Individuals looking to continue working after retirement could be thwarted by age discrimination, a study by the Federal Reserve of San Francisco has cautioned.

The study notes that “the aging of the population will pose fundamental public policy challenges, as the ‘dependency ratio’–the ratio of nonworkers to workers–rises sharply and labor force growth slows.” In response to concerns about the financial viability of Social Security and other benefits programs for older persons, incentives are being created to keep them in the workforce, such as the increase in the age at which full Social Security benefits can be drawn, it says.

Meanwhile, increasing numbers of older workers are hoping to continue work, at least on a part-time basis, for financial, personal satisfaction and other reasons. However, it says that “age discrimination in hiring may be critical to whether older people can work substantially longer, because many seniors transition to part-time or shorter-term partial retirement or ‘bridge’ jobs at the end of their careers,” it says.

The study involved creating fictitious resumes with the same work qualifications but differing in the listed age of the applicant–29-31, 49-51 or 64-66–and sending them in response to some 13,000 job ads. Overall, the callback rate from the employer with the vacancy was the highest for the younger “applicants” and lowest for the oldest.

“While both middle-aged and older applicants experience discrimination relative to younger applicants, older applicants–those near the age of retirement–experience more age discrimination,” it said.