The evidence linking psychosocial factors to sustained blood pressure elevation is highly suggestive and comes from a variety of sources. Hypertensives show increased responsiveness to emotional and mental stimuli. The hemodynamic characteristics of unstimulated hypertensives are similar to those of normotensives under emotional stress. It is likely that the sympathoadrenomedullary system partly mediates these responses, but the evidence in humans is mixed. Stranger evidence comes from studies of mice showing that symbolic stimuli in the form of disordered social relations lead to hypertension and increased heart size. In humans, the evidence linking psychological traits to hypertension is inconsistent. The prevalence of hypertension varies by social class and ethnic group and increases with acculturation from rural, traditional to modernized societies. One possible explanation for this is the attendant psychosocial changes. A variety of stress-management techniques have been shown to lower blood pressure, adding weight to a stress hypothesis.