First described by Hungarian court dentist Georg Carabelli (December 11, 1787 - October 24, 1842) the cusp of Carabelli is an extra anatomical structure found on certain maxillary molars. Also known as Carabelli's cusp, Carabelli's trait, Carabelli's tubercle and, tuberculum anomale, this landmark is typically a small bulge. One source I read indicated that the negative cusp is not defined much as we do not define a valley as "the absence of a mountain" however other sources reveal that the cusp of Carabelli can be either positive or negative and referred to as a pit, ridge, groove, furrow or fissure. Studies indicate that more males than females possess the cusp, it is more prevalent in those of European descent and least common among Pacific Islanders. Nowhere does it state that the cusp of Carabelli interferes with normal occlusion since the cusp receives the same amount of wear and tear as its neighboring cusps. Typically the cusp appears bilaterally although asymmetrical cases have been documented.
My introduction to the cusp came last year when our general dentist informed me that my youngest daughter had a very small cavity. Initially she was very upset however he explained that it can be difficult for a toothbrush to reach deep enough into the grooves created by the extra cusp. Thankfully her cavity was not significant, it took less than thirty seconds to fill and that one filling is all either of my children have. Carabelli's cusp may be an inheritable trait, as far as I know I do not have it however exploring your own molars is a task that is best left to those with the proper professional equipment. If you do have an additional cusp, ridge or furrow extra care should be taken to clean the areas around it as it creates an ideal environment for bacteria and may lead to subsequent decay.
Sources I found interesting:
Occurrence in First Upper Molars
Ethnic Analysis of Chinese patients