PROMOTING POTENTIAL 301very big (2–5 percent in most studies). Moreover, changes to cortical vol-
ume and thickness may not necessarily refl ect learning. Th e tissue changes
may simply refl ect the metabolic demands of heightened neural activity.^21
In an impor tant sense, all assimilation of cultural tools in humans,
from early socialization to specifi c skill learning, is “enrichment train-
ing.” As mentioned in chapter 6, studies on taxi drivers, violin players,
jugglers, and others all found that training was refl ected in gray matter
changes in cortical areas. It follows that individual diff erences in brain
volumes and associated intelligence might simply refl ect diff erences in
access to such cultural tools, not in the potential for learning as such.
Th ese studies all imply direct cause- and- eff ect relationships arising
from environmental deprivation, in a simple model. But they are compli-
cated by other fi ndings. For example, it now seems clear that brain devel-
opment is specially protected against deleterious eff ects by physiological
buff ering. Th e concept of “brain sparing” is based on evidence from epi-
demiological studies in humans and experimental studies in animals.
Starvation in adults is refl ected in loss of body mass and reduction of
other organs, whereas the brain and cognitive functions remain relatively
untouched. Likewise, studies have revealed that malnourishment or other
deprivations during pregnancy yields, as expected, off spring whose phys-
ical growth is retarded. Yet the brain growth seems to be much less af-
fected: the ratio of brain weight to that of other organs, and general body
weight, increases.
A physiological basis for brain sparing has been well established. It in-
volves neural refl exes that ensure re distribution of blood fl ow away from
other parts of the body to essential organs, including the brain. However,
the issue is not free from controversy: some studies have suggested long-
term brain and cognitive impairments, at least for severe deprivations in
pregnancy. Timing of the deprivation in relation to critical periods of
brain development may be crucial.
Fi nally, this category includes surveys of very general environmen-
tal experiences in humans. In these studies, mea sures of putative envi-
ronmental factors are taken and then correlated with cognitive and/or
educational test scores. Th e research has ranged from small- scale obser-
vational studies to large- scale surveys involving thousands of children. Th e
methods have mostly used parental questionnaires; parent- completed
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