Take care in selecting your thesis. This is really a type of persuasive essay, but you don't want to be stuck either just repeating someone else's opinion, or citing all the same sources. Try to come up with an original thesis or take an aspect of someone's thesis and develop it. You can also take a thesis and "transplant" it into different circumstances. For example, use tools of modern economics to argue about the role of medieval guilds in the development of early European settlements. Or take a study done on children in France and try to show it is/isn't applicable to elderly Florida residents. An original thesis is the best start you can make to get a high grade in a research essay.