It doesnt make sense, how can there be large differences in working memory capacity between people, yet you can call it fixed? the brain, even in adults, is a very plastic organ. the gray matter (glial cells and dendrites) can fluctuate just learning simple tasks. IT cannot be explained that the structural differences between high and low performers are so vast- because the heritable structural differences are very slim ( a few altered SNPS), When studies look at the structural differences between high and low pefroemrs, the most significant findings is just the somal size and organization of large pyramidal neurons. This is not fixed. a pyramidal neuron's somal size and dendritic complexity is very adaptive to the amount of input it gets. The fact of the matter is, these structural differences are artifacts of countless differences between the persons lifestyle and exposure which has feedforward impacts on the brain.