An annulment differs from a divorce in that divorce ends a legally valid marriage while an annulment ends a marriage by establishing that the marriage in question was never a valid one. The duration of a marriage is not a factor in determining the availability of an annulment. If certain legal requirements are met, you can obtain an annulment whether you have been married for three hours or three years.
Biology

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If you and your spouse are close biological relatives, you are entitled to an annulment. The definition of “close” will vary among jurisdictions.
Age

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If one of the spouses was below the legal age for marriage, annulment of the marriage is a valid option.
Duress or Threat

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If you were married under duress (at gunpoint, for example), the marriage can be annulled.
Mental Capacity

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If one of the spouses lacked the mental capacity to consent to marriage, whether because of a permanent intellectual handicap or because of temporary intoxication, annulment is a valid option.
Bigamy

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If one of the spouses was already married at the time of the marriage in question, annulment is proper. In certain jurisdictions, bigamy laws will automatically render invalid the second marriage, making an annulment unnecessary.