Ethnic comparison of twinning rates in California

A meaningful comparison of the twinning rates of different populations can be hindered by several factors that may influence the rates. Chief among these factors are the sources of data from which the rates are derived and the maternal age structure of the populations. The effects of such factors were minimal when the twinning rates of 14 ethnic groups in California were compared. After standardization for maternal age, twinning rates per 1000 maternities for the main ethnic groups were 13.20 for blacks, 10.05 for whites, and 7.18 for Asians. There were significant differences within the Asian group. The highest rate was for Cambodians (8.57). This rate compares with rates of approximately 6 for Koreans, Thai, and Vietnamese. These rates are similar to those found in Asia but lower than those for Californian Chinese, Japanese, and Filipinos, who have been established in the state for longer. In general, the results provide support for previous reports that twinning rates are modified by both migration and interethnic mixing. Rates for Indians from India were lower than those reported from the Indian subcontinent, whereas the twinning rates for Native Americans (10.15) and Pacific Islanders (10.60) were similar to the rate for US whites.