We continue to look at idioms using in and on. Editor Ben Korzec writes about these prepositions and their use with the noun expert.
The noun expert is more commonly used with on, a preposition used to indicate the subject of something:
an expert on American history
an expert on copyright law
an expert on viruses
an expert on treating diabetes
In is also often used in the same way but is slightly less common. Both on and in are correct.
However, there is a slight detail in meaning to consider when actually choosing between the two words: in makes the emphasis slightly heavier on the person who has expertise, while on places some emphasis on the subject in which expertise is shown.
My brother is an expert in foreign policy.
We've asked to interview an experton foreign policy.
