On social issues, Americans are more liberal than ever before. What's behind the sudden shift?

From acceptance of same-sex relationships and premarital sex to having a baby outside of marriage, Americans are more socially liberal than ever before.

That's according to a new Gallup survey that finds Americans are becoming more liberal on several social issues – and more liberal in general.

Some 63 percent of Americans now say gay or lesbian relations are "morally acceptable." That's a 23-point jump since 2001, when only 40 percent felt that way, according to Gallup. That jump reflects the greatest shift in opinion to the left of any issue Gallup measured in its survey. 

Americans today are also more accepting of having a child out of wedlock: 61 percent said it's OK, compared with 45 percent in 2001. And some 68 percent of Americans now say sex outside of marriage is acceptable, compared with 53 percent in 2001.

In fact, increasingly liberal attitudes on social issues reflects a wider leftward shift as Americans become more liberal in general. For the first time in Gallup's records on this subject, an equal number of Americans now identify as socially liberal and socially conservative. Thirty-one percent of Americans describe their views as liberal, matching the number who describe their views as conservative, according to Gallup.

What's driving this national shift to the left? Is it because people have changed their minds, or is it because people died and were replaced by younger people with different beliefs?

"I would think that a lot of it has to do with the aging of our population," says Lou Manza, professor and department chair of psychology at Lebanon Valley College. "People growing up in the 1940s and 1950s ... were raised in a broad culture that was way more conservative than people coming of age in the 1960s and beyond. And with each passing generation, our culture, collectively, has become more and more accepting of ideas that, many years ago, had negative connotations."

In fact, according to Gallup, population replacement was a major factor in explaining changes in overall ideology.

"These longer-term trends ... may be a result of population changes, with younger, more liberal Americans entering adulthood while older, more conservative adults pass on," Gallup said in its report.

But the shift in attitudes is so dramatic that demographics alone cannot be responsible, says Jeff Bosworth, associate professor of political science at Mansfield University.

Professor Bosworth speculates that a certain social snowball effect is at work, as increasing acceptance breeds increasing acceptance.

"As more Americans have LGBT friends, relatives, and others, they become far more accepting of different lifestyles," he says. "And as more states legalize gay marriage, the purported threat of gay marriage simply dissipates. In other words, familiarity breeds acceptance."

The rise in popularity of libertarianism – a political philosophy that emphasizes autonomy and freedom of choice – may also play a role.

"I think a related factor is that the legalization of gay marriage has been largely cast as a civil rights issue," says Bosworth. "Most everyone understands the desire to be free and make personal choices to live their lives."

And while Americans largely remain conservative on economic issues, the leftward shift on social issues will have significant implications in marriage and in politics.

As Gallup said in an apparent message to Republicans running for office, "It may be more important now than in the past for the GOP nominee to be a bit less conservative on social issues in order to appeal to the broader general electorate."