[angryprofessor]

So you are proposing that chemicals cause aggresion in rats, and aggression in humans, but for a different reason? Seems unlikely to me.

Double blind experiments do show causation (ignoring metaphysical discussions).

There are 3 possibilities: Chemical -> Behavior, Behavior -> Chemical, or ??? -> (Chemical and behavior).

A randomized trial eliminates the latter two possibilities (since chemical is correlated only to a coin flip). Double blind eliminates experimenter bias.

[curi]

So suppose we are studying sad people, and we randomly give some of them a chemical, in a controlled and double blind way. And those people become happy, and the others don't.

Then, it could be that the chemical somehow causes happiness. Or it could be that the chemical causes memories of one's children, and everyone in the study has happy memories of their children.

So, the causation is still in doubt. Many explanations are consistent with the observed data.

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Yes I am proposing that chemicals correlate with "aggression" in humans and rats for different reasons. But I don't accept that the thing called "aggression" is the same thing in humans, and in rats. To help illustrate why not, consider aggressive behavior by video game characters. Same word, but not the same phenomenon -- in that case, no emotion is present, and the cause is some C code.