I was listening to a podcast entitled Very Bad Wizards, hosted by two university academics. One of the hosts, namely David Pizarro who is a psychologist at Cornell made the claim that most psychological mechanisms have small effects. I have heard this too mentioned by Daniel Lakens on the small likelihood of psychological effects being very large > d =1.6. That made me curious. What are examples of interventions in Psychology that reliably induce long and large changes (large effects that last longer than ~3 months)? The only example that comes to mind are mood states like trait anxiety being changed nearly indefinitely through CBT with pretty big effects. A little background, I was a Psychology major and researcher in undergrad. Enjoy reading about meta research so I can go pretty technical.

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    $\begingroup$ How do you define a "large change"? Note that these Cohen's d effect size rules of thumb are arbitrary, and Cohen warns against cross context use. $\endgroup$
    – mflo-ByeSE
    Aug 6, 2017 at 13:43
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    $\begingroup$ Since you are Psychology major and researcher in undergrad your teachers should have provided you with this information at the beginning of each type of therapy. I do not know if to vote for closing this question since to elaborate an answer would have to review many subjects and the question is that it is necessary to compare the psychological interventions with placebo and non-psychological alternatives. What I can clarify is that many interventions have an effect size greater than treatment with psychopharmacology but without adverse effects. $\endgroup$ Aug 6, 2017 at 16:49

2 Answers 2


Skill acquisition: If you take a novice on a task and get them to practice that task, performance will tend to improve. See the Power Law of Practice. In many cases that I've seen, the difference between between initial and final performance is similar to the standard deviation of individual differences at baseline. So I guess you could call that a d of around 1. Of course, the exact numbers will depend on the domain, but that is indicative.

Of course, you also get forgetting effects which in some sense reduce the effect of skill acquisition over time, but there is also an associated faster relearning effect. And the degree of forgetting is related to a range of factors. In particular, motor-learning seems to have less forgetting than learning related to declarative facts.


From my optional subject Treatment in phobias:

According to the meta-analyzes of Ruhmland and Margraf in 2001, the pre-post effects of behavioral treatment on specific phobic measures ranged from 1.42 to 2.06 for the different treatments applied.

  • Effektivität psychologischer Therapien von spezifischer Phobie and Zwanggsstoerung: Meta Analyze auf Stoerungsebene (Efficacy of phychological treatments for specific phobia and obsessive compulsive disorsers). Verhaltenstherapie, 11, 14-26.

Powers and Emmelkamp found in 2008 an effect size of 1.05 for exposure techniques compared to waiting lists.

  • Powers, M. and Emmelkamp, ​​P. (2008). Virtual reality exposure therapy for anxiety disorders: The meta-analysus. Journal of Anxiety Disorders, 22, 561-569.

Regarding anxiety and dental phobia, the effect size of behavioral treatment on control groups in self-reports of dental anxiety has been 1.79 and the size of the pre-post effect 2.25; The effects of cognitive therapy have been similar. As for dental care, the size of the effect has been 1.4; On average, 80% and 77% of the patients have made at least one visit to the dentist within 6 months and 4 years, respectively (anxiety and dental phobia) (Kvale, Berggren and Milgrom, 2004).

  • Kvale, G., Berggren, U. and Milgrom, P. (2004). Dental fear in adults: A meta-analysis of behavioral interventions. Community Dentistry and Oral Epidemiology, 32, 250-264.

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