Katherine Phillips, a seventeenth-century writer, lamented, "We may generally conclude the Marriage of a Friend to be the Funeral of a Friendship." Many co-authors might be tempted to say something similar about a collaborative writing project. Fortunately, the inverse can be true as well, providing the co-authors approach their project with open hearts and clearly articulated goals. We found collaboration to be a marvelous, perspective-expanding adventure over the two years we spent together writing The Social Sex: A History of Female Friendship (Harper Perennial, 2015).

            Marilyn Yalom, an established author of best-selling social histories (A History of the Breast, A History of the Wife, How the French Invented Love) had been thinking about writing a history of women's friendships for several years. She had her contract for the book in hand when she approached Theresa Donovan Brown, a fellow writer and friend of nearly twenty years, about co-authorship.

            We both had some reservations about how the complicated process would work. We have different writing and research styles. Who would write what parts? How would we speak with one voice? Theresa is an early-bird, Marilyn a night-owl -- when could we work together?      

             Rather than dwell on potential stumbling blocks, we immediately executed a simple legal contract that clearly defined our roles and share of the proceeds. Though some might find a legal agreement anathema to friendly interaction, it's essential in an all-absorbing project that has a tail of many years. That done, we could stop thinking about how things would divvy up, and get on with the big work.

            We often disagreed on many crucial issues -- things like voice, structure of the book, what characters to include, and what aspects of women's friendships to emphasize. But these disagreements were not allowed to simmer. We talked and edited one another's writing and talked some more. We met weekly, and communicated by email between meetings. Our respective egos found ways to balance between what we each wanted and the overall excellence of the book we were creating.

             As the book took shape, a quietly amazing collateral transformation took hold of our friendship. It deepened. It strengthened. Its importance expanded in the scopes of our lives.

            Eleanor Roosevelt, who warranted her own chapter in The Social Sex, provided an object lesson. Even as her husband and she were riding a wave of unprecedented popularity as a power couple, c. 1927, she was finding her true self among her girlfriends. A group of them ran a crafts-work factory in upstate New York and a prep school in New York City. Eleanor declared her engagement in these enterprises "one of the most satisfactory ways of making and keeping friends."

            We consider ourselves very lucky, both in our choice of co-author, and in our subject -- female friendship across 2500 years. As much as we grappled, we also shared awe before the stories we uncovered about the friendships that have warmed women through the ages. Friends writing about friendship became a self-reinforcing dynamic. At the end of the process, we not only can offer the world a book of which we are proud, but in our private moments, we can relish the newly discovered depths of our friendship.