The prevalence and severity of striae gravidarum varies among populations. Considerable variability among countries in the prevalence of bullying has been reported. There are significant differences in regional prevalence. Researchers have commented on the unusual prevalence of older males among H7N9-infected patients. There are cultural differences in the prevalence of somatization disorder. This suggested a high correlation between ALAN and the prevalence of breast cancer. Aging populations are often stigmatized for having a high prevalence of disability. PPV is directly proportional to the prevalence of the disease or condition. The prevalence of sadomasochism within the general population is unknown. Studies on time trends in the prevalence of bulimia nervosa have also yielded inconsistent results. Proper prenatal nutrition is responsible for this small prevalence of low birthweight infants. Some research has pointed to the link between astigmatism and higher prevalence of migraine headaches. Many feel this incline is due to the widespread prevalence of cell phones. Women in nations with high pathogen prevalence also show greater preferences for facial masculinity. Demand is expected to continue or increase as the prevalence of electric vehicles increases.