On temperament

Knowledge of a person's temperament allows one to predict with confidence what they will not become, but is less able to predict the persona they will develop. Predicting the exact location where a rock will land after rolling down a mountain provides an analogy. Knowledge of the location, size, and shape of the rock, analogous to a temperament, permits an observer to eliminate many places where it will not rest, but it is impossible to know its final location because one cannot know the gullies and obstacles it will strike on its way down. These events are analogous to the diverse experiences each person encounters as they grow to maturity.
Jerome Kagan in Born worried: Is anxiety all in your genes?