Recently, when I was assessing my career options in terms of impact I prepared a rough estimate of the social impact of psychotherapy. I haven't encountered such an estimate before - other than in a google sheet calculation that I received from an fellow EA via FB. I thought this might be helpful to people who are interested of doing a more comprehensive review of mental health as a cause.
This is an attempt to roughly estimate the impact of a psychotherapist in units of QALYs added per therapist-hour. One QALY corresponds to value of one year lived in perfect health. This estimate is an upper bound for the social impact because it does not take into account the effects of diminishing returns and replaceability. Similar estimate has been earlier compiled by Lynette Bye .
Following data are from a study on long-term cost-effectiveness of CBT in treating depression [2,3]:
- Mean annualized QALYs in intervention group 0.596 vs. 0.544 control group (usual care including antidepressants)
- Follow-up 3-5 years after intervention, on average 40 months after therapy ended
- Number of CBT sessions received: 12 – 18 (duration 50-60min)
Using these data is possible to estimate additional QALYs gained per therapist-hour:
- Additional QALYs: 0.596 – 0.544 = 0.052 for, on average 40/12 = 3.33 years resulting in 0.052*3.33 = 0.173 QALYs gained.
- 15 sessions lasting on average 55 minutes means 15*55/60 = 13.75 hours
- The resulting estimate is 0.173/ 13.75 = 0.0126 QALY/therapist-hour
A second study on cost-effectiveness of interventions for social anxiety disorder  (this is one of the studies included in earlier estimate ) contains the following data:
- Best CBT treatment (ICBT, C&W) in resulted in average 3.75 QALYs gained vs. waitlist average 3.37 QALYs gained
- Best treatment (ICBT, C&W) included 14 individual sessions, 90 minutes each, resulting in 21 therapist-hours per intervention
These data result in an estimate of (3.75-3.37) / 21 = 0.0181 QALY/therapist-hour.
The two studies resulted in estimates in the range of 0.012-0.018 QALY/therapist-hour. This is in the same ballpark as the result of earlier calculation : roughly 0.157 QALY / 10 sessions = 0.016 / session.
Even though these calculations are based on two, in my opinion high quality studies, the estimate is rough because the basis is just two studies whose QALY estimates contain a fair bit of uncertainty. Secondly, it is unclear how long the effects of therapy persist. Based on these studies, they might persist for a long time (5 years) but it is unclear what is the average effect. Thirdly, improved mental health might have positive secondary network effects for the well-being of patient’s immediate social circle, e.g. children, spouse and close relatives. These hypothetical network effects are not included in the estimate.
To assess the impact of psychotherapy alone, it is better to compare psychotherapy to treatment as usual without psychotherapy (i.e. pharmacotherapy) rather than no treatment. Estimate #1 compared psychotherapy to usual care whereas estimate #2 compared it to waitlist which makes estimate #1 somewhat more credible. Overall, taking into account the high level of uncertainty involved, I estimate that the impact of psychotherapy is in the range of 0.005-0.03 QALY/therapist-hour for mental health problems with good treatment response to therapy such as depression and social anxiety.
Notes regarding impact over career
By estimating the number of therapist-hours a psychotherapist works over his or her career it is possible to gauge the overall impact. For example, assuming 20 therapist-hours per week, that is 27 45-minute sessions per week, for 45 weeks per year for 30 years one gets an estimate of 20*45*30 * [0.005 – 0.03] = 135 – 810 QALYs or 5 – 27 lives saved using conversion rate of 30 QALYs = “1 life saved” .
 Wiles, N. J., Thomas, L., Turner, N., Garfield, K., Kounali, D., Campbell, J., … & Williams, C. (2016). Long-term effectiveness and cost-effectiveness of cognitive behavioural therapy as an adjunct to pharmacotherapy for treatment-resistant depression in primary care: follow-up of the CoBalT randomised controlled trial. The Lancet Psychiatry, 3(2), 137-144.
 Wiles, N., Thomas, L., Abel, A., Ridgway, N., Turner, N., Campbell, J., … & Kuyken, W. (2013). Cognitive behavioural therapy as an adjunct to pharmacotherapy for primary care based patients with treatment resistant depression: results of the CoBalT randomised controlled trial. The Lancet, 381(9864), 375-384.
 Mavranezouli, I., Mayo-Wilson, E., Dias, S., Kew, K., Clark, D. M., Ades, A. E., & Pilling, S. (2015). The cost effectiveness of psychological and pharmacological interventions for social anxiety disorder: a model-based economic analysis. PloS one, 10(10), e0140704.