Is there a difference between these references to a specific person:

> 
> "The implication is clear: as **the** psychologist Jonathan Haidt said..."
> 
> 
> 

and

> 
> "The implication is clear: as psychologist Jonathan Haidt said..."
> 
> 
> 

The first one uses the definite article and is a quote from Daniel Kahneman's book 'Thinking, Fast and Slow', where the article is consistently used in such references. In the second one I've omitted the definite article. It seems more intuitive for me, a non-native speaker, and is something I would use in my own writing.

Are both expressions equally correct? Are there situations that require the article and vice versa?