30 CHAPTER ONE
study (Woollett and Maguire 2011) comparing test- takers, the researchers
measured the hippocampi of those who took the test at both the beginning
and end of their studies. Drivers who passed the test showed an increase in
posterior hippocampus volume, whereas those who failed did not.^16
This research program has been hailed as evidence that the adult brain
can change based on the “demands its owner places on it” (Schwartz and
Begley 2002, 252), but it also suggests a number of limitations to plasticity.
First, when it is understood in morphological terms, plasticity is con-
strained by the limited neural real estate available in the brain. Adding
connections in one area seems to reduce them in others; in the initial study
by Maguire et al. (2000), for example, the anterior region of the hippo-
campi of taxi drivers appeared to shrink in volume as the posterior region
increased. Second, more connections or greater volume does not equate to
greater ability. The taxi drivers in the second study, surprisingly, performed
less well than bus drivers on a battery of tests that measured their ability to
acquire new spatial information (Maguire et al. 2006). Finally, not every-
one gets to be a taxi driver. Some of the test- takers pass and others fail (and
some hippocampi increase in volume and others do not). Woollett and
Maguire (2011) speculate that the deciding factor could be genetic dispar-
ities between individuals. Those who were successful, they suggest, could
“have had a genetic predisposition toward plasticity that the nonqualified
individuals lacked” (2113). While the developmental research cited earlier
suggests that plasticity is not equally distributed among areas within a sin-
gle brain, here plasticity is proposed to be a biological advantage afforded
unequally to persons.
There are many aspects of plasticity research I have not addressed.^ But
even in this very brief description, it should be clear that plasticity is con-
ceptualized, measured, and enacted in multiple ways. While functional or
synaptic plasticity as tied to learning and cognition is easily described in
global and unlimited terms, the morphological or structural plasticity of
the brain can seem to have a stratified economy, being unevenly distributed
across various stages, regions, and even groups of persons. The proposed
temporal, spatial, and genetic variability of plasticity, along with the brain’s
proposed selective vulnerability to various social influences, suggest that its
implications for human agency are far from straightforward. A brain that
expresses a universal configuration of developmental plasticity at adoles-