In [this](http://arxiv.org/abs/physics/9902072) paper, there's the following sentence:

> 
> ...and the factor 1/2 takes into account that the dipole moment is an induced, not a permanent one.
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> 
> 

Without any further explanation. I looked through Griffiths' electrodynamics to see if this was a standard sort of thing, but couldn't find anything. I was thinking it might be because the field of the dipole itself opposes the inducing field, but that doesn't quite seem right for some reason.