OBJECTIVE: To identify factors contributing to the declining prevalence of hearing impairment in more recent generations. METHODS: We used data on hearing thresholds and potential risk factors of hearing impairment collected from studies in Beaver Dam, Wisconsin, the Epidemiology of Hearing Loss Study (1993-1995, n=3753; 1998-2000, n=2800 and 2003-2005, n=2395), the concurrent Beaver Dam Eye Study on the same cohort, and a subgroup (n=2173) of the Beaver Dam Offspring Study (2005-2008). RESULTS: Educational attainment significantly reduced the odds ratio (OR) of the birth cohort effect on hearing impairment from 0.90 to 0.93, while a history of ear infection had a reverse effect on the decreasing trend (significantly changing the OR from 0.93 to 0.94). Occupational noise exposure, smoking, and a history of cardiovascular disease, while associated with hearing impairment, did not attenuate the cohort effect. The cohort effect remained significant after known risk factors were adjusted (OR=0.93; 95% confidence interval, 0.89-0.97). CONCLUSION: These data provide strong evidence that environmental, lifestyle, or other modifiable factors contribute to the etiology of hearing impairment and add support to the idea that hearing impairment in adults may be prevented or delayed.