The child's temperament is more effective in constraining the development of the opposite profile than in determining a particular profile. The principle that a temperamental bias eliminates more possibilities than it determines also applies to the effects of environments. If all one knows about a group of children is that they were born to economically secure, well-educated, loving parents, one can be confident that they are unlikely to become criminals, psychotics, drug addicts, or homeless beggars, but one cannot predict what they will become. Similarly, among children born in poverty to single parents who did not graduate from high school, it is possible to predict the adult occupations they are unlikely to choose--curator of a museum, Wall Street stockbroker, or cellist--but not those they will select. Imagine a stone rolling down a steep mountain over a five-minute interval. An observer of this scene can eliminate a great many final locations after each few seconds, but not until the final second will the onlooker be able to predict exactly where the stone will come to rest.

source: Jerome Kagan, An Argument for Mind, 184 tags: Destiny, Personality