found significantly more grey matter in the posterior hippocampus than in the matched control group
the posterior hippocampus is the part of the brain that is associated with the development of spacial and navigation skills in humans and animals
the longer they (the taxi drivers) had been doing their job, the more pronounced the structural difference was - positive correlation
pros:
Control group allows us to say there is a significant difference (research design), use of scientific objective measurements (MRI), and an attempt to study a real-world phenomenon.
Cons:
We can't be sure the difference is due to knowledge - they weren't tested before, only on an already existing difference.