Progressive age-related loss of hearing acuity or sensitivity can start as early as age 18, primarily affecting the high frequencies, and men more than women. Such losses may not become apparent until much later in life. Presbycusis is by far the dominant cause of sensorineural hearing loss in industrialized societies. A study conducted in Sudan, with a population free from loud noise exposures, found significantly less cases of hearing loss when compared with age-matched cases from an industrialized country. Similar findings were reported by a study conducted of a population from Easter island, which reported worse hearing among those that spent time in industrialized countries when compared with those that never left the island. Researchers have argued that factors other than differences in noise exposure, such as genetic make up, might also have contributed to the findings. Hearing loss that worsens with age but is caused by factors other than normal aging, such as noise-induced hearing loss, is not presbycusis, although differentiating the individual effects of multiple causes of hearing loss can be difficult. One in three persons have significant hearing loss by age 65; by age 75, one in two. Age-related hearing loss is neither preventable nor reversible.