“Isn’t it strange that they are so different?” people often say to me of our twins, in response to the way they look, behave with strangers, move, eat, babble or laugh. But I do not find it strange at all. Why should twins be alike? I suppose there are some genetic reasons which we do not yet understand for why identical twins may be more likely to be alike than other siblings, but for non-identical boy and girl (or any non-identical) twins, who genetically are no more alike than siblings, this seems a significant assumption.

It is this assumption and its possible consequences I find interesting. One explanation is that when you mention twins most people think of the identical type, not remembering that this cannot be possible with a boy and a girl – an unpractised mental leap. That aside, our experience tells that most people expect any twins to be alike and, what is more, are surprised when they are not.

Do we expect siblings to be alike? When they are older perhaps and have the grooming of shared experience, but even then we do not go out of our way to express incredulity when they are not. So why then twins, and particularly as babies?

I wonder whether this will continue throughout their childhoods and what impact this may have. The natural rebel would suggest that you are tempted to try much harder to be different when everyone thinks you should be alike. So, I suppose the least I can do is never assume that they will be similar in anything, and always offer to make two birthday cakes each year, whatever their favourite may be.