Define Average

but now we don't know what the results mean?

It means they rate themselves according to whatever works for them. (Optimism bias for the scale that they used.)

can you explain your reasoning for this "rejection of bad outcomes" rather than "preference for good outcomes"? finding it hard to grasp sorry!

Preference for good outcomes means leaning towards the good outcome.
Rejection of bad outcomes means leaning away from the bad outcome.

There's actually a difference in the two even if they lead to the same thing.

forgot it is possible it is also self-selection bias, as pointed out above, ie "only the people who care will vote" - there is no reason why we can attribute it to either optimism bias or self-selection bias, or both, or neither. yay research

Yes, there's also that. Unfortunately, I can't control that variable. However, I do think people won't see this thread and just say they "don't care". Rather, people will see this thread, rate themselves, then decide whether or not they will vote according to their perception of themselves.

Thus, there are two possible explanations presented by this poll:
1) People who perceive themselves as average or above average would choose to vote more often than those who perceive themselves negatively.
2) People just think highly of themselves.

just a note in response to
i quote wikipedia, but was also taught this during my psychology degree:

"Depressives tend to be more accurate, and less overconfident in their assessments of the probabilities of good and bad events occurring to others but they tend to overestimate the probability of bad events happening to them."

this has been taught in my psychology course for at least 5 years, but not sure where to find a reference, maybe a textbook rather than an article by now

I know of this, and I'll say now that I think this is questionable. Not all things written in textbooks should be taken for absolute truth. If depressives tend to be more accurate, then how do you explain the fact that people who live happier lives tend to live fuller lives compared to depressives? (in other words, how can people who aren't accurate live very "happy" lives?)

If depressives were more accurate, then genetically speaking, the depressives would have more survival value and therefore would be more likely to pass their genes to the next generation. Then we'd ALL be more depressed than we should be. (emotional state "can" be passed on, as evidenced from the fact that animals can be "domesticated" and be more passive rather than aggressive)

In addition, the "definition" of a depressive is someone who has lower self esteem and loss of interest in normally pleasurable activities. The key words here are "lower" and "normally". Who's to say that depressives aren't supposed to have such a state and that we're all just high (which would be an explanation of why we are more optimistic)?