What exactly happened on board the Costa Concordia? How did a luxury cruise go so wrong? Expertise in the psychology of risk perception, the explanation for why our perception of and response to risk sometimes seems pretty irrational. We’re too afraid of some things, and not afraid enough of others, which sometimes leads to new risks all by itself. David Ropeik is a Harvard instructor and author of “How Risky Is It, Really? Why Our Fears Don’t Always Match the Facts.”