 2005 08:29 PDT 
Politicalguruga, You are absolutely right on your last remark: I knew a girl whose brother was the seventh son of a seventh son  not that there weren't some daughters in between (my good luck). Although many native English speakers don't know it, "dice" is the plural of "die" in your context (Würfel), whereas "dies" is the the plural when referring to a die in any other meaning. Myoarin 
Subject:
Re: probability of having a boy
From: kaps_mr2ga on 13 Oct 2005 02:53 PDT 
According to Bayesian stats, it will depend on whether or not the events : Event 1  Having a boy Event 2  Having second and subsequent child are statistically independent or not. If the events are statistically independent  then your brother is right. If the events are statistically independent, then you are right. Regards Kaps 
Subject:
Re: probability of having a boy
From: kaps_mr2ga on 13 Oct 2005 02:54 PDT 
oops spotted a typo ... According to Bayesian stats, it will depend on whether or not the events : Event 1  Having a boy Event 2  Having second and subsequent child are statistically independent or not. If the events are statistically independent  then your brother is right. If the events are NOT statistically independent, then you are right. Regards Kaps 
Subject:
Re: probability of having a boy
From: agmpiniaga on 19 Oct 2005 10:39 PDT 
From the CIA Factbook Sex ratio at birth at birth: 1.06 male(s)/female so the probability of having a boy is 51.4563% This ratio is based on world statistics. In USA the sex ratio at birth is 1.05 male(s)/female so the probability of having a boy is 51.5122% statistics about the world http://www.cia.gov/cia/publications/factbook/geos/xx.html statistics about the USA http://www.cia.gov/cia/publications/factbook/geos/us.html 
Subject:
Re: probability of having a boy
From: randomehga on 19 Oct 2005 17:42 PDT 
Consider this: There are three possibilities for the twins: (as noted above the fact that they already have a child makes no difference) 2 girls 2 boys or 1 boy, 1 girl Assuming that the chance of having a boy is roughly the same as having a girl, there are 2 out of 3 possibilities of having a boy. Therefore the odds are 2/3. Or about 67% 
Subject:
Re: probability of having a boy
From: drhouse19ga on 09 Mar 2006 08:17 PST 
There is a bit of a problem with these kinds of answers: While in the population, the probability of having a boy or a girl is roughly 50/50 (a little higher for boys, a little lower for girls); that in no way implies that a particular person(or couple)has the same likelihood. With no past history, the best guess would be 50% or so. But there is history here. There are some women (or couples)who have a real propensity to have one or the other. That propensity is only revealed by having kids. Having a girl first says something about your propensity to have girls. It doesn't say much, but it does say something. Babies are not pennies, and the biological building blocks are not tossed in the air to mingle together freely. There can be real reasons why a couple would have lots of boys or lots of girls, not just random chance. Having a girl first changes the expected likelihood of having more girls only very slightly. For most people, having a girl reveals nothing real about a physical predisposition. But since some people do have differnences in real propensities, the expected likelihood has to move. If I recall correctly, having a girl first raises the probabilty of having another girl by half a percent or so. Having two girls in a row is stronger evidence of a predisposition, so it moves it around 3% I believe. None of that says a woman who has had 3, 4, or 5 in a row of anything cannot have the other. Random chance cannot be eliminated as being responsible for an unusual chain. Nor does a real physical predisposition mean the other sex is impossible. I am friendly with a family that had a girl after seven boys. The chances that they have a predisiposition toward boys are high, but not complete. Random or real, they have their girl. Also, with twins you have alot more to consider. Just because there are three possible outcomes, it doesn't follow that they have the same probability. Identical twins will be of the same sex, fraternal twins can be the same, or opposite. There are five possible outcomes (BB Identical, GG Identical, BB Fraternal, GG Fraternal, and BG Fraternal) If identical twins are more or less common than fraternal twins, you have to take that into account. What is true in average is not necessarily true in each case, but only experience tells you if you have a different likelyhood. Consider this: you are going to be far more likely to have another set of twins after this than would a woman who had not yet given birth, but less likely than a woman who had twins the first time. There ya go. dhouse 
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