One of the
questions I commonly receive about MRI research on pedophilia is: How
do we know what causes what? Comparing the brain scans of
pedophiles with non-pedophiles gives us a correlation, and correlation does not
necessarily mean causation.
It is true
that correlation does not equal causation, but that does not mean we are
powerless about it either. To understand how to attack this kind of
problem, we need to recall a few important (but pretty rarely discussed) principles
of science:
First, no
scientific statement will ever be 100% proven. There is no such thing as
a perfect study. That is, anyone will always be free to reject any
result, claiming the lack of perfection of whatever study(ies).
Next is
parsimony: Because we cannot prove anything, the best science can do is
explicate each of the possibilities and to pick the best answer available. In
science, the best means the most parsimonious; that
is, the simplest explanation for the data.
Finally,
we need to remember that there exist only three possible interpretations of a
correlation:
- X
causes Y.
- Y
causes X.
- Some
third variable, W, causes both X and Y.
Although
it is true that we cannot conclude that X causes Y directly from a correlation,
we can come to that conclusion indirectly, using a process of
elimination to show that the other two possibilities are inferior (less
parsimonious) explanations for the correlation. This is best shown by
concrete example:
For folks who do not
already know about them, my team and I have found that pedophiles differ from
teleiophiles (i.e., "adult-ophiles") in terms of their IQ
and memory test scores, physical height, handedness, history of
school grade failures, history of head injuries before (but not after) age 13,
and on MRIs (in large regions of white matter).
In
conducting our work, we purposefully focussed on variables that had a very
specific property: We chose characteristics that happened or were
set before the sexual offenses occurred. Although we can
calculate only a correlation between handedness (for example) and having
committed sexual offenses, we know that handedness is present even before
birth. (Fetuses have a preference for which thumb they suck in utero, for
example.) So, even though we have only a correlation, we know that sex
offenses cannot cause handedness. Thus, of the three causal
possibilities, we can already rule one out. It was this logic that led us
to study height (cannot be changed by commiting an offense), experiencing
childhood head injuries (can't go back in time), etc.
So, we now
have two possibilities: X causes Y, or some still unknown variable causes both
X and Y. The still-unknown variable explanation is still in the cards, but
it is a much less parsimonious explanation. That is, although it is certainly
remains possible that some third variable causes both the brain differences and
pedophilia, there is no known behavior capable causing the white matter
differences that we detected. So, for the "third-variable"
explanation to be true, we would have to posit the existance (on the basis of
no evidence) of factors unlike any ever reported before. So, despite that
that is possible, it is not the most parsimonious explanation.
So, that's
where we are today. We do not have proof of causality, and we never
will. (It isn't as if we will ever be able to randomly assign subjects to
groups.) Rather, X causes Y is the best explanation we
have.
And that's
the best a scientist can hope for.
