Abnormal is a deviation from ‘norm.’
But when it comes to human behaviour, no norm qualifies as the statistical mean
Physics, yes. Nature yes. Human behaviour, No.
We can talk about Social acceptability here – what is normal for society. But even that varies from culture to culture and country to country. So there are no clear and objective benchmarks for normal.
On the other hand, social abnormals have been the deviations we have treasured and celebrated (apple ad) – Artists, Scientists, Poets, Philosophers.
We don’t start from the middle of the bell-curve and deviate out. We start from who we are.