Can we ever really understand what goes on in someone else’s marriage? In “Love Lockdown: Dating, Sex and Marriage in America’s Prisons,” Elizabeth Greenwood reports on relationships in America’s prisons, mainly through the lens of women who become romantically involved with men while they are incarcerated.

The questions the book asks — how do these relationships work? Are they real? Why do some women find love in prison when they struggle to find it on the outside? — underscore the complexity of any connection between two humans. How do any of our relationships work? Could a prison relationship be more “real” than a loveless marriage on the outside?

In an early scene, Greenwood is having drinks with a group of prison wives, and afterward one of them says, “Did you see the couple at the other table? They were on their phones the whole time, not even looking at each other. That’s the kind of thing that drives me crazy.”

You can understand why. For women forced to pay hundreds of dollars a month to send a text or receive a call from their partners, who go through ritual humiliation every time they visit (“I cannot cut off my ass,” one prison wife says when told to change outfits), watching another couple take the simple act of being together for granted is hard. Even as many of these prison wives would argue that they receive the kind of fully present attention — free of Facebook and phones — that women in the free world don’t get.

This is not to glamorize these relationships. One of the central tensions of the book is how many MWI (Met While Incarcerated) relationships can be a way for men to manipulate women in order to get money, phone calls and other favors in prison. Almost every woman interviewed in “Love Lockdown” worries about this risk. The only person who doesn’t is Jacques, the lone husband on the outside. (He also admits he would not have married Ivié if the state of New York did not allow conjugal visits.)

Jo, a veteran who served as a medic in Iraq, and her husband, Benny, whom she married while he was doing 10 years for assault and attempted murder, are the central couple in the book. They share love, trust and commitment, but the brutal fact remains that Benny tried to kill his ex-girlfriend. We see Jo try to work through this (“What am I doing marrying a man who, in a fit of rage, tried to run over his girlfriend?” she says, reflexively asking the question she knows her friends and family are thinking). We also see author Greenwood, a frequent Chronicle contributor, try to work through this as she becomes concerned for Jo, but it can’t really be worked through. (Greenwood acknowledges the thorny journalistic territory of forming strong bonds with her sources, a very human but complex space for a reporter.) A woman we are rooting for is in love with a man who has harmed other women. By the end of the book, Benny is out on probation, they are married and happy, but the relationship still feels precarious.

And yet, flaws and all, these couples feel real. The pat explanations for why women might start a relationship with a man in prison — it’s “safe” because they can’t actually be with him; it serves some martyrdom complex; it’s because women fetishize tough, dangerous men (the actual groupies who write fan mail to murderers like Scott Peterson are loathed by prison wives) — may have some salience, but they are not the whole story.

By taking this lens to the prison system, Greenwood also cracks open the absolute awfulness of this subworld — the greed of charging prisoners exorbitant fees to have any contact with loved ones; the exploitation of paying inmates $12 a month for full-time work, forcing women to shoulder the financial burden of caring for families; the constant humiliation that is the point, not the byproduct. As Greenwood explains, the way we do prisons in this country is not normal. Perhaps if we approached incarceration differently in America, we’d have a different understanding of the relationships that come out of it as well.

“Love Lockdown: Dating, Sex, and Marriage in America’s Prisons”

By Elizabeth Greenwood

(Gallery Books; 272 pages; $27)