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In this study, the authors investigated the potential contribution of environmental factors and testosterone levels on androgenic alopecia in women.Ninety-eight identical female twins were recruited from 2009 to 2011. Subjects were asked to complete a comprehensive questionnaire, provide a sputum sample for testosterone analysis, and pose for standardized digital photography. Frontal, temporal, and vertex hair loss were assessed from the photographs using Adobe Photoshop. Hair loss measures were then correlated with survey responses and testosterone levels between twin pairs. Two independent, blinded observers also rated the photographs for hair thinning.Factors associated with increased frontal hair loss included multiple marriages (p = 0.043); longer sleep duration (p = 0.011); higher severity of stress (p = 0.034); positive smoking history (p = 0.021); higher income (p = 0.023); absence of hat use (p = 0.017); and history of diabetes mellitus (p = 0.023), polycystic ovarian syndrome (p = 0.002), and hypertension (p = 0.001). Factors associated with increased temporal hair loss included divorce or separation (p = 0.034), multiple marriages (p = 0.040), more children (p = 0.005), longer sleep duration (p = 0.006), and history of diabetes mellitus (p = 0.008) and hypertension (p = 0.027). Lack of sun protection (p = 0.020), consuming less caffeine (p = 0.040), history of skin disease (p = 0.048), and lack of exercise (p = 0.012) were associated with increased vertex hair loss. Higher testosterone levels were associated with increased temporal and vertex hair loss patterns (p < 0.039). Increased stress, increased smoking, having more children, and having a history of hypertension and cancer were all associated with increased hair thinning (p < 0.05).This study implicates several environmental risk factors in the pathophysiology of female alopecia.