Generally, the grounds will be that the norms for marriage, as expressed in the Roman Catholic Code of Canon Law, were lacking wither in form or in substance.
For example, Can. 1095, §1 et seq, provides:
It follows, then, that if the party seeking an annullment can show that he or his spouse not know what was happening during the marriage ceremony because of insanity, mental illness, a lack of consciousness, a serious lack of discretionary judgment with respect to essential matrimonial rights and duties, or a psychological problem that caused an inability to assume the essential obligations of marriage, the marriage may be annulled.
Similarly, the following section of Canon Law have been used as the basis for decrees of nullity:
Can 1096 - Ignorance about the nature of marriage
Can 1097 - Error about a person (the “mail order bride” syndrome) or about the quality of a person
Can. 1098 - Fraud
Can. 1099 - Error concerning the unity or the indissolubility or the sacramental dignity of marriage that overbore the will
Can 1101 - Lack of intent (to contract a real marriage, to be faithful, to be open to the conception of chidlren, to remain married forever, reservation of some past or future condition as a necessity to the marriage, reservation of present condition that did not exist)
Can. 1103 - Duress, force, or fear
Can 1157 - 1160 Lack of new consent during convalidation
It’s not a rubber stamp by any means. I have done marriage tribunal work and I can tell you there are cases that are rejected.
Why should they be?
The annullment is a legal process, a finding that declares, “We thought, at the time, that this marriage was valid – it had all the appearances of a valid marriage. But now, after close investigation, we discover a flaw that means it was never valid.” The kids were produced during the time that everyone thought the marriage was valid. Why, then, should they be considered illegitimate?
Canon law directly addresses this. Can. 1137 provides that children who are conceived or born of a marriage are legitimate, even if the marriage is later found not to be valid.
I’ve heard the claim (from my father, who is still in denial about his annulment, 25 years later) that in the US, 99% of annulment requests are granted, while elsewhere in the world, it’s only 1%. I have no doubt that those numbers are exaggerated, but do you happen to know what the actual numbers are?
And I’ll second the answer about legitimacy of children. Being in such a situation, I was curious myself, and looked into it. So long as the marriage was believed to be valid at the time, the children are legitimate.
The National Catholic Reporter said that in 1999, nearly 80 percent of the requested annulments in the United States were granted, and contrasted that with slightly less than 40% in Italy that same year (no on-line cite; sorry).
I don’t have a figure handy for world-wide decrees of nullity, but I would not be surprised to learn it’s closer to the Italian figure than the US figure.
There are probably many reasons for this. I suspect in many European countries, where the Church’s influence may not be as strong as it once was, there simply less concern for getting an annulment – a couple may chose to simply remarry, and not worry about it. I personally think that the American attitude towards litigation has something to do with it – people here are more willing to embark on what amounts to a court case, and more attuned to protecting their “rights” if you will.
Finally, just like our secular justice system, there is no real question that if you’re wealthy, and you can afford to hire experienced counsel, your anullment process will not suffer as a result. As a relatively wealthy nation, the US’s anullment rates may tower over third world countries for that reason.
I wonder if the Church was thinking about the kind of abuses that put Richard III on the throne when they wrote this rule. (Richard was the “protector” of boy-king Edward V. He had Edward’s parents’ marriage declared invalid, making Edward V illegitimate, and thus not eligible to be king. He then locked Edward and his brother up in the Tower of London, and probably had them killed.)
Could you be confusing this with divorces granted by the Anglican church (Church of England)? Catholicism is not particuarly prevalent in the UK.
Also, another viable grounds for an annulment is, of course, incest- the grounds on which Henry VIII attempted to dissolve his marriage to his first wife, since she had formerly been married to his brother.