Most living kidney donors are women and, according to a recent study, men are donating even fewer kidneys than ever before.

As of today, there are more than 95,000 people on the kidney transplant waiting list, according to the United Network for Organ Sharing (UNOS). While living donor transplant is the preferred treatment for patients with kidney failure, the number of living donor transplantations has been dropping since 2005.

A team of Canadian researchers sought to determine the factors causing this decline. The study, published online in the Journal of the American Society of Nephrology, showed that living kidney donations from men decreased between 2005 and 2015. Living kidney donation among women during that time was stable.

Women have traditionally outpaced men when it comes to kidney donations. One 2016 retrospective study of 713 living-donor kidney transplant procedures performed from 1987 to 2014 showed that women were 54.7 percent more likely to be living donors and 39.4 less likely to be a recipient. The newest study suggests this trend will continue, and that economics has something to do with it.

Jagbir Gill, M.D., MPH, from the University of British Columbia led the study to investigate the longitudinal changes in living kidney donations between 2005 and 2015. The research team collected data from the Scientific Registry of Transplant Recipients and the U.S. Census. They determined that the unadjusted donation rate was 30.1 per million in women, but only 19.3 per million in men during the study period.

After adjusting for differences in age, race, demographics and incidence of kidney failure, the incidence of living kidney donation was 44 percent higher in women. Incidence of donation has been stable for women over time but has declined in men, widening the gender gap in donations.

Income seemed to play an important role in the discrepancy. The findings suggest that strategies to remove financial barriers to living donation may be key to maintaining an adequate number of living donations.

"We found that the biggest drop in donation has been in men, while women have continued to donate at similar rates," Gill said in a press release accompanying the findings. "We also found that the drop in donation was greatest in lower income populations suggesting that the financial costs of donation may be preventing more men from donating in the current economic climate, leaving more women to donate instead."

The rates of living donations varied by income in both genders, but the decrease in living donation was most significant in males from lower income groups. The research team also found that living related donations dropped in both genders in all income groups. The only group donating at a greater rate were high-income women.

Doctors typically recommend that kidney donors take eight weeks off for work postoperatively; this time off does not include additional days for preoperative evaluation. Complications, which are always a possibility with major surgery, can also keep a donor from working.

"The prospect of a male donor losing his job may more frequently result in loss of health insurance for him and his dependents," Gill said in a telephone interview with Reuters Health. "This research strongly supports initiatives to reduce or reimburse all costs associated with donation so that people who come forward to donate don't have to worry about incurring any out of pocket expenses."