The fifth of the six outcome types, subjective impact of the intervention, was measured by 33 different studies, with a mean unweighted effect size of .159 and a mean weighted effect size of .152. This unweighted effect was significant in a random effects analysis with a significance level of .000035. Because subjective impact of the intervention is a rather broad domain, it was further broken up into four subcategories: positive attitude about intervention (26 studies), attempts to process/make sense of event (21 studies), intervention had no effect (1 study), and negative attitude about intervention (11 studies). Of these four subcategories, two were significant in a random effects analysis: positive attitude about intervention (r ϭ .270) and attempts to process/make sense of event (r ϭ .132). No other subjective impact outcomes approached significance in either the random or the fixed effects approach......Optimism was also found to be a significant moderator for psychological health (r ϭ .340) and reported health effect sizes (r ϭ .157), such that pessimists benefited more from the intervention, but it did not moderate overall or subjective impact effect sizes. The rest of the within-study moderators (age, gender, mood, neuroticism, alexithymia, and emotional inhibition) did not moderate any of the effect size categories....It is interesting that even though disclosure did not affect many objectively measured indicators of disease status, self-reported measures of disease activity were improved. For whatever reason, perhaps because they have improved psychological health, patients feel that they are doing better, even though lab results might indicate otherwise.

(Around page 22.) The psychological health, physical health, reported health were all around 0.05 weighted effect size or less, compared to 0.15 (3x); 'health behaviors' may've had a negative effect size. 'General functioning/life outcomes' was smaller, at 0.036 weighted (most from 'work' and 'social relationships').

So, from the sound of it, the writing therapy helped most with personal relationships, but people considerably overestimate how much it helps. Which is interesting. I was thinking that this wasn't sounding terribly impressive, but the author cover that point:

Some may argue that an effect of .075 is considered to be quite small by traditional standards (e.g., conventions by Cohen, 1988), as it accounts for only 0.56% of the variance in the measured outcomes. However, even Cohen (1988) himself stated that “there is a certain risk in offering conventional operational definitions for [the terms of small, medium, and large] for use in power analysis in as diverse a field of inquiry as behavioral science” (p. 112). Rather than relying on Cohen’s conventions, researchers have argued that the practical importance of an effect depends entirely on its relative costs and benefits (Glass, McGaw, & Smith, 1981). When one considers that the act of disclosing has virtually no costs—it is a free, noninvasive, independent activity and is perceived by participants to be helpful—it seems that any effect that is nonzero and in the positive direction is worth noting (see Prentice & Miller, 1992)...As an example, consider the act of taking a daily aspirin after a heart attack to prevent death from a second heart attack: This treatment is widely regarded in the medical community as extremely valuable, and it has a r-effect size of .034 (Rosenthal, 1994), less than half of the effect size found for experimental disclosure. Similarly, because a number of experimental disclosure studies have been conducted on college students and some of the outcomes were measures of scholastic achievement, it would also be appropriate to consider the size of the effect in light of the educational literature. In particular, educators have recently argued that effect sizes as small as .050 are reasonable effects to expect in educational research and, although small, are nonetheless important in terms of improvements in learning and achievement (Lanahan, McGrath, McLaughlin, Burian-Fitzgerald, & Salganik, 2005); our r-effect size of .075 is even a bit higher than the reasonable and important effect of .050...M. L. Smith and Glass (1977), in a review of approximately 500 studies on psychotherapy, found the equivalent of a r-effect size of psychotherapy to be about .322. Clearly, this is quite a bit larger than the effect size of .075 that was found in the present study for experimental disclosure. However, given that psychotherapy typically takes place for 1 hr per week over the course of several months (sometimes years) and is conducted by a therapist who has had many years of education and training, of course it should be the case that spending only 20 min a day for 3 days on an independent writing (or talking) activity should have an effect size that is quite a bit smaller than months of time-consuming and expensive psychotherapy. To have arrived at any other result should cause the reader to be suspicious. Indeed, it does seem quite impressive that an intervention that is so easy (requires one only to write or talk), so brief (a total of about an hour), so cost efficient (completely free), and so well received by participants (most participants enjoy it or report it to be helpful) can improve so many facets of a person’s life (psychological, physical, social, academic), even if it is considered a very small improvement by conventional standards.

The publication data is fun:

Published studies had significantly higher overall effect sizes (published, r ϭ .095; unpublished, r ϭ .054) and reported health effect sizes (published, r ϭ .141; unpublished, r ϭ .064). Publication status also marginally moderated the subjective impact effect size (published, r ϭ .213; unpublished, r ϭ .123), but there was no significant difference between published and unpublished studies for the psychological health effect size. Across the 146 studies, 52% were published...However, studies in which participants were paid had significantly higher subjective impact effect sizes than studies in which participants were given no payment (paid, r ϭ .167; unpaid, r ϭ Ϫ.006)...With 112 studies containing psychological health effect sizes and a sum of zs of 40.31, there would need to be 488 studies with null effects hidden away in file drawers to make this psychological health effect size non-significant; it seems highly unlikely that such a large number of these studies exists. In addition, it is encouraging that this relation was found only with psychological health effect sizes and not with any of the other effect size categories.

I was worried because early on it talked about the effect sizes being bigger in college students, but in the end:

No participant variables significantly moderated any of the effect size categories in the between-studies analysis.

Some useful advice:

Furthermore and perhaps more important, a number of moderators of experimental disclosure were identified with a random effects approach; effect sizes tended to be larger when studies

• included only participants with physical health problems,
• included only participants with a history of trauma or stressors,
• did not draw from a college student sample,
• had participants disclose at home,
• had participants disclose in a private setting,
• had more male participants,
• had fewer participants,
• paid the participants,
• had follow-up periods of less than 1 month,
• had at least three disclosure sessions,
• had disclosure sessions that lasted at least 15 min,
• had participants who wrote about more recent events,
• instructed participants to discuss previously undisclosed topics,
• gave participants directed questions or specific examples of what to disclose,
• gave participants instructions regarding whether they should switch topics,
• and did not collect the products of disclosure.

So it sounds like for us, the basic idea should be to "write at home in private for non-distribution for half an hour every few days for a week about something you haven't dealt with before".

This actually sounds kind of similar to Alicorn's Luminosity journalling stuff, going on my vague memories.