You have made an excellent point.

The problem with the nature/nurture debate is that it eliminates a combination of the two. Certain developmental disabilities such as Down syndrome we know are caused by a brain abnormality. However, many mental disorders have no known cause (such as schizophrenia). Also, homosexuality has no known cause.

A more accurate hypothesis is to view a brain abnormality as a predisposition that when combined with environmental factors (maybe more accurately the person's interpretation of the environmental factors) causes a disorder such as psychopathic behavior.

Excellent point.