An earlier post looked at children of divorce and single motherhood. That was less feasible in the 19th and early 20th-century period of rising inequality because on an absolute level, they lived more precarious lives. But that didn't stop them from committing adultery.
I admit that the data here are hard to estimate directly. Indirectly, we have how common of an obsession it was in the literature of the time. Here is an unobjectionable list from About.com of the Top 10 Adultery Novels. Madame Bovary and Anna Karenina are the best known, and most clearly show adultery as a form of status striving on the part of the woman who isn't content with the modest life provided by her husband, and wants higher status, more conspicuous consumption and leisure, and so on. Effi Briest didn't make the list, but is also from the late 19th C.
The theme was a staple of Victorian, Gilded Age, and Edwardian / Ragtime literature, then started to peak and fade during the '20s and especially the '30s, becoming more or less unheard of during the mid-century. Divorce was also not a common theme during the mid-century, taking off only during the '70s. Victorian adultery and Millennial divorce are both forms of hypergamy, hence of status striving, so we shouldn't get too caught up in whether one version was more common in the 19th C. and another in the 21st. We should also not lose sight of the fact that hypergamy in either form fell during the Great Compression, reaching a minimum in the mid-century. *
How does rising adultery lead to rising inequality? It's just the counterpart of men competing more viciously in the economy. When men become discontent with a modest life, striving begins, and a few climb up, up, up, while the losers stay put. The losers will probably try to then push others down, so that they can at least enjoy a little relative status boost. They might want those below them not just below them, but way below them -- locked up in debtor's prison, serving time in the work house, or whatever.
For women in an age before they earned much income, their status was tied to who they could marry, or sleep around with in exchange for a finer material life. When women become discontent with modest living, they strive to marry higher and higher-status husbands, or at least become their mistresses. This raises the ceiling of the female status distribution. As for the women below them, they try to marginalize them even further down the hierarchy, ostracizing them to ensure that they can only get jobs as prostitutes, which is as bad for their status as being locked up is for a man's. Or at least tolerate the existence of red light districts -- "If there are women working there, it automatically raises my status in comparison." This causes the floor to fall on the distribution.
The net effect is to widen inequality among women, similar to the hollowing out of the middle class in the paid-labor economy. Among men there are more vagrants and more billionaires, among women there are more prostitutes and more wives-of-billionaires.
My hunch is that during the Progressive Era, reducing inequality was not only a matter of reining in the dog-eat-dog striving of men in the economy, but also the bitch-eat-bitch striving of women in the domestic world. We ought to remember that as we try to undo the way society has been heading. As we burn the bailed-out bankers at the stake, yea shall we also brand yon materialistic hussies with the scarlet letter. (Not out of self-righteousness, but to contain social instability.)
Then there's adultery / divorce on the part of the husband who isn't content with his wife and wants to trade up -- gotta boost your status by 1 point with an incrementally prettier babe at your side. The time trends appear to be the same for male as well as female adultery / divorce. It's not as though men become more and more likely to trade up during one time, and women during another.
I do wonder, though, about the magnitude of the changes for men and women. It's more in man's nature to cheat, so it probably doesn't rise and fall as dramatically across time periods. Women tend not to cheat as much, so they have much more room to rise during a period of increasing adultery.
What little we can tell from real-world estimates supports this. The General Social Survey asks a question if you've ever cheated on your spouse while married. The data only go back to 1991, so we can't see a fuller picture of how it does or does not track the trend for status competition and inequality. Still, men show no change from 1991 to 2012, at a little over 20% saying they ever have. Women began at around 10% and have steadily risen to around 15% now saying they ever have. (Data restricted to ever-married whites.)
Men's adultery stems from being horny, women's from wanting access to finer things in life. Even in the rare case where she's married to a wealthy man and cheats with a lower-status but better loving man, that's still a form of not being content with a modest lot in life -- of wanting to have her cake and eat it too.
* I reject the argument that popular culture tells us nothing about the society it came from, let alone that it gets things backward -- as though there were few prostitutes and mistresses in the Gilded Age, and soaring levels during the Fifties. People are generally tuned into what the major problems are of their day, and don't worry about problems from 200 years ago or 100 years into the future. If writers want to find an audience, they have to cater to contemporary concerns.
GSS variables: evstray, year, sex, race