Tom W. Smith, director of the General Social Survey at the National Opinion Research Center at the University of Chicago, said the rise in support for same-sex marriage has been especially dramatic during the past two decades.
It went from 11 percent approval in 1988 to 46 percent in 2010 -- when 40 percent were opposed and there was, for the first time, a narrow plurality in favor of same-sex marriage.
The report is based on findings of the latest General Social Survey, conducted in 2010 with a cross sample of more than 2,000 people.
"There is a large generation gap on the issue of same-sex marriage," Smith said in a statement.
Sixty-four percent of those age 30 and under back same-sex marriage, compared with 27 percent of those age 70 and older, Smith said.
In 2010, 26 percent of those age 30 said they felt same-sex behavior is always wrong, while 63 percent of the people age 70 and older held that opinion.
In 1973, 70 percent said they felt same-sex relations are always wrong, and in 1987, 75 percent held that view, but by 2000 that number dropped to 54 percent and by 2010 dropped further to 43.5 percent, Smith said.
"Public opinion is highly polarized on this issue, with few people sharing the middle ground," Smith said.