Members of Congress of both parties have begun questioning the rate-differences' justification and proposing the possibility of giving people tax credits or other assistance to buy coverage on their own, The New York Times reported.

The findings come as people are increasingly buying individual health plans because they've lost jobs that provided coverage, the newspaper said.

For instance, a 30-year-old Chicago or Denver woman having a Humana Inc. Portrait plan with a $2,500 deductible pays 31 percent more than a man of the same age, Humana rates data show.

A same-age Columbus, Ohio, woman with Anthem's Blue Access Economy plan pays 49 percent more than a man, rates data show.

"Premiums for our individual health insurance plans reflect claims experience -- the use of medical services -- which varies by gender and age," Humana Senior Vice President Thomas Noland Jr. told the Times.

"Females use more medical services than males, and this difference is most pronounced in young adults," he said.

Women ages 19 to 55 tend to cost more than men because they typically use more healthcare, especially in the childbearing years.