Which preposition would a native speaker use in the following sentence?
He was genius at/by birth.
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
I'm not sure I agree with StoneyB's analysis.
To me, by birth usually implies that a trait is conveyed on the person by their family - for example:
Prince Charles is Duke and Heir to the throne by birth.
In this case John is a Duke and Heir to the Duchy of Cornwall because the title is hereditary and they were born into the correct position in their family to automatically be granted the title:
This contrasts with
Alfonso XIII became King at birth
Which states that Alfonso XIII became King at the time he was born, without requiring that the reason he became king at birth be because of a hereditary link. For example, he could have become King because his mother was Queen, and she died during childbirth. The causal link here being between the death of his mother (which happened at the time he was born) rather than because he was born per se.
The at refers to the events being linked in terms of time without requiring a causal link. By on the other hand implies causation.
In the specific case of the question at hand, "he was a genius at birth" is perfectly valid, although it is always meant in a figurative sense. The child probably wasn't a genius (in the sense that an adult might be a genius) "at" the point when he/she was born, but the implication is that the person was unnaturally smart even from a very young age.
That being said, the term "genius at birth" and "genius by birth" are both quite unidiomatic. A native speaker would be much more likely to describe someone who was extraordinarily smart or intelligence at a very young age would more likely be described as a child prodigy.
He was a genius at birth (note that you must use the article a) is very unlikely; it would mean "He was already a genius at the moment he was born".
He was a genius by birth is probably what you intend: it means that he was a genius by virtue of something inherent, possibly genetic, rather than by virtue of his efforts and studies.