Parent-Adult Child Relations Studied

Having an adult child with problems negatively influences parents' mental health, even if they also have a successful child. This is among the interesting findings from the annual meeting of the American Psychological Association, as reported in USA Today. The article also reports findings from other studies of parent-child relations.

I have to wonder if the researchers have the famous "causal arrow" pointing in the right direction. That is, what if having parents with negative mental health causes some children to have problems instead of vice versa?

Technical note: if it is a correlational study, as it appears to be, X can influence Y or Y can influence X when the two are found to be correlated. For completeness sake, I should add that some third force, Z, might influence both.