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The financial crisis of 2008 may have driven many people to betray their wedding vows, according to data from Ashley Madison, an unusual and apparently very popular dating Web site for those seeking extramarital relations.

Ashley Madison has expanded rapidly, but 2008 was a banner year for the company. According to the site, membership swelled 166 percent worldwide that year and 192 percent in the United States, compared with average yearly growth of 50 percent worldwide and 71 percent domestically since the site's launch 12 years ago. Each month, around 130 million people around the world visit Ashley Madison.

Analysts at Ashley Madison found evidence of a relationship between the economy and infidelity when they examined user data in individual states. They compared the change in the number of employed people in each state with the growth in Ashley Madison's membership there. The tentative conclusion: People who've lost their jobs might be more likely to cheat -- or, at least, are more likely to sign up for an adultery dating site.

From the end of 2007 to the end of 2009, in states where the economy remained relatively strong, fewer people signed up for Ashley Madison. Here's the chart:

There was a lot of variation among the states, and it's easy to think of other possible causes besides economic ones for those differences. Ashley Madison's marketing campaigns might explain membership increases in some areas, for example. Also, 74 percent of Ashley Madison's users have a college degree, so a lower concentration of highly educated people might correspond to a smaller target audience for the site. During those two years, Ashley Madison's membership nearly quadrupled in Massachusetts, the state with the highest level of educational attainment, even though the economy there fared relatively well during the crisis.

All the same, in states where the employment level fell further, more people signed up for Ashley Madison. People out of work might find they have more time to pursue an affair. Also, a reduction in family income might spark stress and conflict between spouses.

"It may very well be that in times of distress, that one's partner could also be viewed with antipathy, leading one to cheat," said Eric Anderson, a sociologist at the University of Winchester in England who is working as a consultant to Ashley Madison. He also said an extramarital affair might look like an easy way for people to distract themselves from worrying about financial problems.

Some other interesting notes from Ashley Madison's data:

Almost all of the activity on Ashley Madison -- around 95 percent -- is heterosexual.

Husbands with 39-year-old wives should be extra nice: Women who are 39 years old are more than four times as likely to sign up for Ashley Madison as women aged 38 or 40 .

. About one-third of Ashley Madison's users seek their first affair from three to five years after their wedding , around the time when the first child is born.

, around the time when the first child is born. A little more than a quarter of Ashley Madison's users first create an account when their children leave the house for college. Men in this group are generally between the ages of 60 and 65, while women are between the ages of 55 and 58.

Ashley Madison hired Anderson to work with scientists and journalists who are interested in the company's data. The site's founder, Noel Biderman, said he believes that adultery is a normal part of human experience and that greater tolerance of infidelity would preserve more marriages and more families. He said he is confident that a rigorous review of the numbers will support his conclusion.

"I see the number of people signing up for the service every day. I see the economics," he said. "It’s few of us who will make it through a life of monogamy."

There are around 60 million American married couples, according to the Census. Around 13 million people in the United States have created an account on Ashley Madison to date, about three-quarters of whom identified themselves as married or in a committed relationship. Many of them might not have exchanged anything more intimate than a message with a potential lover on Ashley Madison, but the site still offers researchers a massive amount of data to explore.

"For the first time that I'm aware of, we have the ability to peek in on people having an affair, and if that doesn't thrill any researcher interested in human sexuality, I don't know what could," Anderson said.

Separate research by Christin Munsch, a sociologist at Furman University, suggests another reason that adultery and the economy might be connected. After controlling for factors such as a person's income, education and satisfaction in his or her relationship, Munsch found that men who earned very little relative to their wives were much more likely to stray. Perhaps as men lost their jobs during the recession, they might have wanted to compensate for what they felt to be their professional failure by asserting their sexual prowess.

Munsch also found that among very wealthy women, those who made significantly more than their husbands were also more likely to cheat. During the recession, a woman who was highly successful in her career might have been more likely to judge her husband's termination by her own experience and lose respect for him, Munsch said.

Munsch, who is preparing her work for publication, says her research is based on survey data from the Bureau of Labor Statistics. While she has confidence in the survey's methods, Munsch she acknowledges that subjects might have lied to the government's pollsters.

Dishonesty among those surveyed has been a real obstacle to research on infidelity. Sociologists and psychologists have long sought to understand how many married people are unfaithful and why they do it, and estimates of the proportion of cheaters based on surveys range widely -- from 25 percent to 75 percent of married people.

To be sure, there are limitations on Ashley Madison's data, as well, since it is a particular kind of person who signs up on a dating Web site for married people. The site's advertising is aimed at well-educated women. Only about 32 percent of all Americans have a college degree, while almost three-quarters of Ashley Madison's users do. (In general, research suggests that people who are well educated are actually less likely to cheat.)

As a result, Anderson cautioned, the Web site cannot settle the question of how often or how rarely people cheat on their spouses. "That is a question that we will never know the answer to," he said.

More analysis would be needed to determine whether the financial crisis truly helped Ashley Madison's success during 2008 and 2009. Yet the preliminary evidence here suggests the economy did have some effect -- another reminder that unemployment can alter a person's sense of who they are.