Probability problemSuppose that it is equally likely for a pregnancy to deliver a baby boy as it is to deliver a baby girl. Suppose that for a large society of people, every family continues to have children until they have a boy, then they stop having children. After 1,000 generations of families, what is the ratio of males to females?
:)[macho (9-10 0:42, Long long ago)] [ 传统版 | sForum ][登录后回复]1楼

probability不变。。因为总是一男一女生小孩。。

这样只会导致男女鄙视失调罢了。。[帅温柔 (9-10 0:53, Long long ago)] [ 传统版 | sForum ][登录后回复]2楼

HeeheeP(1 M) = 0.5
P(1 F 1 M) = 0.5 ^2
P(2 F 1 M) = 0.5 ^3
...

E(Male) = 0.5 + 0.5 ^ 2 + 0.5 ^ 3 ... = 1
E(Female) = 0.5 ^ 2 + 2 * 0.5 ^ 3 + 3 * 0.5 ^ 4 = 1

[香陵居士 (9-10 15:46, Long long ago)] [ 传统版 | sForum ][登录后回复]3楼

(引用 香陵居士:HeeheeP(1 M) = 0.5 P(1 F 1 M) = 0.5 ^2 P(2 F 1 M) = 0.5 ^3 ... E(Male) = 0.5 + 0.5 ^ 2 + 0.5 ^ 3 ... = 1 E(Female) = 0.5 ^ 2 +)掉了个省略号E(Female) = 0.5 ^ 2 + 2 * 0.5 ^ 3 + 3 * 0.5 ^ 4 ... = 1[香陵居士 (9-11 8:51, Long long ago)] [ 传统版 | sForum ][登录后回复]4楼

IMHO this pattern of reproduction doesn't exhitbit any kind of preferencefor or against a particular sex of new born babies. in other words, if families choose to give births in this way, it doesn't mean that they prefer boys over girls.

consider this thought experiment:
in a remote village, people believe that all pregnant women ARE MEANT TO give births to girls only. the moment any pregnant woman gives birth a boy, it is considered that she is cursed and thus is not allowed to give birth anymore. if a woman has a perfect record in the past being able to deliever girls repeatedly, then she is allowed to give birth again and this will only stop once she gives birth to a boy.
after 1000 generations, what is the ratio of males to females in this village?

what i was trying to say with my above thought experiment is that: if you say the birth pattern discribed by you shows any preference for boys, same pattern can be argued to show an appreciation for girls too, which is contradictory. and in fact, personally, i don't think the pattern says anything about sex preferences.

香陵居士's calculations showed that if all family obey the rules, then on average, each family would expect that they will have one girl and one boy.

i first met this problem in my Science Foundation module in year 2001, i was from nus. the problem was raised by a math professor, he used this example to conclude that sex preference doesn't hurt, it is totally natural. the increasingly serious gender gap in China is not a result of traditional gender preference for boys, "it is because of the most recent use of pre-labour screening and illegal abortions ". he said something like sex preference was by no means agains nature course, there are so many more baby boys in china nowadays is because they (people in china) killed female new born babies, and killing is agains nature. I was extremely upset by the way he phrased the issue, he actually stressed the word "killed"! there are many chinese students in the lecture, and many more for all over the world.

i had fierce arguments with him over emails after class, but the prof insisted that he's got the point, and end up telling me, abortion is killing. but that's not my point. my initial intention is try to dissuade him from including this example in the next semester and in the semesters to come, and said it is an inappropriate example and is a very sensitive issue as well. HOwever, He did not take my suggestion.
3 years later, he was my HYP sup. zao wu nong ren. i did not mention the whole incidence to him ever again, he seemed to have forgotten it too, for I was rewarded with an A grade for my HYP in the end. whether he really remembered it or not, i never knew. [stevevai (9-11 11:01, Long long ago)] [ 传统版 | sForum ][登录后回复]5楼

(引用 stevevai:IMHO this pattern of reproduction doesn't exhitbit any kind of preferencefor or against a particular sex of new born babies. in ...)Hehe, long story!!!Nice point! preference doesnot make sense to the nature.
But the abortion/killing does.
[macho (9-12 22:23, Long long ago)] [ 传统版 | sForum ][登录后回复]6楼