Over the past half century, rising individualism among Americans has coincided with ever loosening of attitudes toward sex relationships of all types. We've also seen an increasing number of lifetime sexual partners—except among Millennials. So say data derived from the General Social Survey (GSS), as reported in a new study by a team of psychologists led by Jean Twenge from San Diego State University. The survey, conducted almost annually by researchers at the University of Chicago, spans from 1972 to 2012 and covers some 57,000 participants.

Twenge's team first reviews data that indicate increasing individualism. For example, top 10 song lyrics are more likely now to use first person singular (I, me, mine) and second person singular pronouns (you, yours) instead of first person plural pronouns (we, ours). Several studies used the Google Books Ngram Viewer as a way to probe American cultural trends. For example, since 1960 the use of first person plural pronouns has decreased 10 percent in all books, while the first person singular increased 42 percent and the second person singular quadrupled. Books since are also less likely to use such words as virtue, modesty, and purity, and more likely to use words emphasizing choice over obligation. Finally, adolescent religiosity has been declining since the 1970s. The psychologists argue that these trends suggest that Americans are ever more willing to throw over traditional strictures, including those related to sex. In general, Americans have become more sexually permissive, both in outlook and practice.

The researchers also aim to tease out any differences in sexual attitudes and behaviors arising from cultural shifts over time, generational divergences, and changes due to aging. They divvy up Americans by generations, sorting us into the Greatest Generation (born 1900-1924), the Silent Generation (1925-1945), Boomers (1946–1964), GenX (1965–1981), and Millennials (1982-1999).

In 1972, the GSS started asking about attitudes toward premarital sex, extramarital sex, and homosexual sex; in 1985, it added a question about sex between 14- to 16-year-olds. Respondents could reply that such behaviors are "always wrong," "almost always wrong," "wrong only sometimes," or "not wrong at all." The study found that "between the 1970s and the 2010s, American adults became more accepting of premarital sex, adolescent sex, and same-sex sexual activity, but less accepting of extramarital sex."

Only 29 percent of American adults (35 percent of men and 23 percent of women) said premarital sex is "not wrong at all" in the early 1970s. Acceptance went up to 42 percent in the 1980s, remained flat in the 1990s, climbed to 49 percent in the 2000s, and surged to 58 percent in 2012. Generationally speaking, 47 percent of Boomers in the 1970s thought premarital sex was not wrong at all, compared to 50 percent of GenXers in the 1990s, and 62 percent of Millennials in the 2010s. Meanwhile, Americans' already strong disapproval of extramarital sex has toughened over the decades. Just 4 percent of the country—5.6 percent of men and 1.9 percent of women—accepted it in 1973. In 2012 the number was 1 percent (2 percent men and 0.6 percent women).

The biggest attitudinal change has been the fast rising tolerance of sexual activity between adults of the same gender. Here acceptance hovered between 11 and 16 percent until 1993, when it shot up to 22 percent (21 percent of men and 23 percent of women). By 2012, it reached 44 percent (35 percent of men and 51 percent of women). In the 1970s, 21 percent of people ages 18 to 29 believed that homosexual activity was not wrong at all, compared to 26 percent in the 1990s and 56 percent in the 2010s.

Adult Americans have long taken a dim view toward sex between young adolescents with only about 4 percent agreeing that it was acceptable as late 2006. That figure rose slightly from 4 percent in 2006 (5.6 percent of men and 3 percent of women) to 6 percent in 2012 (6.4 percent of men and 5.7 percent of women).

So much for attitudes. Do Americans sexually practice what they preach (or tolerate)? The average number of lifetime partners increases from 2.12 for those born in the early part of the 20th century to 11.68 for those born in the 1950s, then declines to 8.26 for those born in the 1980s and 1990s. "Thus," the authors conclude, "Millennials report 6 more sexual partners than G.I.s [Greatest Generation], Boomers reported 9 more partners than G.I.s, and Millennials reported 3 fewer partners than Boomers." They add, "The median number of partners was 1 for those born in the 1910s–1920s, 2 for the 1930s cohort, 3 for the 1940s, 4 for the 1950s–1960s, and 3 for those born in the 1970s–1990s."

If Millennials are so permissive, why do they have fewer sexual partners than their Boomer parents? It is possible that the advent of AIDS inculcated more sexual restraint. Millennials also report more casual sex, which the researchers speculate might mean that they are enjoying "regular contact between a limited number of individuals, perhaps reducing the overall number of partners."

The study concludes that the shift toward greater sexual permissiveness (with the exception of extramarital sex) occurs generationally. In other words, people form their views around sexuality as adolescents and young adults, and they don't change them much as they grow older. As the Boomers begin to shuffle off this mortal coil, the Millennials look set to let it all hang out.