Dear Economist,

I have twin sons. One gets cheap beer and cigarettes as a soldier in the army but he is teetotal and does not smoke. The other is a milkman who drinks too much beer and smokes like a trooper. Should I tell them to swap jobs?

-- S.M., Sheffield

Dear S.M.,

I can’t believe that the milkman’s bills for booze and fags would fall by enough to justify retraining for the army, unless a soldier’s life has always appealed to him for other reasons.

Still, you are onto something here. Soldiering is a dangerous activity, and so is smoking and excessive drinking. You should be asking yourself whether your portfolio of sons is appropriately diversified.

Let’s say that soldiers have a 5 per cent chance of dying in their 30s, while boozy smokers have a 20 per cent chance of dying in their 40s. (Both figures are illustrative only.) For simpler maths, also assume that your sons are otherwise indestructible.

As the situation currently stands, your chance of losing both sons is 5 per cent times 20 per cent, or 1 per cent. The chance of both surviving to the age of 50 is 95 per cent times 80 per cent, or 76 per cent.

Imagine you persuaded the solder to quit and the milkman to take his place. Now your teetotal son will certainly make it to 50. The chance of losing one of your sons has not increased, since your dissolute son will only get the chance to drink himself to death if he survives army life.

This is a rare thing: a genuine free lunch for you as a parent.

By getting your sons to swap jobs you eliminate the chance of losing both without raising the chance of losing one. How you plan to persuade your sons to play along is quite another matter.

This week’s Economist is Tim Harford, www.timharford.com.

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