This looks highly interesting, and the sort of article I was looking for during the last few months. Empathy can be over- or under-measured, suggesting that empathy and the emotion one empathizes with are quantifiable things that can be over- or under-estimated by somebody. There are real answers to questions about emotion and empathy that don't reduce to "it's all an illusion" or "you can't measure emotion" - there are real world phenomena one can point to, like these mirror neurons. Better still, we've identified a mechanism that makes cooperative behaviour not only possible but akin to a secondary sense (empathy) obtained by inferring from the primary senses (seeing, hearing, etc.), getting one step closer to explaining how altruism evolved and how it works.