What are the most frequently used grounds for annulment?

The grounds can vary from state to state, but generally all the grounds for annulment include trickery or deceit about the desire or ability to have children; bigamy; forced marriage ("shotgun wedding"); under age; an impairment as a result of drugs, alcohol, or mental incapacity.

In short, annulments happen in cases where one party would or should not have married had he or she known all the facts.