Gabriel delves into the tumultuous private life of Karl Marx in Love and Capital: Karl Marx and Jenny Marx and the Birth of a Revolution.

Your book is a biography of five people—Karl Marx; his wife, Jenny; and their three daughters who managed to survive into adulthood. Was it difficult to immerse yourself in so many distinct personalities?

The Marxes were a unit, their lives so intertwined, so mutually dependent, I felt it would be impossible to write a book about life in the household without writing what would amount to a biography of all five family members. That realization was initially daunting—at times I felt like a conductor faced with a full orchestra, when in previous books I had only had to corral a string section of sorts. But I grew to enjoy so many noisy characters. Each Marx family member was so distinctive and strong they were relatively easy to draw, and because each was so prolific in their correspondence, I was able to "hear" what each had to say at critical junctures.

Despite Marx's devotion to his wife, he had an affair with Lenchen, the family housekeeper, and even fathered a child with her. How does a biographer remain sympathetic to subjects who display such less than endearing qualities?

Marx made many terrible personal decisions, and more often than not he was exasperating, immature, reckless, and selfish. But I came away from the project liking him more than I did going into it. An intrinsically compassionate man, he believed his words would alleviate the plight of the working poor, and for the sake of this idea, he toiled virtually alone for decades without recognition or reward, experienced countless personal losses and defeats, and endured years of grinding poverty. Amid that desperation and uncertainty it would have been difficult not to occasionally go off the rails. The fact that he did so, so unforgivably in Lenchen's case, would have made him at best a trial to live with. But as a biographical character, his private transgressions only made him richer and more intriguing.

Jenny was a remarkably freethinking and radical woman for her time—a member of an aristocratic family marrying far below her station and committing herself to a life of significantly less comfort. Add to this her unwavering support of Karl and the challenge his ideas posed to all that she had come from. What made her choose such a life?

Jenny von Westphalen was a child of the Romantics, and she saw the world through the lens of that movement, which championed individual freedom and, above all, the power of creativity to reinvent the world. She saw in Marx a warrior who would use a pen rather than a sword to destroy the rotting monarchies of Europe. For her part, she would do what Westphalen women had always done: stand beside her husband and support him in his noble endeavor. I am certain, however, that she had no idea what she was getting into—nor did Marx himself, for that matter. One has the sense that not until Jenny arrived in London in 1849 with three children in tow and a fourth on the way did she come face-to-face with the reality of what it meant to be Mrs. Karl Marx. The filth, depravity, stench, and hopelessness of Victorian London would have made even the most optimistic Romantic shudder. But I also believe that if she could have glimpsed her difficult future with Marx from the security of her parents' home in Trier, she would have married him anyway. Jenny was brave and bold, and not inclined to surrender.

Two of Marx's daughters committed suicide. Why do you think Laura and Eleanor were driven to such a fate?

All three had been raised with the promise of a glorious future. They had lived and breathed big ideas, revolutionary ideas, from the time they were children. But by the time they were adults that glorious future had not materialized, and their lives had devolved into ones of struggle, seemingly without end or purpose. As to why the Marx daughters were prone to suicide, first, they were committed atheists and therefore under no obligation or stricture that required them to continue their lives. They could, as it were, come and go as they pleased. Second, their family's personal philosophy, aside from being atheist, was also based on the power of the individual to chart his or her own course.

The unofficial sixth member of the Marx family is, of course, Friedrich Engels, who provided crucial financial support for the Marx family. Why was he so generous?

The key to understanding Engels is that he was not poisoned by greed and had absolutely no interest in property or possessions. He did not marry because, in the 19th century, marriage would have made his wife his possession. The only way of life that made sense to him was a communal one, in which property was shared, people worked for the benefit of the group, and when their work was finished, they were free to do as they pleased. He was one of those rare individuals whose happiness depended upon the well-being of others.