Members of the same animal species – both human and non-human – vary in their behavior across time and space. Some Labradors bark more than others, some sparrows sing less than others, and some children run frenziedly around as others sit still with their toys. Meanwhile and less visibly, individuals also vary in how they process information – their “cognitive styles”. But charting the causal arrows linking these two is no easy task. Cognitive abilities – such as speed in learning about associations, rewards and categories – are tricky things to measure, because, in order to capture an individual’s true ability hidden beneath day-to-day fluctuations, they need to be tested repeatedly. Unfortunately, every new measurement may be influenced by previous ones, as an individual becomes familiar with the task. And if researchers, despite these practical challenges, were to find that a trait indeed correlates with a cognitive ability, such as shyness with learning difficulties, they will have to find a way to exclude the possibility that they both are caused by a third factor, like stress. Griffin and her colleagues discuss how personality psychology can best overcome these hurdles to illuminate why no two Labradors, sparrows or children are the same.