Formation of monopolies
Government license (e.g. electricity, post)
Ownership of resource (e.g oil industry in Saudi Arabia; Windows)
Copyright, patents (over designs, ideas, characters, images, sounds or names)
High starting cost (e.g. natural resources)
Advantages
Maintaining super-normal profits in the long run
For ‘natural’ monopolies may be best to remain monopolies (to avoid the wasteful duplication of infrastructure that would happen if new firms were encouraged to build their own infrastructure)
Valuable export revenues (e.g. Microsoft)
Disadvantages
Restricted output
Higher prices
Reduced consumer surplus (whenever the price a consumer actually pays is less than they are prepared to pay)
Restricted choice
Reduced consumer sovereignty
…
