How Do They Know Which Direction The Causation Runs?

This yahoo science story carries the implication that some people are more sociable because they have a greater amount of grey matter "in the orbitofrontal cortex (the outer strip of the brain just above the eyes) and in the ventral striatum (located in the center of the brain)."

But who is to say that some people don't have more grey matter in these areas because they are more sociable? If, as would be the case under Thomistic metaphysics, the soul is the form of the body, then the direction of causation assumed by "modern science" is precisely backwards.

A scientist is quoted in the article:

"Sociability and emotional warmth are very complex features of our personality. This research helps us understand at a biological level why people differ in the degrees to which we express those traits," he said.

But why couldn't one just as well say,

"The structures in the brain supporting sociability and emotional warmth are very complex features of our neurophysiology. This research helps us understand how the personality of the soul causes people to differ in the degrees to which such traits are given a material expression via brain structure."

To the credit of the quoted scientist, he does understand that such correlations do not actually prove the direction of causality:

However, Murray noted that this research is "only correlational and cross-sectional" and "cannot prove that brain structure determines personality. It could even be that your personality, through experience, helps in part to determine your brain structure."