Selecting a mate is the most crucial decision of our lives. We spend a huge amount of time and energy trying to find that special someone. Our appetite for a relationship fuels a billion-pound industry of matchmaking services. Yet we’re often not satisfied. A 2005 survey of more than 900 people who had been using online dating services revealed that three-quarters had not found what they were looking for. We seem as much in the dark as ever about who is a suitable match. As a scientist studying human behaviour, I am not too surprised by the mysterious nature of...