The anthropologist Helen Fisher, in Why we love, has beautifully expressed the insanity of romantic love, and how over-the-top it is compared with what might seem strictly necessary. Look at it this way. From the point of view of a man, say, it is unlikely that any one woman of his acquaint- ance is a hundred times more lovable than her nearest competitor, yet  that is how he is likely to describe her when 'in love'. Rather than the fanatically monogamous devotion to which we are susceptible, some sort of 'poly- amory' is on the face of it more rational. (Polyamory is the belief that one can simultaneously love several members of the opposite sex, just as one can love more than one wine, composer, book or sport.) We happily accept that we can love more than one child, parent, sibling, teacher, friend or pet. When you think of it like that, isn't the total exclusiveness that we expect of spousal love positively weird? Yet it is what we expect, and it is what we set out to achieve. There must be a reason.

Helen Fisher and others have shown that being in love is accompanied by unique brain states, including the presence of neurally active chemicals (in effect, natural drugs) that are highly specific and characteristic of the state. Evolutionary psychologists agree with her that the irrational coup de foudre could be a mechanism to ensure loyalty to one co-parent, lasting for long enough to rear a child together. From a Darwinian point of view it is, no doubt, important to choose a good partner, for all sorts of reasons. But, once having made a choice - even a poor one - and conceived a child, it is more important to stick with that one choice through thick and thin, at least until the child is weaned.

Richard Dawkins: The God delusion,  A Mariner Book, Houghton Mifflin Co., New York, 2008.