"All truth passes through three stages.  First, it is ridiculed.  Second, it is violently opposed.  Third, it is accepted as being self-evident."  (mis) attributed to Arthur Schopenhauer.

It's hard to improve psychotherapy outcomes.  Typically, different well structured therapies produce very similar outcomes in head to head trials.  This makes the results of the recent trial by Sanne Bruijniks & colleagues (including Pim Cuijpers, Arnoud Arntz & Steven Hollon) even more impressive.  Their paper "The effects of once- versus twice- weekly sessions on psychotherapy outcome in depressed patients" was published online first by the British Journal of Psychiatry on February 7th.  The abstract reads "Background: It is unclear what session frequency is most effective in cognitive–behavioural therapy (CBT) and interpersonal psychotherapy (IPT) for depression.  Aims: Compare the effects of once weekly and twice weekly sessions of CBT and IPT for depression.  Method: We conducted a multicentre randomised trial from November 2014 through December 2017.  We recruited 200 adults with depression across nine specialised mental health centres in the Netherlands.  This study used a 2 × 2 factorial design, randomising patients to once or twice weekly sessions of CBT or IPT over 16–24 weeks, up to a maximum of 20 sessions.  Main outcome measures were depression severity, measured with the Beck Depression Inventory-II at baseline, before session 1, and 2 weeks, 1, 2, 3, 4, 5 and 6 months after start of the intervention.  Intention-to-treat analyses were conducted.  Results: Compared with patients who received weekly sessions, patients who received twice weekly sessions showed a statistically significant decrease in depressive symptoms (estimated mean difference between weekly and twice weekly sessions at month 6: 3.85 points, difference in effect size d = 0.55), lower attrition rates (n = 16 compared with n = 32) and an increased rate of response (hazard ratio 1.48, 95% CI 1.00–2.18).  Conclusions In clinical practice settings, delivery of twice weekly sessions of CBT and IPT for depression is a way to improve depression treatment outcomes."

Reading through the full text, I note that the authors commented "Participants who were randomised to the condition with twice weekly sessions received 16 sessions during the first 8 weeks of treatment, and 4 sessions during the last 8 weeks (up to 20 sessions over a period of 16 weeks). Patients who were randomised to the condition with once weekly sessions received 16 sessions during the first 16 weeks and 4 sessions during the last 8 weeks (up to 20 sessions over a period of 24 weeks) ... 43.3% of the sample experienced recurrent depression and self-reported duration of symptoms was 40.87 months (s.d. 68.31; median 20) ... patients received a mean number of 16.54 (s.d. 4.73) sessions.  There was no difference between frequency conditions in the total number of sessions ... There were no differences in change over time on the BDI-II between CBT and IPT ... In summary, the present study demonstrates that in clinical practice settings both CBT and IPT are best provided twice weekly. This finding implies that reorganisation of the specialised mental healthcare services for depression (going from once a week to twice a week) can lead to less attrition, quicker response and better outcomes across the course of treatment." 

This is exciting, but not completely unexpected.  As far back as 2013, Pim Cuijpers was reporting - in the abstract of his paper "How much psychotherapy is needed to treat depression? A metaregression analysis" - that "Background: Although psychotherapies are effective in the treatment of adult depression it is not clear how this treatment effect is related to amount, frequency and intensity of therapy. Methods: To fill this gap in knowledge, the present metaregression analysis examined the association between the effects of psychotherapy for adult depression and several indicators of amount, frequency and intensity of therapy. The analysis included 70 studies (92 comparisons) with 5403 patients, in which individual psychotherapy was compared with a control group (e.g. waiting list, care-as-usual). Results: There was only a small association between number of therapy sessions and effect size, and this association was no longer significant when the analysis adjusted for other characteristics of the studies. The multivariable analyses also found no significant association with the total contact time or duration of the therapy. However, there was a strong association between number of sessions per week and effect size. An increase from one to two sessions per week increased the effect size with g=0.45, while keeping the total number of treatment sessions constant. Discussion: More research is needed to establish the robustness of this finding. Based on these findings, it may be advisable to concentrate psychotherapy sessions within a brief time frame."

And a couple of years later we had another paper that focused on this issue - "The relationship between session frequency and psychotherapy outcome in a naturalistic setting".  The paper's abstract read "Objective: The dose-response relationship in psychotherapy has been examined extensively, but few studies have included session frequency as a component of psychotherapy "dose." Studies that have examined session frequency have indicated that it may affect both the speed and the amount of recovery. No studies were found examining the clinical significance of this construct in a naturalistic setting, which is the aim of the current study. Method: Using an archival database of session-by-session Outcome Questionnaire 45 (OQ-45) measures over 17 years, change trajectories of 21,488 university counseling center clients (54.9% female, 85.0% White, mean age = 22.5) were examined using multilevel modeling, including session frequency at the occasion level. Of these clients, subgroups that attended therapy approximately weekly or fortnightly were compared to each other for differences in speed of recovery (using multilevel Cox regression) and clinically significant change (using multilevel logistic regression). Results: Results indicated that more frequent therapy was associated with steeper recovery curves (Cohen's f2 = 0.07; an effect size between small and medium). When comparing weekly and fortnightly groups, clinically significant gains were achieved faster for those attending weekly sessions; however, few significant differences were found between groups in total amount of change in therapy. Conclusions: Findings replicated previous session frequency literature and supported a clinically significant effect, where higher session frequency resulted in faster recovery. Session frequency appears to be an impactful component in delivering more efficient psychotherapy, and it is important to consider in individual treatment planning, institutional policy, and future research."

So comparing once weekly to once fortnightly sessions seems to indicate an unsurprising difference in rates of improvement but not much difference in final outcomes.  When comparing twice weekly with once weekly sessions however, there seems to be a more important difference ... that final outcomes tend to be better for those seen twice weekly.  Fascinating. 

                                      (This post is downloadable both as a PDF file and as a Word doc.)

And here is a 2022 paper - Individual differences in response to once versus twice weekly sessions of CBT and IPT for depression - looking at who might benefit more from twice weekly sessions and who from once weekly.