Developers read more code than they write, so it would be great if we could just throw source code into an algorithm and have it tell us how complex it is. Therefor Dror made a fun game called Get the Codes (http://getthecodes.com/ seems to be offline though) where 222 developers explained 40 different code snippets. This way the authors could measure the impact of different source code concepts on comprehension.
In total, 2326 snippets were explained, and they found several factors impacting understandability, for example, negation is harder than simpler formulation:
And a loop counting up is easier than a loop counting down! Read the full paper it has a lot more interesting comparisons.