The most active mechanism is under the Human Rights Committee. There have been over 2000 complaints filed under this mechanism since its inception (36 years). The procedure is described in the First Optional Protocol to the International Covenant on Civil and Political Rights (CCPR). This Committee issues approximately 100 decisions per year.
The other available mechanisms come under the Convention Against Torture (CAT), the Convention on the Elimination of All Forms of Discrimination Against Women (CEDAW), the Convention on the Elimination of Racial Discrimination (CERD), the Convention on the Rights of Persons with Disabilities (CRPD), and the Convention on Enforced Disappearances (CED).
The chart below shows the number of states that have ratified the relevant mechanism under each treaty.
Three other treaties also have individual complaint mechanisms, but are waiting for the necessary number of ratifications before coming into effect: the Convention on the Rights of the Child (CRC), the Covenant on Economic, Social and Cultural Rights (CESC), and the International Convention on the Protection of the Rights of Migrant Workers and Members of their Families (CMW).
16 mechanisms in total
But there are also various special procedures available under some of the treaties which can be thought of as complaint mechanisms. When viewed from this perspective, there are actually 16 possible complaint mechanisms under the treaties (3 of which have not yet come into effect).
Here are the 16 mechanisms:
1. CCPR complaint (civil & political rights). Currently in effect in 114 states. As noted above, this mechanism has so far been the most popular, most used mechanism in the system.
2. CEDAW complaint (women). Applies to 104 states. While it is the 2nd most ratified mechanism, it has so far not seen the usage that the CCPR mechanism has. An important decision was issued in May 2012 under this procedure, on the rights of an aboriginal woman in Canada, involving a housing dispute and right to legal assistance.
3. Inquiry procedure under CEDAW (women). Applies to 101 states. This is a procedure available under article 8 of the Optional Protocol to the treaty. It applies to all states that have ratified the Optional Protocol, unless they expressly "opt out" at the time of the ratification by making a declaration under article 10. 3 states have opted out by making this declaration. The procedure has been used a few times so far -- the most notable being a major investigation into an uncommonly high number of unsolved abductions, rapes and murders against women in the Chihuahua, Mexico region in 2005.
4. CERD complaint (racism). Currently in effect in 53 states. Even though this complaint option has been available for over 20 years, it has been rarely used. This is probably because of the racial discrimination focus of the treaty (most complaints involving racial discrimination also involve other types of human rights violations, making it more convenient to file the complaint with one of the more broad-scoped treaties). Since the CERD Committee has only a few cases a year to review, complaints filed under this mechanism are usually heard and decided much more quickly than under the other mechanisms.
5. CERD early warning procedure (racism). Applies to 175 states. This procedure was adopted by the Committee as a response to several genocide/mass killing events. The Committee will act under this early warning, urgent action procedure to address serious violations brought to its attention, including potential genocide, mass discrimination events, significant flows of refugee or displaced persons, forced removal of indigenous groups from a region, etc. It applies to all states that have ratified CERD.
6. CAT complaint (torture). Applies to 64 states. This mechanism has received the 2nd highest number of complaints filed each year in the treaty body system, approximately 20 to 40 complaints per year. It applies to all states that have accepted the competence of the Committee under article 22 of the Convention Against Torture (CAT).
7. CAT special inquiry (torture). Applies to 145 states. The Convention Against Torture (CAT) also has a procedure in article 20 that permits the Committee on its own discretion to investigate particular issues of concern, where torture is a risk. The procedure applies to all states that have ratified CAT unless the state has expressly "opted out" at the time of ratification by making a declaration under article 28. Eight states have opted out by making this declaration. The procedure has been used in 2012, for example, to investigate conditions in the crisis in Syria.
8. CRPD complaint (disability). Applies to 73 states. This is one of the 6 main complaint procedures now available in the system. The first decision under this new mechanism came out in May 2012 against Sweden, addressing reasonable accommodation requirements under a building code, to permit a disabled woman with fragile skin and bone conditions to build a hydrotherapy pool in her home even though it was not permitted by the local building ordinance.
9. CRPD inquiry procedure (disability). Applies to 72 states. This procedure is described in articles 6 and 7 of the Optional Protocol to the CRPD. It applies to all states that have ratified the Optional Protocol unless the state has expressly "opted out" by making a declaration under article 8 at the time of ratification. So far one state has opted out (Syria). Thus the procedure applies to 72 of 73 states that have ratified the protocol. The procedure has not yet been used by the Committee but it is under review, according to a UN press release from the Committee's April 2012 session.
10. CED complaint (disappearances). Applies to 14 states. The newest individual complaint mechanism, this procedure applies to all states that have accepted the competence of the Committee to hear individual complaints under article 31 of CED. It came into effect earlier this year once the 10th ratification had been received.
11. CED urgent action (disappearances). Applies to 35 states. This mechanism is a special procedure described in article 30 of the treaty which applies to all states that have ratified the treaty. If a person has disappeared, possibly at the hands of the authorities, a relative or legal representative may submit a request to the CED Committee to seek and find the person on an urgent basis. When it receives such a request, the Committee must investigate, contact the state concerned, keep the relatives informed, and continue its efforts as long as the fate of the person remains unknown.
12. CED inquiry procedure (disappearances). Applies to 35 states. This mechanism is another inquiry procedure, described in article 33 of the treaty, which applies to all states that have ratified the treaty. If the Committee receives reliable information indicating that serious violations have occurred, it may, after consulting the state concerned, send one or more of its members to undertake a visit to the country, investigate the circumstances, and report back without delay.
13. CED widespread/systematic violations (disappearances). Applies to 35 states. CED also has a special procedure under article 34 that permits the Committee to bring a matter to the attention of the UN General Assembly on an urgent basis, if it receives well-founded information of widespread or systematic enforced disappearances that are occurring in any country which is a party to the treaty.
14. CESC complaint (economic, social, cultural rights). This mechanism is not yet in effect. Eight states have ratified the optional protocol creating this mechanism. Two more are needed before it enters into force. It would appear that the mechanism will come into effect very soon.
There is also a new inquiry procedure that comes into effect once the optional protocol to CESC is in force. This procedure is described in articles 11-12 of the protocol. It only applies if a state has "opted in" by making a declaration under article 11. So far one state, El Salvador, has opted in. The procedure permits the Committee to investigate any situation where reliable information indicates grave or systematic violations. The Committee must contact the state concerned, to seek its cooperation. It may then appoint 1 or 2 members to investigate and report back, including a country visit if appropriate.
15. CMW complaint (migrant workers & their families). This mechanism is also not yet in effect. It requires that 10 states accept the procedure under article 77 before it enters into force. So far 3 states have accepted the procedure. These ratifications have been slow in coming, so it would seem that it is not close to coming into effect at this time.
16. CRC complaint (children). This mechanism is not yet in effect. It just opened for signature in February 2012 and requires 10 ratifications before entering into force. As of this date there are 2 ratifications to this optional protocol (OPIC). It is likely that the remaining 8 ratifications needed to become effective will be received relatively quickly, perhaps by the end of this year.
There is also a new inquiry procedure that comes into effect once the OPIC is in force. The procedure is described in article 13. It applies to all states that have ratified OPIC unless a state has expressly "opted out" at the time of signing or ratification. So far 35 states have signed OPIC and two states have ratified it. No states so far have exercised the "opt out" declaration. Like the other inquiry procedures mentioned in this post, the Committee is permitted to investigate any situation where reliable information has been received indicating grave or systematic violations. In any such case, it must first contact the state concerned to attempt to obtain its cooperation in the examination. It then may appoint 1 or 2 Committee members to investigate and report back, including a country visit if appropriate.
Conclusion
The utilisation of these complaint mechanisms under the UN human rights treaties, many of which are very new, is likely to change the workload and nature of the outputs of the treaty body system in the coming years. I would encourage interested persons to become familiar with each mechanism, how it works, what types of circumstances where it applies, and how to effectively bring cases under it. It will become an important part of the human rights "toolbox" for seeking justice for the victims of human rights violations.