This is particularly true for young and middle-aged blacks, who have a substan- tially higher risk of subarachnoid hemorrhage (SAH) and ICH than whites of the same age.36,37 In the Atherosclerosis Risk in Communities (ARIC) study, blacks had an incidence of all stroke types that was 38% (95% confidence interval [CI], 1.01 1.89) higher than that of whites.42 American Indians have an incidence rate for stroke of 679 per 100000 person-years, which is high relative to non-Hispanic whites.43 It remains unclear whether these racial differences are genetic, environmental, or an interac- tion between the two. Possible reasons for the higher incidence and mortality rates of stroke in blacks include a higher prevalence of prehypertension, hypertension, obesity, and diabetes melli- A higher prevalence of these risk factors, however, may not explain all of the excess risk.50 Several studies have suggested that race/ethnic differences may be the result of social determi- nants, including neighborhood characteristics,51 53 language, access to and use of health care,35 Genetic Factors A meta-analysis of cohort studies showed that a positive family history of stroke increases the risk of stroke by 30% (OR, 1.3; The Framingham study showed that a documented parental history of stroke before 65 years of age was associated with a 3-fold increase in the risk of stroke in The odds of both monozygotic twins having strokes is 1.65-fold higher than for dizygotic twins.55 Stroke heritabil- ity estimates vary with age, sex, and stroke subtype.57,58 Younger stroke patients are more likely to have a first-degree relative with Women with stroke are more likely than men to have a parental history of stroke.58 Recent estimates of heritability using genome-wide common variant single-nucleotide polymorphism (SNP) data show similar heritability for cardioembolic (32.6%) and large-vessel disease (40.3%) but lower heritability for small- vessel disease (16.1%).59 These estimates, however, do not con- sider the potential contribution of rare variants. 