"The acceptance of gays and lesbians has really been the civil rights issue of the last few decades," said study co-author Jean Twenge, professor of psychology at San Diego State University and author of the book "Generation Me." Twenge, who studies the way social behavior changes over time, considers the acceptance of LGBTQ individuals to be the best example of such a change in recent history. "It’s been a social change that’s happened relatively quickly," she said.

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Between 1973 and 1990, the percentage of adults who believed "sexual relations between two adults of the same sex [was] not wrong at all" only rose from 11 percent to 13 percent. But by 2014, 49 percent of all adults and 63 percent of millennials expressed tolerance of these relationships.

"Millennials are markedly more accepting of same-sex behavior than Gen Xers were at the same age — but then, so are most adults," co-author Ryne Sherman of Florida Atlantic University said in a statement. "The change is primarily one of time period, where all adults shifted in their attitudes."

That increase in tolerance likely helped drive an increase in sexual behaviors (and willingness to report them). The percentage of men who have had sex with at least one man increased from 4.5 percent to 8.2 percent between 1990 and 2014. Women reporting at least one female sexual partner increased from 3.6 percent to 8.7 percent of the population during the same period. But Twenge and her colleagues say that bisexual behavior drove this change: The percentage of survey respondents who had all same-sex partners didn't increase significantly during that time, but the percentage of adults with both male and female partners increased from 3.1 percent to 7.7 percent.

"That's what I found surprising," Twenge told The Post. "When we looked more closely at the change, it was mostly due to people having sex with partners of both genders."

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But all of these factors — the increased acceptance, the increased behavior and the trend toward sexual fluidity — make Twenge think she knows the real driving force behind this cultural shift: We care more about ourselves. She believes this comes from an increased sense of individualism.

"Some thinkers have made the case that individualism has been increasing in Western culture since the Renaissance, but that this change accelerated beginning around 1965 or 1970," she explained. As societies become more comfortable in terms of resource availability, one doesn't need to worry as much about fitting in to the rules and expectations of the larger group.

"Think about what an enormous group effort it used to be to make a meal. Now you just need two bucks and a microwave," Twenge said. This security means less motivation to follow cultural "rules" that don't suit an individual's personal desires.

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And that individualism could make us more tolerant, too.

"Individualism says basically that you do what you want to do and let other people do what they want to do," she said. "People are more willing to accept behaviors they have no wish to engage in. There's more of a sense of, you know, I need to do what's right for me."

The researchers tried to figure out how much individual generations were driving the change, but this is difficult — even using the latest statistical methods.

Does the world look this way simply because millennials are ushering in an era of tolerance and free love? Do people get more or less straight or narrow as they age? Are we just changing overall as a society over time?

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"It’s very tough to separate those questions, because at any one time a person's age is the product of both the year it is and the year they were born," Twenge said. And there are other limitations: It's hard to track how these things might change in an individual with age, because Boomers weren't being asked about their partners when they were 20 and we can't fast forward to see what millennials will be in their 60s.

Some of the data suggests that young women are more likely to have same-sex relationships than older women, which Twenge and her co-authors note could support the idea that many women are "gay until graduation." But this will likely be a controversial finding: As Twenge and the other researchers readily admit, it's difficult to separate this spike in same-sex behavior in young women from generational changes. And at least one previous study showed evidence that this phenomenon is a myth. A 2013 Pew Survey on sexuality did find that bisexuals were very likely to marry members of the opposite sex, but one could easily argue that this is a result of (dwindling) societal pressure to do so, as opposed to a decrease in sexual fluidity with age.

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Twenge looks forward to examining the data set in a few years. She has no doubt that acceptance will continue to increase —after all, the younger generations are notably more tolerant, and the oldest survey respondents are going to slowly remove themselves from the data pool — but how much sexual behavior will increase remains to be seen.

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We aren't sure what it would mean for the country to be so tolerant that everyone with fluid sexuality acted upon their same-sex attractions — because we don't know what percentage of the population is naturally attracted to more than one sex. Some researchers have argued that bisexuality doesn't exist, but simply serves as a placeholder identity for those unwilling to admit to an exclusive attraction to the same sex. A surge in funding from the American Institute of Bisexuality has helped support more comprehensive studies on sexual attraction, and many results — including some from scientists who used to conclude that the orientation was a false label — have shown that bisexual individuals are truly sexually fluid. And bisexual behavior can be found outside of our own species, too; including in our closest cousin, the freewheeling bonobo. But while we can be fairly confident that many individuals are fluid in their sexual attractions, it's hard to say how just how many.

"We don't know what the natural limit is for bisexual attraction," Twenge said. "So how much that will increase is more of an open question."