(USA TODAY) -- What's "normal" and what's not when it comes to human behavior, sexuality and relationships?
Researchers who solicited responses to an online survey of almost 100,000 people from around the world, including 23,000 in the USA, get at that question and more than 1,000 others in a new book called The Normal Bar, out Feb. 5.
Among their findings, based on responses from individuals 18 and older who are in relationships (both heterosexual and same-sex):
-- 40% say they have sex three to four times a week.
-- 48% of men and 28% of women report having fallen in love at first sight.
-- 43% of men and 33% of women say they are keeping a major secret from their partner.
"This 'normal' is different from most normals," says co-author Pepper Schwartz, a sociologist at the University of Washington in Seattle. "Most normals look at the average - if a big clump of people do it, they call it 'normal.' What we want to know is which normal is correlated with happiness."
Even among the happiest couples, the survey found 27% were keeping some secrets.
Co-author James Witte, who directs the Center for Social Science Research at George Mason University in Fairfax, Va., says the team looked at behaviors reported by couples who said they were happiest to see which might help others be happier. Of U.S. respondents, Witte says 62% were in the "happiest" category and 14% were "extremely happy."
"If they're really unhappy, they would have ended that relationship," he says. "We have a pretty satisfied group. Otherwise, they would have split up."
Still, Schwartz says, it appears many couples are "somewhat romance starved."
In the USA, the survey found that of 1,218 respondents answering a question about romance, almost 29% of women and 44% of men say it bothers them "a lot" that their partner is not more romantic.
"We make a big deal of Valentine's Day because I think people are doing catch-up," she says. "If you look at the happiest couples, they do have date nights. They hold hands. They do PDAs (public displays of affection). That whole package of romance that some couples preserve - that shows how important it is."
In the USA, 44% of Americans report that they "hardly ever" or "never" go out on a date - but that's still better than 53% in Italy, 54% in England and 55% in France.
The authors suggest two ways to improve sexual satisfaction: go to bed nude (34% of U.S. women and 38% of men sleep nude with their partner) and kiss more as a sign of affection, not necessarily while making love. Those who kiss for affection rather than as part of a sexual act report being more sexually satisfied.
Among U.S. respondents, the happiest couples identified communication as the most fulfilling aspect of their relationship (40%), followed by friendship and then affection. Sex came in fourth, parenting last.
The book's third co-author is Chrisanna Northrup, a California wellness entrepreneur who created the concept and worked with the sociologists to make it happen.
Witte, of Clemson, S.C., says all participants completed 31 questions and then selected any of 16 categories of additional questions to answer. He says for any question, there were at least 600 respondents. The overall sex category had more than 2,200 respondents.
Psychologist David Buss of the University of Texas-Austin studies sex differences and urges people not to think of this survey as a benchmark of sexual frequency. "People do want to know if they're having sex as much as everyone else," says Buss, co-author of the 2009 book Why Women Have Sex. "If they're not, they may feel deficient in some way or that something's wrong."
But, he adds, "there are huge individual differences in sex drives and individual differences in sexual chemistry within relationships and all sorts of other things - job stress, kids - all sorts of things influence it. It would be alarming if people got too overly concerned with where they stack up in terms of frequency."
The U.S. survey sample, which is not nationally representative nor randomly selected, is 89% white, 68% women, and 56% ages 35 and older.
"Probably at best, it tells us something about the white, probably better-educated, somewhat higher-income population in the U.S., which is a population we know a fair amount about already," says sociologist John DeLamater of the University of Wisconsin-Madison. "What we really, really need are studies that look at these diverse groups in the U.S. That would go a long way in addressing the whole issue of 'normal.' "
DeLamater, co-editor of the book Sex for Life, out last year, says he's also concerned about suggesting that the findings in this book represent normalcy. "That really worries me when people use these surveys as a benchmark for what's normal, because these populations being studied are often not representative of the diversity in the United States."
Still, psychologist Sam Gosling, also of UT-Austin, who co-edited the 2010 book Advanced Methods for Behavioral Research on the Internet, says online samples have an advantage in that the anonymity affords greater honesty than the old-style phone survey. "Internet samples specifically are shown to be good at things that you're asking that people might not like to tell other people, such as sexual behavior," he says.
Witte says he doesn't worry much about respondents lying, based on the open-ended responses received.
"The level of detail - often about trivial things - lets me know they couldn't be making this up," he says.