Testing random

[Laura Creighton]

In a message of Wed, 17 Jun 2015 22:47:37 +1000, "Steven D'Aprano" writes:
>There are magicians who are capable of forcing coins to land the required
>way up, and somebody once built a machine capable of tossing a coin with
>the precise equal force and velocity every single time. Dice are rarely
>unbiased, and neither are roulette wheels. Nevertheless, we ignore those
>factual biases for the sake of simplicity.

Simlplicity is fine.  But if you use this for real world decisions,
such as 'Let's try one more time, I really want a daughter!'
because you didn't know that the simplicity is there, or rather
that what is true in populations isn't true because it is also
true for each individual in a population .. well, that can be tragic.

>Correct. And more Y sperm die than X sperm. Together, these two factors lead
>to a small but real bias towards girl children, at least on countries that
>don't practice wide-spread sex-specific abortion.

It is not clear, last I checked, that 'more Y sperm die' matters, because
most of the time, for reasons we still don't know, the ratio of male
and female sperm produced is not 50/50 either.  So it may matter.  But
maybe not.

>> But when you look at breeding age men and women you will find 
>> that the ratio is a lot closer to 50/50 than the birth age ratio.
>> Human beings, as a population currently produce more males than
>> females.
>
>Surely that depends on where (and when) you are?
>
>I know that, today, both India and China both abort far more female fetuses
>than male ones, leading to a large excess of men. But in countries that
>don't practice selective abortions, my understanding is that there is a
>small excess of women at virtually all ages, especially among the elderly.
>Men tend to die earlier than women in every age bracket.

No.  Before breeding age there is an excess of males.

I am excluding the notion of abortion here.  (Another reason why humans
make lousy experimental subjects for this sort of study.)  What I am
saying is that a whole lot more boy babies get born than female ones.
If infant mortality was sex-neutral, we would expect that, when we
took a census of 16-year-olds we would find that there continued to
be an excess of boys at that level, too.  Instead, we have a small
excess of females, enough to believe that:

       For populations of human beings, when they hit breeding age, it
       is desirable to have a sex ratio near 50/50 but with a small
       excess of females.

'Desirable' here just means that this is the answer that pops out when
you study real populations (or model them with computer simulations).

And to get this Desirable outcome, you have to produce many more male
babies than female ones.

Laura