For far too long, scholars studying adult relationships have been matrimaniacal. They have focused overwhelmingly on marriage and romantic relationships. Other kinds of relationships that can be profoundly meaningful and long-lasting, such as close friendships, have been mostly neglected.

Friendship is likely to be especially significant in the lives of people who are single, and more people are single than ever before. In the U.S., for example, there are nearly as many unmarried adults who are 18 and older, as married ones. Even counting people who do marry, Americans spend more years of their adult lives not married than married – something that has been true for well over a decade.

The rise of single people in the U.S. and many other nations around the world should be reason enough for scholars to turn their attention to friendship. Slowly, whether for that reason or others, researchers are beginning to take adult friendship seriously.

I was delighted to discover an indicator of that new seriousness, a recently-published scholarly book. The Psychology of Friendship, edited by Mahzad Hojjat and Anne Moyer, features 19 chapters (including the foreword and introduction) by 40 contributors. As soon as I got my copy, I immediately looked for the chapter on friendship in the lives of people who are single.

There was no such chapter.

In a book on friendship, romance once again triumphed. There was a chapter on “Friendship and Romance” and another on “Friendship after Romantic Relationship Termination.”

Other demographic categories did get special attention. There were chapters on friendships in childhood and adolescence, in young and middle adulthood, and in old age. There was another chapter on “Friendships Across Race, Ethnicity, and Sexual Orientation.”

It was not that single people were entirely neglected. Most of the chapters addressed issues relevant to all friendships, including, of course, the friendships of single people. They included, for example, chapters on friendship and social-media use, the implications of friendship for physical health and mental health, and a chapter on “Maintaining Long-Lasting Friendships.” Themes relevant to conflict got some attention, too, with chapters on competition as well as revenge, transgression, and forgiveness. I was happy to see chapters devoted to friendships involving coworkers as well as mentors, and even a chapter on pets as friends.

I also appreciated a chapter by Michael Monsour, challenging the longstanding tradition of contrasting “same-sex” and “opposite-sex” friendships. As our growing cultural awareness of transsexuality should have told us, not all friendships fit so easily into those categories.

The “same-sex” vs. “opposite-sex” way of thinking, Monsour argues, is part of another unfortunate tradition – the “heteronormative bias” in which heterosexuality gets outsized attention and friendships between men and women are sexualized. That bias has led to “dozens of studies focusing on romantic and sexual attraction and tension in other-sex friendships” and hundreds of studies of “friends with benefits.”

Monsour’s main objection is that people who are not heterosexual get marginalized by those kinds of studies. But they are not the only ones who get shoved to the sidelines. Anyone interested in friendship for the sake of friendship, and not as a potential source of sex on the side, is marginalized by such research. Anyone who maintains friendships with other-sex individuals with none of the culturally-anticipated drama about romantic or sexual tension or potential threats to the romantic partners of those friends, is, perhaps, above this sort of research. It is fine to have some research on such topics, but I agree with Monsour that there is way too much, relative to the other meaningful topics worth addressing.

In my search for insights on the psychology of friendship in the lives of people who are single, I browsed every page of The Psychology of Friendship and read some chapters very closely. Then, to try to catch anything I may have missed, I also used the online “search inside” feature to look for keywords such as “single” and “unmarried” as well as the names of authors who have published research relevant to friendship in single life.

In the chapter on friendships in young and middle adulthood, I found one sentence on single people, “Among single people, friends and best friends were more important for emotional needs (companionship, disclosure, reassurance) and support (advice and tangible help) than among married people or people with children…,” then one more sentence reiterating the same findings. The chapter on romantic break-ups included a sentence about a study comparing those who do and do not begin to date after a romantic relationship ends. One of the chapters includes an acknowledgment of the growing number of single people. The concluding chapter points to the increase in people living alone and mentions the Going Solo finding that “single women are more likely than married women to have weekly face-to-face as well as other mediated forms of contact (e.g., phone calls or e-mails) with a best friend.”

The growing literature on the ways in which single people (and not just single women) do more than married people to maintain their friendships is mostly missing. That body of work now includes research that follows the same people over time as well as research that compares different people (e.g., married vs. single) at one point in time. (Some innovative experimental research on the benefits of friendship gets no mention, either.)

Despite my disappointment with the short shrift given to single people in The Psychology of Friendship, I think the book is in other ways a very valuable contribution and I welcome it with enthusiasm. Friendship, finally, is making its mark in academic research.

[Click here and here for links to writings on friendship in single life. Some research papers on nonverbal sensitivity and deception-detection among friends are collected in Friendsight: What Friends Know that Others Don’t. Also take a look at my TEDX talk, “What no one ever told you about people who are single,” if you are interested. This collection of articles on all sorts of topics relevant to single life is always available. Check out my website, too, if you’d like.]