The Dodo Bird Verdict Is Alive and Well—Mostly

Abstract

We examined 17 meta-analyses of comparisons of active treatments with each other, in contrast to the more usual comparisons of active treatments with controls. These meta-analyses yielded a mean uncorrected absolute effect size for Cohen's d of .20, which is small and non-significant (an equivalent Pearson's r would be. 10). The smallness of this effect size confirms Rosenzweig's supposition in 1936 about the likely results of such comparisons. In the present sample, when such differences were corrected for the therapeutic allegiance of the researchers involved in comparing the different psychotherapies, these differences tend to become even further reduced in size and significance, as shown previously by Luborsky, Diguer, Seligman, et al. (1999).

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coming, but in 1975 Luborsky, Singer, and Luborsky
examined about 100 comparative treatment studies and
found that Rosenzweig’s hypothesis was essentially right:
There was a trend of only relatively small differences from
comparisons of outcomes of different treatments. Around
that time researchers began to call such small differences
by the title from Rosenzweig’s quote from Alice in Wonder-
land: “everybody has won, so all shall have prizes” which
was the “Dodo bird’s verdict” after judging the race. The
term “Dodo bird verdict” has since become commonly
used, and researchers have continued to write articles for
or against the existence of, or the meaning of, that trend.
In this study we aimed to survey and then to evaluate
whether Rosenzweig’s (1936) hypothesis is still fitting and
still flourishing. We examined the exact amount of sup-
port for this trend, for the task is still very necessary; even
expert psychotherapy researchers have different opinions,
and even high affect, about the expected results. For a
brief sample of these many opinions, see Beutler (1991);
Crits-Christoph (1997); Cuijpers (1998); Henry (1998);
Howard, Krause, Saunders, and Kopta (1997); Luborsky
(1995); King (1997); King and Ollendick (1998); Lubor-
sky, Diguer, Luborsky, and Schmidt (1999); Nietzel, Rus-
sell, Hemmings, and Gretter (1987); Reid (1997);
Tschuschke et al. (1998); Wampold, Mondin, Moody, and
Ahn (1997); and Wampold, Mondin, Moody, Stich, Ben-
son, and Ahn (1997).
THE USE OF A COLLECTION OF META-ANALYSES TO
CHECK THE DODO BIRD VERDICT
Before we present results of our collection of meta-
analyses of studies on this topic we must review our rea-
We examined 17 meta-analyses of comparisons of ac-
tive treatments with each other, in contrast to the more
usual comparisons of active treatments with controls.
These meta-analyses yielded a mean uncorrected abso-
lute effect size for Cohen’s
d
of .20, which is small and
nonsignificant (an equivalent Pearson’s
r
would be.10).
The smallness of this effect size confirms Rosenzweig’s
supposition in 1936 about the likely results of such
comparisons. In the present sample, when such differ-
ences were corrected for the therapeutic allegiance of
the researchers involved in comparing the different
psychotherapies, these differences tend to become
even further reduced in size and significance, as shown
previously by Luborsky, Diguer, Seligman, et al. (1999).
Key words:
comparisons of active psychotherapies,
psychotherapy outcomes, correction for researcher’s
treatment allegiances, empirically validated treatments,
Dodo bird verdict.
[Clin Psychol Sci Prac 9:2–12,
2002]
Saul Rosenzweig’s seminal survey of 1936, “Some implicit
common factors in diverse methods of psychotherapy,”
launched the field of psychotherapy’s lasting interest in
this topic. He supposed that the common factors across
psychotherapies were so pervasive that there would be
only small differences in the outcomes of comparisons of
different forms of psychotherapy. It was a long time in
Address correspondence to Lester Luborsky, 514 Spruce Street,
Philadelphia, PA 19106. E-mail: luborsky@landru.cpr.upenn.
edu.
The Dodo Bird Verdict Is Alive and Well—Mostly
Lester Luborsky, University of Pennsylvania
Robert Rosenthal, University of California
Louis Diguer, Laval University
Tomasz P. Andrusyna, University of Pennsylvania
Jeffrey S. Berman, University of Memphis
Jill T. Levitt, Boston University
David A. Seligman, Access Measurement Systems
Elizabeth D. Krause, Duke University
2002 AMERICAN PSYCHOLOGICAL ASSOCIATION D12 2
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