The survey was conducted last summer, according to the AP, and the results were given to Defense Secretary Leon Panetta before he announced last month that the ban on women in combat would be lifted.
Amos said at a defense conference in San Diego on Thursday that infantry units are among the most skeptical about having women join, according to the AP.
"I think from the infantry side of the house, you know they're more skeptical," Amos said. "It's been an all-male organization throughout the history of the U.S. Marine Corps, so I don't think that should be any surprise."
Amos said earlier this week that some of the 28 Marine positions currently closed to women would remain closed, though most would become open to female Marines, he said.
The military services have until 2016 to determine what positions should remain closed to women. The chairman of the Joint Chiefs of Staff and the secretary of Defense must sign off on any decisions to keep positions closed.
The Pentagon’s lifting of the ban opens up as many as 237,000 positions to women that have been closed due to the ban on women serving in ground combat units. The majority of those positions are in the Army and Marines.