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The aims of this study are to describe the frequency of occurrence and degree of expression of the parastyle in six different ethnic groups; to assess inter- and intra-observer errors when scoring the feature; and to compare the expression of the feature in a small number of twin pairs. Dental casts were examined for evidence of the parastyle from samples available in the Adelaide Dental School. A dental plaque developed by Katich & Turner was used to standardize scoring. The highest percentage frequency of parastyle occurrence was found in a sample of European twins with a value of 1.7%. The buccal aspect of the mesiobuccal cusp of the permanent maxillary right second molar was the most common site for the parastyle. Inter-observer reliability in scoring was lower than intra-observer reliability. In 10 pairs of twins (seven pairs of monozygotic (MZ) twins and three pairs of dizygotic (DZ) twins) only two pairs of MZ twins showed concordance for presence of the parastyle. The expression of parastyles likely results from a complex interaction of genetic, epigenetic, and environmental influences during dental crown development. There may be a relationship between parastyles and supernumerary teeth that are occasionally located buccally to the maxillary molar teeth.