Adam Gopnik reflects on what makes a happy marriage. Darwin, Gopnik writes, when first thinking about marriage, made a list of pros and cons. Cons included the expense and anxiety of children and the odd truth that a married man could never go up in a balloon.

On the plus side, he noted, marriage provided a constant companion and friend in old age and, memorably, that a wife would be better than a dog.

Gopnik's own formula for a happy marriage is lust, laughter and loyalty.

Via Samuel Beckett, Monty Python and The Big Lebowski, Gopnik concludes that loyalty is a much-underrated quality. Loyalty is not, he argues, a passive state that holds two people together when all else has failed.

Rather, he explains, loyalty is a wholly active state, as a new family dog has demonstrated. Dogs are there, he writes, "to remind us that loyalty is a jumpy, fizzy emotion - loyalty leaps up at the door and barks with joy at your return, and then immediately goes back to sleep at your side".

Producer: Adele Armstrong.