EXCEPT THAT NUMEROUS STUDIES DEMONSTRATE...:

My theory is that happy marriages, from the Darwins on down, are made up of a steady, unchanging formula of lust, laughter and loyalty.

The Darwins had lust, certainly - 10 children in 17 years suggests as much anyway - and they had laughter. Emma loved to tease Charles about his passion, already evident in youth, for obsessive theorising.

"After our marriage," she wrote to him early on, "you will be forming theories about me, and if I am cross or out of temper you will only consider: 'What does that prove?' which will be a very philosophical way of considering it."

And loyalty? Well, despite Emma's Christian faith, she stood by him through all the evolutionary wars, and did for him the one thing only a loyal spouse can do - pretend he wasn't in when German journalists came calling.

So, marriages are made of lust, laughter and loyalty - but the three have to be kept in constant passage, transitively, back and forth, so that as one subsides for a time, the others rise.

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