More than one-third of US marriages begin with online dating, and those couples may be slightly happier than couples who meet in other ways, a US study said on Monday.

Online dating has ballooned into a billion-dollar industry and the Internet "may be altering the dynamics and outcome of marriage itself", said the study by US researchers in the Proceedings of the National Academy of Sciences.

The research is based on a nationally representative survey of 19,131 people who married between 2005 and 2012.

"We found evidence for a dramatic shift since the advent of the Internet in how people are meeting their spouse," said the study, led by John Cacioppo of the University of Chicago's Department of Psychology.

People who reported meeting their spouse online tended to be 30 to 49 years old and in higher income brackets than those who met their spouses offline, the survey found.

Of those who did not meet online, nearly 22 percent met through work, 19 percent through friends, 9 percent at a bar or club, and 4 percent at church, the study said.

So who is happier?

When researchers looked at how many couples had divorced by the end of the survey period, they found that 5.96 percent of online married couples had broken up, compared with 7.67 percent of offline married couples.

The difference remained statistically significant even after controlling for variables like year of marriage, sex, age, education, ethnicity, household income, religion and employment status.

Among couples who were still married during the survey, those who met online reported higher marital satisfaction - an average score of 5.64 on a satisfaction survey - than those who met offline and averaged 5.48.

The lowest satisfaction rates were reported by people who met through family, work, bars or clubs, or blind dates.

"These data suggest that the Internet may be altering the dynamics and outcomes of marriage itself," Cacioppo said.

"It is possible that individuals who met their spouse online may be different in personality, motivation to form a long-term marital relationship, or some other factor."

But not all experts believe that online dating translates into instant bliss.

Eli Finkel, a professor of social psychology at Northwestern University, led an extensive review of the science published about online dating last year.

He said he agreed with the proportions found in the PNAS study. His research showed about 35 percent of relationships now start online.

"The overreach occurs when the authors conclude that meeting a partner online is better than meeting a partner through offline avenues," Finkel said.

"Nobody's surprised when a minuscule effect reaches statistical significance with a sample of 20,000 people, but it's important that we don't misunderstand 'statistical significance' to mean 'practical significance'."

According to New York City psychologist and author Vivian Diller, the seven-year study was too short to assess the long-term outcomes of relationships that begin online.

"Success in marriage is largely about how you negotiate differences, not just compatibility," she said, adding that online dating can raise expectations and result in greater unhappiness.

"I find that younger people who use online dating seem to use it more as a playground, especially guys looking at profiles of young women. They look at it as an opportunity to meet as many people as possible, and women fatigue of that."

Agence France-Presse