Group therapy appears to be gaining popularity for two reasons: More clients are seeking it out as a more affordable alternative to one-on-one psychotherapy, and more research is demonstrating its effectiveness, say psychologists who practice it.

"Group therapy is more popular than it has been in the past because of the many studies that show its efficacy," says Nina W. Brown, EdD, a professor at Old Dominion University in Norfolk, Va.

For many conditions, group therapy works as well as individual therapy, says Gary Burlingame, PhD, a professor of psychology at Brigham Young University. He points to the results of more than 50 clinical trials that have compared patients who were randomly assigned to individual or group treatment. All of those studies "overwhelmingly support the equivalence of the two formats in producing the same degree of improvement for several disorders," Burlingame says.

In addition, he notes, group therapy exceeds APA Div. 12 (Society of Clinical Psychology) standards for efficacy for major depressive disorder, bipolar disorder, panic disorder, post-traumatic stress disorder, social phobia, obsessive-compulsive disorder, bulimia nervosa, binge-eating disorder, substance use disorder, schizophrenia, borderline personality disorder and general personality disorder.

But what makes groups work? "Only recently have we been able to demonstrate how the group influences individual group members," adds Dennis M. Kivlighan Jr., PhD, professor at the College of Education at the University of Maryland, in College Park.

Among that research is the finding that the most effective groups have a common identity and a sense of shared purpose, according to a meta-analysis of 40 studies by Burlingame and others, published in the International Journal of Group Psychotherapy in 2009.

When it comes to a group format, new research shows two leaders are better than one. Members of co-led groups experience greater benefits than those of individually led groups. That second set of eyes and ears makes a big difference when group leaders are trying to follow multiple interactions, Kivlighan's research has found (Group Dynamics: Theory, Research, and Practice, 2012).

"An individual group therapist, no matter how skilled, cannot keep up with the richness of the group experience," says Sally H. Barlow, PhD, professor of psychology at Brigham Young University. Important cues, particularly non-verbal cues, are in danger of being missed with only one leader at the helm.

Research is also shedding light on how, exactly, groups help people heal. One important factor is the ability to interact with peers. Numerous studies, including Kivlighan's 2012 work in Group Dynamics, have found that peer interactions tap into many therapeutic factors.

"In cases of abuse or trauma, groups provide social support, they improve social networks and they can reduce stigma, isolation and feelings of alienation among members," says Brown.

In fact, according to Stanford University's Irvin David Yalom, MD, in the 2005 book "The Theory and Practice of Group Psychology," hearing from peers may be more helpful than receiving guidance from a therapist since peers can identify with one another. Those peer interactions appear to translate to real-world gains. In a meta-analysis of five studies, Kelly L. Callahan of Harvard Medical School found that sexual abuse survivors improved markedly after participating in group therapy (Journal of Group Psychotherapy, 2004).

And as participants improve, the group as a whole benefits, says Brown. "Members can be agents of change for each other," she says. "Seeing others' progress can help group members realize they, too, can cope and feel better."

Group therapy also offers advantages for the psychologist: The approach allows therapists to observe relational patterns, says Kivlighan. Rather than rely on the accuracy (or inaccuracy) of self-reports, patients reveal their problems through interactions with other members.

"The group becomes a mini-re-creation of the patients' universes," says Kivlighan. "You have so much more data available to you in the group setting. All you have to do is watch."

Amy Paturel is a writer in Murrieta, Calif.