What is the difference between disposition and predisposition?

What exactly is someone's disposition? And is it different from a predisposition? A person's disposition is his or her usual mood or attitude. Are you typically pretty happy? You could be described as having a happy—or cheerful, or sunny—disposition. Animals have dispositions too; a dog with a nervous disposition doesn't easily relax into a restful pup curled up at someone’s feet. In this use, disposition is a synonym of temperament; both words refer to the complex set of attitudes and inclinations that guide behavior.

Disposition can also mean "tendency" or "inclination," and in such cases it has a surprising synonym: predisposition. A disposition to exaggerate is the same as a predisposition to exaggerate. A disposition toward humility is likewise the same thing as a predisposition toward humility. The fact of being "in advance" that the prefix pre- implies hardly matters when tendency and inclination are concerned, since both concern what is likely to happen in the future.

While phrases like "a disposition to cooperate" are about as common as "a predisposition to cooperate," when the context is medical, predisposition is far more common. Phrases like "a genetic predisposition to nearsightedness" appear much more frequently in published, edited text than similar phrases employing disposition.

Predispose usually means putting someone in a frame of mind to be willing to do something. So a longtime belief in the essential goodness of people, for example, will predispose us to trust a stranger. Teachers know that coming from a stable family generally predisposes children to learn. And viewing television violence for years may leave young people with a predisposition to accept real violence as normal. The medical sense of the word is similar. Thus, a person's genes may predispose her to diabetes or arthritis, and malnutrition over a long period can predispose you to all kinds of infections.