Someone, however, who can reliably predict another person's behavior is likely a reliable judge of that other person's motivation.
You are using the word "likely" in a manner that is inconsistent with the probability calculus.
Often times, multiple people will reliably predict that a comment will be downvoted, though each attributes a different motivation to the downvotes. They can't all be "likely a reliable judge" of that motivation. The respective probabilities of their being correct have to sum to less than one, so all but one (at most) is probably wrong.
A response essay written by Eliezer Yudkowsky posted at Cato Unbound for the issue Brain, Belief, and Politics:
Is That Your True Rejection? by Eliezer Yudkowsky
The lead essay has been written by Michael Shermer:
Liberty and Science by Michael Shermer