Schoenemann, P. Thomas. 1997. An MRI Study of the Relationship Between Human Neuroanatomy and Behavioral Ability. Dissertation, University of California, Berkeley.
Abstract
Hominid brain volume increased
more than 3-fold in ~2.5 million years. From an evolutionary
perspective, natural selection for one or more behavioral abilities
is the most likely explanation. A large number of studies have
demonstrated that brain volume is related to performance on
standardized intelligence tests. The present study expands on
this work in three important ways. First, a sibling-pair design
is used, allowing for the calculation of independent within-family
(WF) and between-family (BF) correlations. Second, a broader
set of cognitive dimensions (specifically guided by an understanding
of human evolution) is investigated. Third, neuroanatomical
components are quantified from higher resolution 3D MR images (whole
brain volume resolution of 1.3 mm3). The subject group consisted of 36 pairs of
sisters.
The results for BF correlations
between neuroanatomical and cognitive variables are consistent with
previous MRI-based studies, but WF correlations are almost entirely
negative or close to zero. The BF correlation of brain volume
to the first principal component of 11 cogntive tests (1st PC) is
r=0.46 (p<0.01), but the corresponding WF correlation is only
r=-0.05 (NS). Social interest variables show a similar pattern,
while throwing ability and maturation rate are unrelated to
neuroanatomic variation both BF and WF.
This pattern is not easily
explained by methodological or statistical artifacts. This
study also shows that family differences in socio-economic status
(SES) do not explain this pattern, because SES is not correlated with
brain volume. It is therefore highly unlikely that the genetic
correlation between neuroanatomy and behavior is zero. Possible
explanations are: 1) Behavioral variability is genetically correlated
only weakly with brain volume, precluding reliable detection in
relatively small sample sizes; 2) The types of tests used might not
assess behavior important for hominid brain evolution; 3)
Within-family environmental effects (e.g., sibling competition) may
mask any underlying genetic correlation. This is consistent
with the finding that individuals more than four years apart from
their sister show a correlation between brain volume and 1st PC of
r=0.65 (p<0.023, N=12). Those closer in age only show a
correlation of r=0.15 (NS, N=60). Furthermore, several
cognitive tests are negatively correlated with WF age
differences.