To begin with, it should be noted that the term "annulment" is not exact. To "annul" implies to dissolve or rescind something that had really existed. A civil contract can be annulled, but not the covenant of marriage. Once a true marriage has been entered on and a real marital bond created, it can only be dissolved by death.
What can at times happen is that an apparent marriage was in fact never a true marital covenant, because some element essential to its constitution was missing from the start, and therefore no marriage ever came into being. If this can be established, then justice demands that such a "marriage" be declared null, i.e. never to have existed as a marriage at all. What the Church does in such cases is not to "annul", but to make a "declaration of nullity".
An apparent bond can be non-existent or null due to various reasons. These include objective factors, such as impediments (e.g. insufficient legal age, an existing married bond, etc.: see cc. 1083-1094), or fear induced by outside pressure which causes a person to consent to an unwanted marriage (c. 1103). A bond could also be null because of subjective intentions, as in simulation (c. 1101), when one (or both) of the parties excludes from his or her consent some essential element of marriage (e.g. openness to having children, the permanence of the bond, etc.); or in purposeful deceit (c. 1098) about some personal quality objectively important to married life (such as concealment of the fact of being sterile). The factor bringing about invalidity could also be unconscious, as when a person's psychic faculties are marked by some grave deficiency which makes him or her incapable of a free act of consent (c. 1095).
In such cases, where there never was a real marriage at all, it would be unjust to hold the parties to a non-existing bond. The matter is so important however that it requires a proper judicial investigation before a church tribunal. The issue is put to the court in the form of a question: is the nullity of this marriage - between X and Y - proved? To which the court, on the basis of the evidence gathered, answers Affirmatively or Negatively.
A single declaration of nullity is not sufficient. It must be confirmed by a higher court before the parties are held free from the former (apparent but invalid) bond, and able to marry someone else. The "first instance" court and the "second instance" (appeal) court may give opposed decisions: say, one Negative (against the nullity) and the other Affirmative (in favor). With two discordant decisions, such a case automatically goes for a final decision to the Roman Rota, the highest appeal Court of the Church. In fact either of the parties can appeal directly to the Rota even after a first decision, whether Affirmative or Negative (c. 1444).
Further, even if two concordant decisions have been given (for or against the bond), the case can always be reopened at a higher level, provided new and serious arguments are adduced, or some clear indications exist of a possible miscarriage of justice (c. 1644).
When a petition for a declaration of nullity is presented to a tribunal, it can happen that both parties favor the petition. Then normally only the Ptr will be assigned an Advocate to plead his or her case. The "Defender of the Bond" is an official of the Tribunal whose mission is to ensure that the arguments in favor of the validity of the marriage are not neglected; a party who opposes nullity can however also request or appoint an Advocate. The Church takes special care to protect the "right of defense" of each party. They must be properly informed of the grounds of nullity proposed, allowed to give evidence, and to present witnesses. Each one personally (and not only his or her advocate) has a right - before any judgment is pronounced - to know the allegations or arguments put forward by the other side and the evidence of witnesses and experts (c. 1598). Violation of this right invalidates the whole process and any subsequent declaration of nullity.
It is sometimes thought that pursuing a declaration of nullity is an expensive process, especially at the level of the Rota. This is not so. Several countries, including the United States, have an agreement with the Rota by which a sum (currently ranging from $300 to $850) to cover part of the rotal costs is paid by the diocese (not the parties). The Petitioner (also the Respondent, if he or she requests it) then gets the services of a rotal Advocate whose fees are paid by the Rota. If a person chooses to appoint his or her own Advocate, then he or she will have to pay their fees, which may be high. Rotal Advocates are so called because they are specially qualified to plead before the Roman Rota; they are not officials or employees of the Rota.
It is often said today that there are too many declarations of nullity. What is more certain is that there are too many broken marriages. Many of these are quite valid (and therefore cannot justly be declared null). There lies the deeper problem: to avoid that such marriages, which began in love, should end in failure. If the spouses pray, have recourse to the sacraments, get sound advice, they can learn to be faithful to their free commitment to each other, to their children, to God. Canon law itself insists that before accepting a petition of nullity, whenever there seems any hope of success, the judge should try to persuade the couple to try again to make the marriage work (c. 1678).
As striking as the growth in declarations of nullity is the fact that almost all cases nowadays are pleaded and judged on the relatively new grounds of consensual incapacity; i.e. that although the person intended a true marriage, he or she was not capable of real consent due to some serious defect in his or her psychic faculties, which made it impossible (and not just difficult) for him or her to understand, freely choose or actually carry out the essential obligations of the married state.
Since marriage is something most natural to men and women, incapacity for marrying is by any standard an extraordinary handicap. That is why only a grave psychic deficiency can incapacitate a person. This is clear from the terms of canon 1095 which deals with these grounds of nullity, from the teaching of the Pope, and is firmly established in rotal jurisprudence. In 1987 the Pope said to the Rota: "An argument for real incapacity can be entertained only in the presence of a serious form of anomaly which must substantially undermine the capacity of understanding and/or of willing of the contracting party".
Simple immaturity, which is not gravely abnormal for the age at which a person married, offers no argument for consensual incapacity. An equally important point, specifically stated in c. 1095, is that the psychic anomaly in question must incapacitate a person for the essential obligations of marriage. Certain psychic disorders, such a claustrophobia, agoraphobia, etc., even if present in a severe degree, no doubt make married life (just as social life) more difficult; but they cannot be said to relate to any essential constitutional obligation involved in marriage.
It must further be shown that the anomaly was present at the moment of consent. A psychic disorder originating after marriage, cannot invalidate the consent already validly given. The fidelity promised at consent - in good times and in bad - also covers the possibility of ensuing psychic illness. The healthier partner has the obligation to be a faithful support for the person suffering the disorder, and the latter has the right to that fidelity.