A mythical city contains 100,000 married couples but no children. Each family wishes to “continue the male line”, but they do not wish to over-populate. So, each family has one baby per annum until the arrival of the first boy. For example, if (at some future date) a family has five children, then it must be either that they are all girls, and another child is planned, or that there are four girls and one boy, and no more children are planned. Assume that children are equally likely to be born male or female.

Let p(t) be the percentage of children that are male at the end of year t. How is this percentage expected to evolve through time?

I worked this little riddle out for myself also so it's probably fairly easy...

Right unusually let me tell you the answer first:-

p(t) ≠ f(t) p(t) = 50%

That is to say the percentage of children that are male is not a function of time and is always 50%. This may seem a little counter intuitive as we know that at some point in time a family could conceivably have 10 girls and one boy, but this is balanced by the fact that half the families will have no girls at all.

The table below is drawn up using the simple rule that of the families who have a new girl one year all will try for a new baby, with half of them having a boy and half having a girl.

End of Year 1 Year 2 Year 3 Year 4 Year 5 Year Infinity

Total Boys 50,000 75,000 87,500 93,750 96,875 100,000

Total Girls 50,000 75,000 87,500 93,750 96,875 100,000

New Boys 50,000 25,000 12,500 6,250 3,125 0

New Girls 50,000 25,000 12,500 6,250 3,125 0

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