From a study by Social Security's Office of Retirement and Disability Policy:
Since 1984, Social Security beneficiaries with total income exceeding certain thresholds have been required to pay federal income tax on some of their benefit income. Because those income thresholds have remained unchanged while wages have increased, the proportion of beneficiaries who must pay income tax on their benefits has risen over time. A Social Security Administration microsimulation model projects that an annual average of about 56 percent of beneficiary families will owe federal income tax on part of their benefit income from 2015 through 2050. The median percentage of benefit income owed as income tax by beneficiary families will rise from 1 percent to 5 percent over that period. If Congress does not adjust income tax brackets upward to approximate the historical ratio of taxes to national income, the proportion of benefit income owed as income tax will exceed these projections.