You believe making less would motivate people to work harder?

No, but I think people who perform worse at their jobs should be paid less the ones that do good job.

As far as I'm aware, research into wages as motivation indicate that high wages don't work well in motivating people who are otherwise unhappy with their work, but paying too low wages can demotivate people who are otherwise satisfied (high enough wages are a condition for job satisfaction, but not a "positive" motivator).

And collective agreement makes you pay everyone the same, so there is a possibility that satisfied worker will get less money then he feels he needs to get and demotivate him.

Secondly, I can only talk personally and anecdotally but knowing that we all earn same amount I would not be motivated to perform better either, and why should I?

And usually what happens with collective agreement is just what you said, good people get paid less bad people get paid more. So we get exactly the problem you described.

Another question is if you think that most hard worker will negotiate a good salary, or if you think good negotiators are more likely to do that? And if you think that being a good negotiator colleagues well with being a good nurse, teacher, cleaner etc?

It is a skill like any other, there will be good negotiators in any kind of work. If someone can get more money for their work by negotiating I don't see anything wrong with that, that is after all how the market works.

Question is how does the individual feel about that. I don't negotiate my salary and I am probably always on the low end and I don't care about it since I make good living. But then again my type of work is specific as I can quit when ever I want and find another job instantly and make that my negotiation tactic.

As a manager you would want to keep good nurses and cleaners and teacher around though and get rid of the bad ones, and one way of doing this is paying them differently even without negotiations.

It certainly could be that the hard workers you want to reward,

Of course, and this is what collective agreement hinders as everyone should be paid the same or nearly the same.

are lifted more by collective bargaining, than the lazy ones?

How so?

This depends so much on what work you do and how you do it and where.

I would argue, if the person in charge of what ever company or service we are talking about wants to keep good people around he will understand the logic behind paying them more then the ones that are lazy. Otherwise the company/service will suffer.

And this is the reason why collective agreements work so well in government as there are no penalties for having bad service, they are after all paid from public money instead of having to perform better to earn the money.

Private company with this approach would fail as it would not be able to provide good service.

Again, having to pay everyone the same will basically make lazy people lazier and better performing people unhappy.

Collective agreements are not just about salary but also about how you can get terminated from your job position. Usually it is very hard to let go of the low performance people. Which makes this a vicious circle of sucking the money out of the (usually) government.