with the highest incidence reported in Great Britain and Ireland (Copp and Bernfield, 1994). Other populations with high incidence include northern Chinese and Australian Aborigines (Bower et al., 1984; Moore et al., 1997). Although Sikhs have a high NTD incidence, the defects are often thoracic and associated with minimal deficit, suggesting a distinct etiology (Baird, 1983). The decrease in NTDs among Irish immigrants to the United States could be explained by genetic dilution through interethnic marriages. However, some studies of migrant populations in which NTD incidence decreases with changes in locale suggest a nutritional etiology (Borman et al., 1986; Carter, 1974).

The causes of these abnormalities have been the subject of intensive research over many decades. Differences in the pathogenesis and the epidemiology of different categories of NTD have led to the idea that NTDs are highly heterogeneous in etiology (Dolk et al., 1991). Substantial familial aggregation indicates that anencephaly, myelomeningocele, and craniorachischisis are related pathogenetically and genetically. Evidence from epidemiological studies of NTDs indicates that heredity is a major contributor. Indeed, the recurrence risk in a sibling birth is 3 to 5 percent (Laurence, 1990). For the most cases the inheritance is believed to be polygenic, potentially involving multiple genes. Such polygenic traits are influenced by environmental factors, thus the etiology appears to be multifactorial (Laurence, 1990). Recently, attention has turned to assessing the genetic basis of NTD and to evaluating the role of