I know I'm being pedantic here, but in psychology, negative motivation is when you take away something averse as a reward for a certain behavior. Negative here relates to the fact you're negating or removing something to act as motivation, whereas if something is positive, you're adding something (like a treat for a dog). What a lot of people refer to as "negative motivation" is actually positive punishment because you are adding something aversive (like a spanking) in response to a behavior you want to decrease.

It's important to keep this in mind when looking at scientific literature so that you know exactly what they're talking about.