Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: CliffordK on 15/11/2012 09:39:35
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There are many families with two children, both male, or both female.
So...
Does the sex of the first child affect the probability of the sex of the second child?
I.E.
If the first child is a girl, is it more or less likely, or just the same that the second will also be a girl.
Likewise, if the first child is a boy, is it more or less likely, or just the same that the second child would also be a boy.
If the sexes of the children were truly randomly distributed, one would expect for 2 child families:
M M
M F
F M
F F
Thus, giving half of the families with one boy and one girl, and a quarter with two boys, and a quarter with two girls, or half having two of the same sex.
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Does the sex of the first child affect the probability of the sex of the second child?
Birth order seems to influence sexual orientation in males ... http://en.wikipedia.org/wiki/Fraternal_birth_order_and_male_sexual_orientation
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Not quite what I was thinking... but interesting nonetheless.
I was just wondering if certain couples would be predisposed to have mostly girls, and others predisposed to have mostly boys. But, probably as I mentioned above, one just notices the half of the families that would have two of the same sex.
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I was taught that for any given pregnancy, a boy is slightly more likely because the gender is determined by the man's sperm, and since the Y chromosome is smaller and lighter, sperm carrying the Y chromosome can get to the egg slightly faster than those carrying the X chromosome, so getting an XY combination is slightly more likely, leading to more males.
Whether the reasoning is right, the statistics bear out that there are more male babies than female: it's roughly 51% male and 49% female at birth in the US.
I was able to find one study that I haven't had time to read, but it probably answers your question:
http://www.isds.duke.edu/~dalene/chance/chanceweb/144.rodgers.pdf
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Thanks JP.
Like my sister, many mothers and fathers believe that a tendency to have boys or girls runs in a family. Informally, we have noted that belief among many of our friends. Pregnant women appear particularly interested in and amenable to the notion that sex composition runs in the family. But research in the statistical and cognitive psychology literature suggests that humans are notoriously bad at distinguishing systematic patterns from random patterns. Even if the sex selection process is purely by chance, some parents will have all boys in families of size 1, 2, 3, or even 10 or 12. For example, mothers of four children who have all boys or all girls must naturally wonder if something systematic contributed to their “unusual sex composition.” Yet, around 1/8th of all four-child families are expected to be a same-sex family under a chance model, not an especially unusual occurrence.
The paper does cite an 1889 German paper that did seem to find a slightly higher than expected number of very large families with all girls or all boys.
The paper does conduct a study, of 6089 families, but apparently not enough to really reach statistical significance.
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Looking at table 2 in the paper, it appears as if the number of homogeneous BB & GG two child families is lower than would be expected. This may actually be significant. However, the cause may be completely unrelated to genetics. It is quite possible that families with a boy and a girl might be more likely to limit their family size to two, while those with two of the same sex might be more likely to have a third (or more) children. This may have also skewed the 18
89 German study of 12 child families.
Looking at only the first two children if two or more child families (table 3), the pattern isn't as apparent, except for a slightly lower than expected rate of homogeneous first two girl families.
Does Hollywood like to present all girl or all boy families? Perhaps that also affects our preceptions.