temperament

Blame Latin.

Like the related noun temper (which most often refers to a person's tendency to become angry, but also has a neutral sense very close to that of temperament, among other meanings) temperament traces back to a Latin word, temperare, which means "to mix or blend." The a in temperare lives on in the modern spelling of temperament.

Temperament is an old word (it dates to the 15th century) with multiple meanings, but in modern use it typically refers to the usual attitude, mood, or behavior of a person or animal. If you're adopting a dog, you might look for one with a sweet temperament; you might describe someone who is often nervous as having a nervous temperament.

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.