

Subject:
Probability of third child being male
Category: Science > Math Asked by: otraubga List Price: $10.00 
Posted:
26 Nov 2006 18:09 PST
Expires: 26 Dec 2006 18:09 PST Question ID: 785797 
For a given couple with two children of the same sex, what is the probability that the third child will also be of that sex? This is not the same as: What proportion of threechild families are all the same sex. Please cite sources.  


Subject:
Re: Probability of third child being male
Answered By: pafalafaga on 26 Nov 2006 18:48 PST 
otraubga, Disregard my comment, above, as I found the actual statistics. There is a small effect of gender "running in the family" for a third birth, if the first two children are of the same gender. There is a wellwritten presentation of the data here: http://www.ingender.com/XYU/Odds/Gender_Odds.aspx The Odds of Having a Boy or a Girl and as you can see from two "Previous Children" tables fairly far down in the article, the odds of the third child being the same gender as the first two are: First two are boys  Odds are 53.3% the third will be a boy, vs 46.7% it will be a girl First two are girls  Odds are 54.0% the third will be a girl, vs 46.0% it will be a boy Thus, there is a shift from the usual pattern of births, which is 51% boys, 49% girls. The shift is fairly small, but noticeable just the same. I trust this information fully answers your question. However, please don't rate this answer until you have everything you need. If you would like any additional information, just post a Request for Clarification to let me know how I can assist you further, and I'm at your service. All the best, pafalafaga search strategy  Google search on [ birth gender statistics "first two" ] 

Subject:
Re: Probability of third child being male
From: liviofloresga on 26 Nov 2006 18:56 PST 
Take a look to the following question (and answer of course): http://answers.google.com/answers/threadview?id=465659 
Subject:
Re: Probability of third child being male
From: ansel001ga on 27 Nov 2006 02:44 PST 
Are you viewing this as strictly a mathematical problem where the probability of a boy and girl are both exactly 50%, or as a real life problem where the probabilities are slightly different? If the problem is simply a mathematical exercise: If you are asking what is the probability that the the third (youngest) child will be the same sex as the first two, the answer is 1/2. The birth of the third child is independent of the first two and will either match the sex of the first two or not with equal probability. If however, you selected two of the three children at random, rather than the oldest two and discovered that they were the same sex, the answer is also 1/2 but the reasoning is different. Consider the possible cases. MMM 3 MMF 1 MFM 1 MFF 1 FMM 1 FMF 1 FFM 1 FFF 3 If you select one of the eight possible combinations at random and it happens to be MMM or FFF. The probability that two of the three children selected at random will be the same sex is 1 or 3/3. For the other six combinations the probability that two of the three selected at random will be the same sex is 1/3. So the likelihood of MMM or FFF being selected is three times as likely as one of the other six given that two of the children selected at random are the same sex. Therefore the probability that the sex of the third child (but not necessarily youngest) is the same as the two selected at random is: (3+3)/(3+1+1+1+1+1+1+3) = 6/12 = 1/2 
Subject:
Re: Probability of third child being male
From: steph53ga on 27 Nov 2006 08:30 PST 
Its the man that determines the sex of the child. So, if you have a third child of the same sex, slap your hubby across the head ;) Steph53 
Subject:
Re: Probability of third child being male
From: barnecaga on 27 Nov 2006 14:21 PST 
steph, multiple children of the same gender leads to drastically reduced clothing costs (assuming they don't wear out). also, you can more easily warehouse all of them in the same bedroom instead of giving each one their own room, saving on housing costs. so instead of smacking your husband for a third kid of the same gender, i recommend thanking him. maybe a set of golf clubs. cab 
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