This is actually an extremely stupid, even fallacious question, it can be taken to mean lack of positive belief in yourself, or religiously as lack of belief in God.
If it is a religious question to Atheists, God is a negative entity so lacking belief in God will add to their motivation to attack those that push God onto the Atheists as if they should believe in God, this is actually the reason that Atheists exist as a reaction to a Negative that they disbelieve in.
So a disbelief in a negative produces a positive motivation.
If I lack belief in my incompetence I will be motivated to be competent. If I lack belief in that my parachute will tangle, I will happily make the jump.
Motivation requires a belief in something. If someone has no belief, then there is no focus, no goal, and no attainable reward. Without such a thing, there is no motivation. Without such a thing, there is just mundane and aimless activity, or lack thereof. Motivation is the belief that if you accomplish something, it will have other affects that you desire. Without belief, then, there is no motivation.
I think people work harder to prove the things that they believe in are true. I think disblief can cause people to sometimes work hard as well, but if you don't believe in whatever it is, I don't think you will have as much motivation as the person who has their ego wrapped up in the belief.
If students do not believe that their efforts are likely to improve their performance, they will not be motivated to work hard. Motivation can be affected, for instance, if a course that has a reputation for being inordinately difficult. Students may also have had discouraging experiences in similar courses or on early assignments in a course that convince them they cannot do the work. Additionally, students have beliefs about intelligence and learning that can affect their motivation. If they believe learning is generally fast and easy (and should not be slow or arduous), they may lose motivation when they encounter challenges. Similarly, if they believe intelligence is a fixed quantity (something you do or do not have, but not something you acquire over time), they may not see the point of extra effort. Finally, if students attribute their success to their innate talents rather than effort, they may not be motivated to work. This can happen whether they believe they possess the necessary abilities (“I’m a good writer; I don’t need to start my paper early”) or lack them (“I’m just no good at math. What’s the point of trying?”)
Strong beliefs motivate action. If you strongly believe in the right to bear arms, you will act - purchasing weapons, joining supportive organizations, and voting for politicians who hold the same beliefs. If you believe in gun control, you will act in the opposite manner - promoting gun buybacks, protesting lax gun laws, and voting for politicians who hold your beliefs. If you lack belief in either side, you will not be motivated to act. Lack of belief leads to a lack of motivation.