The current body of epidemiologic and experimental evidence suggests that, in addition to environmental and behavioral factors, genetic predispositions significantly influence refractive development. Genetic epidemiologic studies in a wide variety of populations have consistently produced high heritability estimates for refraction.
1 2 3 4 5 6 Though refraction is highly heritable within populations, the prevalence of refractive errors varies among ethnic groups and geographical locations (see Morgan and Rose
7 for an excellent review). For example, East Asian populations, particularly the Chinese ethnic groups in Singapore and Taiwan, have some of the highest reported prevalences of myopia in the world.
8 9 10 11 12 13 These high rates are thought to be, in part, due to high levels of exposure to environmental and behavioral risk factors in these populations (myopia is associated with higher education,
11 13 14 greater urbanization,
15 16 17 intensive studying and reading habits,
18 19 20 and a visually demanding work environment
21 22 ). Exposure to intense near-work activities from an early age is also common in the Orthodox Jewish community, which suffers from a high prevalence of myopia. Zylbermann et al.
20 has suggested that unique visual demands and intense study habits account for the high rates of myopia among Orthodox Jewish male students. Conversely, myopia tends to be less common in rural and agrarian societies in which access to formal education is limited.
4 23 24 25 Recent work also suggests that outdoor activity during childhood has a protective effect against myopia
26 which would partially explain the lower rates of myopia in rural areas. A recent study by Peet et al.
4 reported lower crude prevalences of both myopia and hyperopia among the Old Order Amish compared with the overall population in the United States. The Old Order Amish live rural agrarian lifestyles that have remained relatively unchanged for centuries and do not educate their children past eighth grade.
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